Download Chapter 1: Seismic Wave Attenuation

Crustal Attenuation in the region of the
Maltese Islands using Coda Wave Decay
By
Raffaella Bugeja
UNDER THE SUPERVISION OF
Dr. P. Galea
Department of Physics
University of Malta
May 2011
A dissertation presented to the Faculty of Science in part fulfillment of the requirements for
the degree of Bachelor of Science (Hons.) at the University of Malta
Statement of Authenticity
The undersigned declare that this dissertation is based on work carried out under the auspices
of the Department of Physics by the candidate as part fulfillment of the requirements of the
degree of B.Sc. (Hons.).
____________________
__________________
Candidate
Supervisor
Abstract
The attenuation property of the region around the Maltese Islands was investigated by
analyzing coda waves from 43 local earthquakes in the south of the Islands and 6 local
earthquakes in the North-West of the Islands using the single back scattering model (Aki and
Chouet, 1975). These were digitally recorded by the WDD station at Wied Dalam, Malta
during the period January 2006 - January 2011. The frequency dependent coda Q values were
calculated through the CODAQ subroutine in the seismic analysis package SEISAN 8.0 by
applying the time domain coda-decay method. The coda Q was computed at central
frequencies from 2 to 12 Hz. Coda Q values obtained show a clear dependence on f according
to the relation
Pantelleria to
. The relationship varies from
in the North-West near
in the South of the Maltese Islands. The average Q values vary
from 229 ± 93 at 2 Hz to 1984 ± 281 at 12 Hz in the south region and 117 ± 11 at 2 Hz to 4028
± 3073 at 12 Hz in the North-West region. The variation of Q with frequency reflects the
structural inhomogenity around the Maltese Islands. The subduction zone near Crete was
chosen as another area of study so that the attenuation results obtained for the Maltese Islands
could be compared to this region. The relationship obtained for this area is
.
Dedicated to
Mum and Dad
Acknowledgements
Would like to thank my tutor Dr. Pauline Galea for her constant guidance and help with this
dissertation. My thanks also go to Dr Sebastiano D’Amico for his support shown regarding
programming. Finally I wish to thank my family and friends for their encouragement and
unlimited patience.
Contents
Chapter 1 Seismic Wave Attenuation ......................................................................................... 1
1.1 Seismic waves ................................................................................................................. 1
1.2 Introduction to Seismic Attenuation ............................................................................... 7
1.2.1 Geometrical spreading .............................................................................................. 7
1.2.2 Intrinsic Attenuation ............................................................................................... 10
1.2.3 Scattering attenuation ............................................................................................. 12
1.3 Coda Waves .................................................................................................................. 14
1.3.1 Coda Analysis ......................................................................................................... 15
1.3.2 Phenomenological Modeling of Coda wave excitation .......................................... 16
1.3.3 Scattering Characteristics ....................................................................................... 18
1.3.4 The AC (Aki and Chouet) Method: Single back-scattering model ........................ 21
1.4 Coda-Attenuation Measurements .................................................................................. 24
1.4.1 Tectonic dependence of Coda Attenuation............................................................. 25
Chapter 2 Tectonics and Seismicity of the Maltese Islands Region ....................................... 27
2.1 History of the Mediterranean ........................................................................................ 27
2.2 Tectonics of the Mediterranean Region and the Maltese Islands ................................. 29
2.3 Pelagian Platform and the Pantelleria Rift System ....................................................... 31
2.4 Seismicity around the Maltese Islands .......................................................................... 32
Chapter 3 SEISAN - Earthquake Analysis Software .............................................................. 36
3.1 Structure of Seisan-Directories ..................................................................................... 36
3.2 Waveform Data ............................................................................................................. 38
3.2.1 Data Format ............................................................................................................ 38
3.3 Programs ....................................................................................................................... 40
3.4 Calculation of coda q, CODAQ .................................................................................... 41
3.4.1 Input ........................................................................................................................ 41
3.4.2 Operating CODAQ ................................................................................................. 44
3.4.3 Output ..................................................................................................................... 47
Chapter 4 Data Processing ...................................................................................................... 49
4.1 Seismic Recording in Malta .......................................................................................... 49
4.1.1 The Wied Dalam Station, WDD ............................................................................. 50
4.1.2 Aims of the Malta seismograph station .................................................................. 53
4.2 Seismic Monitoring and Research Unit at the University of Malta .............................. 54
4.2.1 Earthquake locations ............................................................................................... 54
4.2.2 The Website ............................................................................................................ 56
4.2.3 The Seismic Database, Online ................................................................................ 56
4.3 The Data Set .................................................................................................................. 60
4.3.1 South of Malta Events ............................................................................................ 60
4.3.2 North-West of Malta Events ................................................................................... 61
4.3.3 Crete Events............................................................................................................ 62
4.3 Calculating the Qc - values ............................................................................................ 63
Chapter 5 Results .................................................................................................................... 66
5.1 The frequency dependence of Q relationship................................................................ 66
5.2 Results for the South of Malta Earthquakes .................................................................. 67
5.2.1 The frequency dependence of Q relationship for the South of Malta Events ........ 71
5.3 Results for the North-West of Malta Earthquakes ........................................................ 72
5.3.1 The frequency dependence of Q relationship for the North-West of Malta Events 76
5.4 Results for the Subduction Zone near Crete Earthquakes ............................................. 77
5.4.1 The frequency dependence of Q relationship for the Crete Events ........................ 81
5.4.2 The variation of Q with depth for the Crete Events ............................................... 82
Chapter 6 Discussion .............................................................................................................. 89
6.1 Analyzing the results..................................................................................................... 89
6.2 Comparing the South of Malta, North-West of Malta and Crete Results ..................... 90
6.3 Comparison with other Areas........................................................................................ 93
6.3 Further Work ................................................................................................................. 94
References ................................................................................................................................. 89
APPENDIX 1 Recorded Events
APPENDIX 2 Coda Q Values
List of Tables
Table 3.1: The main subdirectories of SEISAN ...................................................................... 36
Table 3.6: Abbreviations of SEISAN ...................................................................................... 45
Table 5.1: Results for the South of Malta Events .................................................................... 67
Table 5.2: Results for the North-West of Malta Events .......................................................... 72
Table 5.3: Results for the Crete Events ................................................................................... 77
Table 5.4: Estimated Q0 at Different Depths for Crete ............................................................ 82
Table 6.3: Frequency dependence of Qc for different tectonic areas……………………........95
List of Figures
Figure 1.1:
Propagation of P and S waves ............................................................................... 3
Figure 1.2:
Reflected and Refracted Seismic Waves .............................................................. 4
Figure 1.3:
Reflection and Refraction of body waves through the Earth ............................... 4
Figure 1.4:
A seismograph showing the different phases and the 3 earthquake components. 5
Figure 1.5:
The forms of ground motion near the ground surface of a Rayleigh wave .......... 6
Figure 1.6:
The forms of ground motion near the ground surface of Surface Love wave ...... 6
Figure 1.7:
Cylindrical area showing the wave energy propagation ....................................... 8
Figure1.8:
Geometrical spreading of body and surface waves ............................................. 10
Figure 1.9:
A representation of seismic scattering. ............................................................... 13
Figure 1.10: Seismogram showing the P wave, S wave and the coda ..................................... 14
Figure 1.11: Modeling a random medium as a distribution of point-like scatte. ..................... 18
Figure 1.12: Differential scattering cross-section of a single scatteres ................................... 19
Figure 1.13: Geometry of the single backscattering model ..................................................... 21
Figure 1.14: Seismograms showing high and low coda attenuation........................................ 24
Figure 2.1:
Bathymetric Map of Central Mediterranean around the Maltese Islands ........... 29
Figure 2.2:
Bathymetry of the Sicily Chanel………………………………………………..32
Figure 2.3:
Seismicity in the Mediterranean region between 1980 and 2000 ....................... 33
Figure 2.4:
More reliably located seismicity, 1990-2003 ...................................................... 34
Figure 2.5:
A seismogram of an earthquake ......................................................................... 35
Figure 3.2:
Structure of SEISAN ........................................................................................... 37
Figure 3.3:
Example of an input file ...................................................................................... 41
Figure 3.4:
Example of a parameter file ................................................................................ 42
Figure 3.5:
Calculating Codaq ............................................................................................... 44
Figure 3.7:
Type line 4 using Nordic Format ........................................................................ 46
Figure 3.8:
An example of an output file ............................................................................... 47
Figure 3.8:
A codaq plot for an earthquake .................................................................... …. 48
Figure 4.1:
Location of the WDD station .............................................................................. 51
Figure 4.2:
Wied Dalam Station WDD in the south of Malta. .............................................. 51
Figure 4.3:
The MedNet Network.......................................................................................... 53
Figure 4.4:
The main page of the Seismic Monitoring and Research Unit website .............. 55
Figure 4.5:
The real-time plot for Februaury 2011 ................................................................ 57
Figure 4.6:
The online database of seismic events ................................................................ 58
Figure 4.7:
Single event displayed online .............................................................................. 59
Figure 4.8:
A hybrid map showing the South of Malta earthquakes ..................................... 60
Figure 4.9:
A hybrid map showing the North-West of Malta earthquakes............................ 61
Figure 4.10: A hybrid map showing the Crete earthquakes .................................................... 61
Figure 4.11: Calcualtion of
.......................................................................................... 64
Figure 4.12: Procedure in calculating Qc ................................................................................ 65
Figure 5.1:
A graph of Qc against frequency for the South of Malta events ......................... 68
Figure 5.2:
A graph of the Average Q values against frequency for the South of Malta ...... 69
Figure 5.3:
A Graph of ln (Qc) against ln (f) for the South of Malta events .......................... 70
Figure 5.4:
A graph of Qc against frequency for the North-West of Malta events ................ 73
Figure 5.5:
A graph Average Q values against frequency for the North-West of Malta. ...... 74
Figure 5.6:
A Graph of ln (Qc) against ln (f) for the North-West of Malta events ................ 75
Figure 5.7:
A graph of Qc against frequency for the Crete events ......................................... 78
Figure 5.8:
A graph of the Average Q values against frequency for the Crete events. ......... 79
Figure 5.9:
A Graph of ln (Qc) against ln (f) for the Crete events ......................................... 80
Figure 5.10: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range
4-15 km ..................................................................................................................................... 83
Figure 5.11: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range
17-25 km ................................................................................................................................... 84
Figure 5.12: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range
26-35 km ................................................................................................................................... 85
Figure 5.13: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range
36-45 km ................................................................................................................................... 86
Figure 5.14: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range
46-53 km ................................................................................................................................... 87
Figure 5.15: A Graph of Q0 against depth for the Crete events ............................................... 88
Figure 6.1:
A Google Map showing different types of crust between the Malta Escarpment
and Ionian Region ..................................................................................................................... 92
Figure 6.2:
A graph of the Average Q values against frequency for the North-West of Malta,
the South of Malta and Crete evens .......................................................................................... 93
Figure 6.3:
Comparing the South of Malta seismograms to the North-West of Malta
seismograms.............................................................................................................................. 94
Figure 6.4:
Comparison of the Qc relations obtained in different tectonic and volcanic are. 97
Chapter 1: Seismic Wave Attenuation
Chapter 1 Seismic Wave Attenuation
In this chapter a brief introduction on seismic wave attenuation is given. A discussion
on coda waves and their properties is also included in this chapter. The back scattering
model (Aki and Chouet, 1975) is also discussed, which is a way to model coda wave
excitation.
1.1 Seismic waves
Seismic waves are waves of energy that travel through the earth for example after an
earthquake. There are two main types of seismic waves, the surface waves and the body
waves.
Body waves are waves that penetrate deeply thorough the interior of the Earth. These
waves represent short pulses of propagating energy. They follow refracted raypaths
determined by the elastic moduli and densities of different regions of the Earth’s interior.
There are two types of body waves generated, the P and S waves (Lay and Wallace, 1995).
The P waves are the fastest moving waves and are simply sound waves. The P wave is a
longitudinal wave made up of a series of compressions and rarefactions. This type of wave
forces the point in Earth from where it passes to vibrate back and forth in the direction in
which the wave is travelling. The equation of P waves is given by:
(1.1)
where
is the density.
is the cubic dilatation given by the sum of the longitudinal strains i.e.
.
1
Chapter 1: Seismic Wave Attenuation
and
are the Lame constants.
is the rigidity modulus and
is defined as
where K is the bulk modulus.
This is a scalar wave equation representing the propagation of the dilatation
This
means that it represents a disturbance in which the material expands and compresses
periodically. This is a case in which there is a change in volume that does not result in a
change in shear. The P-wave velocity is
given by
(1.2)
where the constants are defined earlier.
The S waves unlike the P waves are transverse waves. They cause the particles of the
medium to move perpendicular to the path along which the wave is travelling. The S wave
being a transverse wave is polarized in two perpendicular planes, the vertically polarized
components, Sv and the horizontally polarized components, SH. These two components are
identical in the case of an isotropic medium but are separate components travelling at different
speeds if the medium not isotropic.
The equation of the S waves is given by:
(1.3)
The quantity that is propagating is
. When the components of this term are
considered it can be shown that this quantity represents a rotational disturbance without a
change in volume. This means it is a shear wave called an S wave. The S-wave velocity is
given by:
2
Chapter 1: Seismic Wave Attenuation
(1.4)
In these equations β is always smaller than α. This means that the S waves always travel
slower than the P waves. It can be shown that in a liquid µ=0. This means that S waves do not
propagate in a liquid.
Figure 1.1: Propagation of P and S waves (Available
http://science.jrank.org/pages/48108/seismic-body-waves.html)
online
from:
P and S waves propagate independently of each other. The seismic body waves travel in
ray paths perpendicular to the wavefront. These are paths that small packets of seismic energy
follow as it travels throughout the Earth. The velocity of the wave changes as it propagates
and so the ray paths are bent according to Snell’s law (Snellius, 1621). This change in the
wave velocity is mainly due to strong discontinuities like changes in the type of rock along
which the wave is propagating. These discontinuities act as interfaces that reflect the seismic
body waves like a mirror and refract them like a lens. This is shown in figure 1.2.
3
Chapter 1: Seismic Wave Attenuation
Figure 1.2: Reflected and Refracted Seismic Waves
http://science.jrank.org/pages/48108/seismic-body-waves.html)
(Available
online
from:
Body waves generated by earthquakes travel from the core to the mantle and may be
refracted from the core-mantle interface. They can also travel through the core and emerge on
the other side of the Earth. Body waves were observed to travel along particular paths. These
paths are referred to as phases and are labeled as PcP, PKP, Pn, PmP in the case of P waves
and ScS ect for the case of S waves (Robertson, data unknown). These phases are shown in
figure 1.3.
Figure 1.3: Reflection and Refraction of body waves through the Earth (Available online
from: http://www.sciencebuddies.org/science-fair-projects/project_ideas/Geo_p018.shtml)
The phase SkS involves the conversion of the S-wave energy at the core-mantle
boundary to form a P wave which travels throughout the core before being converted to the
opposite side of the core. As a seismic disturbance reaches the surface of the earth from the
interior, the motion of the ground surface is a combination of both types of waves. A
seismograph usually records three components of ground motion. One vertical, Z component
and two horizontal components aligned NS and WE. A seismograph showing the phases and
these 3 components is shown in figure 1.4.
4
Chapter 1: Seismic Wave Attenuation
Figure 1.4: A seismograph showing the different phases and the 3 earthquake
components. (15/02/2007, recorded by the Maltese Station, WDD)
Another type of Seismic waves is the surface waves. Surface waves are confined to a
surface layer of the earth usually crust and the upper mantle. The lower the frequency, the
larger the depth sampled by the waves. The amplitude of the waves decreases exponentially
with depth. There are two types of surface waves that are the Rayleigh waves and the Love
waves.
The Rayleigh waves, Lp are confined to a vertical plane containing the direction of
propagation (Rayleigh, 1887). They are a combination of P and SV displacements, in which
the particle motion is retrograde ellipse, with the major axis vertical. Retrograde ellipse
motion is a combination of a transverse and a longitudinal wave. This means it is a
combination of back and forth and up and down motion. Rayleigh waves can be thought of
arising from the constructive interference of multiple reflected P and S waves. As the depth
increases the size of this ellipse gets smaller until it decreases to zero. The amplitudes of these
waves diminish slowly with distance. Surface waves are usually the prominent feature on
seismograms, since they propagate as to encircle the Earth many times.
5
Chapter 1: Seismic Wave Attenuation
Figure 1.5: The forms of ground motion near the ground surface of a Rayleigh wave
(Earthquakes, B.A. Bolt, 1999)
Seismic surface waves of the second type are the Love waves, Lo are transverse waves
confined to the horizontal plane (Love, 1911).This means that particle motion is perpendicular
to the direction of propagation. Love waves are actually produced by SH waves guided in a
surface layer in which the S-wave velocity is smaller than the underlying medium.
Figure 1.6: The forms of ground motion near the ground surface of Surface Love wave.
(Earthquakes, B.A. Bolt, 1999)
6
Chapter 1: Seismic Wave Attenuation
1.2 Introduction to Seismic Attenuation
The main concern in discussions is usually the elastic properties of the earth. The
amplitude of a seismic pulse in an idealized, purely elastic earth is controlled by the reflection
and transmission of energy at the boundaries and by geometric spreading. These seismic
waves can propagate indefinitely once they are excited. But this would be true if the earth was
perfectly elastic. It is known that the real earth is not perfectly elastic. This causes the waves
that are propagating to attenuate with time as they travel. This attenuation in the propagating
waves is caused due to various energy loss mechanisms.
Due to the conservation of energy, it is known that the energy switches from being
potential to kinetic exactly without any losses. However this is only true if there is no other
form of energy involved. As the wave travels, its energy experiences a continuous conversion
between potential energy due to the particle position and kinetic energy due to the particle
velocity. This energy conversion is not perfectly reversible as the wave propagates. Apart
from these types of energies that are continuously being exchanged as the wave propagates,
there is also work being done. This work done can take many forms such as work done as the
wave travels along mineral dislocations. Work is also done as shear heating at the grain
boundaries. These processes are described collectively as internal friction. These will all affect
the energy of the wave as the wave travels away from the seismic source. The simplest way to
describe attenuation is by using an oscillating mass attached to a spring.
1.2.1 Geometrical spreading
Seismic wave amplitudes suffer changes as they travel across the earth. As the
wavefront moves out from the source, the initial energy released in the earthquake is spread
over an ever-increasing area and thus the intensity of the wave decreases with distance.
7
Chapter 1: Seismic Wave Attenuation
Energy intensity is the total energy flow through a unit area in a unit time. The wave
energy propagation direction coincides with the area of the cylinder
Figure 1.7: Cylindrical area showing the wave energy propagation
(1.5)
where v is the propagation velocity of the waves. By the conservation of energy the total
energy at any moment should be constant.
Consider two wave fronts. These form two spherical shells whose centers coincide at the
source. The greater radius of the outer shell is r2 and the radius of the inner shell is r1. The
surface areas of the outer and inner shells are
and
respectively. By the
conservation of energy the total energy flowing through the inner and outer shell should be the
same and so
(1.6)
(1.7)
(1.8)
8
Chapter 1: Seismic Wave Attenuation
(1.9)
(1.10)
(1.11)
Generalizing thus gives that
(1.12)
The amplitude decays as . This is the geometric spreading for spherical waves.
The same can be done for an infinitely long line source, the shape of the wavefront is a
cylinder and so this is referred to as the cylindrical wave.
The same is repeated and the conclusion is that
(1.13)
The amplitude decays as
for the waves generated by a line source.
This can be generalized to seismic waves. In body waves the energy spreads over a
hemisphere. The intensity therefore varies as:
(1.14)
Since the intensity is proportional to the square of the amplitude, then the amplitude of
body waves is proportional to 1/r. In the case of surface waves, the energy spreads out
approximately along the curved side of a cylinder, whose height is equivalent to the
penetration depth of the surface wave. Thus
9
Chapter 1: Seismic Wave Attenuation
(1.15)
and therefore the amplitude of the surface waves is proportional to
This shows that body
waves attenuate faster than surface waves. This is in fact shown on a seismogram where at
long distances from the earthquake the surface waves are the dominant feature.
The geometric spreading alone cannot describe the complete attenuation of seismic wave
energy. The decrease of the kinetic energy of seismic waves is also due to the energy
absorption caused the imperfections in the earth. This is the case when the elastic energy is
completely transferred to the mantle.
Figure1.8: Geometrical spreading of body and surface waves
1.2.2 Intrinsic Attenuation
There is another factor that affects seismic amplitudes. This is energy loss due to
anelastic processes or internal friction during wave propagation. This is called intrinsic
attenuation (Shearer, 1999). The strength of intrinsic attenuation is given by the dimensionless
quantity Q in terms of the frictional energy loss per cycle
(1.16)
10
Chapter 1: Seismic Wave Attenuation
where E is the peak strain energy and
is the energy loss per cycle. Q is usually known as
the quality factor. It is often needed to talk about the inverse of the quality factor, Q-1. Q is
inversely related to the strength of the attenuation. This means that in regions where Q is
found to be low are more attenuating than regions where Q is found to be high. An
approximation may be derived which is valid when considering that Q >> 1. This
approximation is better suited for seismic application:
(1.17)
where x is measured along the propagation direction and c is the velocity. This equation shows
that for a constant value of Q, the higher the frequency the higher the attenuation. This is
because for a given distance the high frequency wave will go through more oscillations than a
low frequency wave. As the wave travels away from the source, the pulse broadens at
successive distances. As the wave propagates, attenuation removes the high frequency
component of the pulse.
The constant c depends whether it is a P wave or an S wave.
attenuation
and c =
for S waves with attenuation
for P waves with
. The amplitude of harmonic waves
may then be written as a product of a real exponential and an imaginary exponential. The
amplitude decay due to attenuation is incorporated in the real exponential while the imaginary
exponential describes the oscillations. These two exponentials are brought together into one
equation that gives the amplitude of harmonic waves:
(1.18)
The exponentials can be combined together and the effect of Q is now found in
.
This is done by adding a small imaginary part to the velocity c.
This equation can also be written in terms of time. This is better suited when considering
a seismic application since the wave is propagating forward in time:
(1.19)
11
Chapter 1: Seismic Wave Attenuation
P waves and S waves have different values of Q with the values of the S waves usually being
larger than the values of Q for the P waves. This is because of the shear motion involved
between particles that lead to more frictional heating. Qα and Qβ are the values of Q for the P
waves and the S waves respectively. Intrinsic attenuation occurs mostly in shear wave motion.
In fact it is associated with lateral movements of lattice effects and grain boundaries. As the
density and the velocity of the material increases, Q increases. It has been found that if the
loses of a material are only due to shearing mechanisms then
(1.20)
For frequencies up to 1.0Hz the quality factor, Q for seismic waves is independent of
frequency. As the frequencies increase, Q becomes frequency dependent and in general it
increases with frequency. There are many ways to determine Q. A common way to determine
Q is by knowing the amplitude and frequency of the seismic wave at some point during its
propagation. A number of seismic rays that have travelled the same path or rather a similar
path are usually considered. Their amplitudes and frequencies are then compared.
1.2.3 Scattering attenuation
There is another different type of attenuation called scattering attenuation. This is the
effect of seismic amplitudes in the main seismic arrivals are reduced by scattering off smallscale heterogeneities. This is different from other types of attenuation since the integrated
energy in the total wave field remains constant.
The region of the earth to about 100 km is known as the lithosphere. This refers to the
solid part of the earth and its thickness varies from one place depending on the tectonic setting
of the area. The heterogeneities of the Earth have been investigated using different methods
both geological and geophysical. Seismic velocities and density of rocks give a geophysical
characterization. On the other hand the evolution of rocks gives a geological interpretation of
such heterogeneities. They analyze the rocks from within the earth that gives a sign of
heterogeneity. There are many factors which contribute to the heterogeneity of the lithosphere.
12
Chapter 1: Seismic Wave Attenuation
These include tectonic processes such as faulting, and large scale crustal movements.
Scattering of high-frequency seismic waves shows the existence of such small scale
heterogeneities in the lithosphere. High frequency waves interact with discontinuities and
small-scale heterogeneities, so that the main arrivals are drawn out into a coda. Low frequency
meaning long wavelength waves are unaffected by small scale reflectors.
In figure 1.9 one can see seismic waves propagating after a seismic disturbance,
propagating away from the source. The S wave travels the shortest path and so arrives at the
seismic station before all the other waves which interact with the heterogeneities. The other
wave amplitudes are scattered off by the small-scale heterogeneities and so they arrive after
the S wave and have smaller amplitude than the S wave. These are in fact the coda waves.
This study is focused on scattering attenuation and will be explained in more detail in the next
section.
Figure 1.9: A representation of seismic scattering.
The quality factor representing the total attenuation, Qt is given by:
where
is the quality factor due to scattering losses
is the quality factor due to intrinsic absorption.
13
Chapter 1: Seismic Wave Attenuation
1.3 Coda Waves
One of the properties used to study the structure of the earth is the attenuation of seismic
waves in the lithosphere at high frequencies ranging from 1Hz up to 20 Hz. The most
important evidence that the earth is heterogeneous is that in seismograms of local earthquakes
there is the appearance of the coda waves. On seismograms this is seen as the direct S wave
being followed by wave trains whose amplitude decrease exponentially as the lapse time1
increases. These wave trains are called S coda waves or simply coda waves. Initially the word
coda didn’t refer to these wave trains but it used to refer to the oscillations of the ground as the
surface waves propagated through it or the tail portion in the seismogram. The definition of
the word coda has recently changed and now coda refers to all wave trains excluding the direct
waves that propagate after a seismic disturbance. This is shown in figure 1.10. There are two
different types of coda. P coda refers for waves between direct P and S waves and S coda
refers to the waves following the direct S waves. As the epicentral distance increases, the
direct S wave amplitude decreases. This is true if you take lapse times large enough. At small
times the average S coda amplitudes are nearly equal independent of epicentral distances. This
is taken into account when conducting experiments and usually twice the lapse time is taken.
Figure 1.10: Seismogram showing the P wave, S wave and the coda
1
The lapse time is the difference in time between the S wave starting time and the origin time
14
Chapter 1: Seismic Wave Attenuation
Rautian and Khalturin (1978) studied coda wave amplitude. In their study they studied
these coda amplitudes at different lapse time and frequency bands. In their study it was found
that the early portions of the coda are different from one station to another. If the data is taken
from a bandpass-filtered seismogram, coda shows no variation in shape from one station to
another after three times the S travel time from the source to the receiver. Coda is quite similar
at all stations when twice the S travel time, lapse time is taken.
The magnitude of local earthquakes can be determined if the amplitude of the direct
wave is known. The magnitude is determined from the average amplitudes of the direct waves
at many stations. This is done after correcting the distance from each station. The value for the
magnitude obtained from the amplitudes was found to be proportional to the logarithm of the
duration of a local seismogram. This duration is the time measured from the P wave arrival to
the time when the S coda amplitude decreases to the level of microseisms (Solov’ev, 1965). In
many studies all over the world, the logarithm of the duration time has been used to find the
magnitudes of each earthquake. A correlation has been found between the magnitude and the
duration time. This correlation is consistent with the similarity in shape of the portion of
seismograms. It was then concluded that coda portions of seismograms are composed of
scattered waves.
1.3.1 Coda Analysis
Many different methods have been developed to determine Q from coda waves (Aki and
Chouet, 1975; Rautian and Khalturin, 1978; Del Pezzo et al., 1983; Rovelli, 1984). As
discussed earlier, coda wave attenuation is caused by two types of effects scattering and
anelastic attenuation. These processes cannot be separated easily. Dainty (1981) has suggested
that in the frequency range 1 to 20Hz, the frequency dependence of coda Q is primarily due to
scattering while anelastic attenuation is almost frequency independent. A strong correlation
between the dependence of Q on frequency and the tectonics of the region was found by Aki
(1981), Roecker (1982) and Pulli (1984). In areas where there is strong tectonic heterogeneity,
a strong frequency dependence of coda Q was found as that compared to stable areas. This
15
Chapter 1: Seismic Wave Attenuation
relation shows that the attenuation of seismic waves as the distance from the source increase is
different for different frequencies. This means that the seismic data has to be bandpass-filtered
first before calculating the attenuation.
The Q factor increases with frequency (Mitchell, 1981) and it follows the following
relation
where
is the quality factor at the reference frequency f0 (generally 1Hz) and
frequency parameter.
is the
vary as the region varies due to the heterogeneity of the
medium (Aki, 1981). This relation shows that attenuation as the wave propagates is different
for different frequencies. Hence seismic data are first bandpass-filtered when calculations of
attenuation are made.
1.3.2 Phenomenological Modeling of Coda wave excitation
The characteristics of high frequency S-coda waves of local earthquakes were
summarized by Aki and Chouet (1975). These characteristics are the following:

The S-coda of seismic waves observed at different stations are almost identical to each
other;

A reliable measure of an earthquake magnitude can be obtained from the total
duration2 of a seismogram;

S-coda traces of different local earthquakes that are first bandpass-filtered and are
recorded within the same region have a common envelope shape. Such traces are
independent of the epicentral distance;
2
defined as the length of time between the P-wave onset and the time when the coda amplitude equals the level
of microseisms
16
Chapter 1: Seismic Wave Attenuation

The temporal decay of S-coda amplitudes is independent of the earthquake magnitude,
for earthquakes having magnitude less than 6;

The S-coda amplitude depends on the tectonics of the area where the recording station
is.
Other studies show more different characteristics of coda waves. These include:

Array measurements show that S-coda waves are not regular plane waves coming
directly from the epicenter (Aki and Tsuijura, 1959).

(Tsujiura, 1978) found that the S-coda waves are composed primarily of S-waves. This
was confirmed as his studies show that S-coda waves have the same site amplification
factor as that of direct S-waves.

S coda waves have been first identified on seismograms which were recorded at the
bottom of deep boreholes drilled in hard rock beneath soft deposits (Sato,1978; Leary
and Abercrombie, 1994).
A phenomenological model has been proposed by Aki and Chouet (1975). This model is
for coda-wave generation and is based on a number of assumptions. The earth’s lithosphere is
viewed as composed of a random and uniform distribution of point-like scatters in a
homogeneous background medium. The wave velocity in the medium is assumed to be
constant. Aki and Chouet (1975) first presented this model for the case the source and the
receiver are at the same location. This model was then extended by Sato (1977) where the
source and the receiver were not collocated. Sato did this extension of the model for body
waves while Kopnichev (1975) did it for surface waves.
Many other phenomenological models have been proposed for the generation of S-coda
waves. Before Aki and Chouet (1975) presented their model, Wesley (1965) explained the
seismogram envelopes by using diffusion – like process. In studies conducted it was found
that the coda wave have long duration. These were studied using the diffusion model, Dainty
and Toksoz, (1981). The propagation in the lunar crust can be explained using the diffusion
model. This can be done since the lunar crust have low intrinsic attenuation and a large
amount of scattering. The energy flux model was developed by Frankel and Wennerberg
17
Chapter 1: Seismic Wave Attenuation
(1987). This model is base on the fact that the energy in the scattered wave is uniformly
distributed.
1.3.3 Scattering Characteristics
To model the randomly inhomogeneous media, homogenous background media with
propagation velocity Vo filled with distributed point- like scatters with number density n are
used. This is seen in the following figure 1.10.
Figure 1.11: Modeling a random medium as a distribution of point-like scatters. (Seismic
Wave Propagation and Scattering in the Heterogeneous Earth: P, 1998).
This distribution is taken to be randomly homogenous and isotropic. The scattering has a
scattering cross section
An incident wave with energy-flux density J0 intersects a
scatterer. This is a stationary process. Due to this intersection of the incident wave with the
scatterer, spherical waves are generated having energy flux density3 J1.
3
The energy flux density is defined as the amount of energy passing through a unit area perpendicular to the
propagation direction per unit time
18
Chapter 1: Seismic Wave Attenuation
Figure 1.12: Differential scattering cross-section of a single scatterer. (Seismic Wave
Propagation and Scattering in the Heterogeneous Earth: Sato, Fehler, 1998).
The amount of energy scattered per unit time into a given solid angle element dΩ is J1r2 dΩ
where r2 dΩ is the corresponding surface element. The differential scattering cross section is
defined as the ratio
(1.21)
The scattering coefficient is the scattering power per unit volume of a medium filled
with scatterers. This is given by the product of the number density and the differential
scattering cross section [Aki and Chouet, 1975]:
(1.22)
This product, g has dimension of reciprocal length. The scattering power may be
characterized using the scattering coefficient only. In this formula there is no distinction
between a small number of strong scatterers and a large number of weak scatterers. The
scattering coefficient may be in all directions and so the total scattering coefficient is the
average over all directions:
19
Chapter 1: Seismic Wave Attenuation
Sc
(1.23)
Where
is the total scattering cross section4,
: is the mean free path5 ,
Sc
is the scattering attenuation that represents the decrease in the incident wave energy due
to scattering as the distance travelled increases. This is defined for waves of wave number k.
The energy flux density at travel distance x decays exponentially for a plane wave. It decays as
Sc
(1.24)
There are many models that are used to represent scattering. But the simplest of these models
is isotropic scattering:
(1.25)
g=g0
4
the integral of the differential scattering cross section over a solid angle
5
the reciprocal of the total scattering coefficient
(1.26)
20
Chapter 1: Seismic Wave Attenuation
The scattered waves are incoherent since the scatterers are considered to be randomly
distributed. Due to incoherence the phase may be neglected and the scattered wave power is
the summation of all the power from each scattered waves.
Figure 1.13: Geometry of the single backscattering model. (Seismic Wave Propagation and
Scattering in the Heterogeneous Earth: Sato, Fehler, 1998).
1.3.4 The AC (Aki and Chouet) Method: Single back-scattering model
This method was developed by Aki and Chouet in 1975. In this method the coda is
considered to be made up of single back scattered waves. These scattered waves are the result
of discrete randomly distributed heterogeneities. This method is a single backscattering model
that explains the coda waves as a superposition of secondary waves from randomly distributed
heterogeneities. In this method it is assumed that the distance between the source and the
receiver is negligible. Therefore this method is valid for signals that arrive long after the
primary waves. The coda wave amplitude decrease with lapse time at a particular frequency.
This is due to energy attenuation and geometrical spreading. It is independent of earthquake
source, path effect and site amplification (Aki, 1969).
Assuming single scattering from randomly distributed heterogeneities, Aki and Chouet
(1975) developed an equation for the coda wave amplitude at frequency f, and elapsed time, t
from the origin for a bandpass-filtered seismogram at central frequency f is related to the
attenuation parameter Q by the following equation:
21
Chapter 1: Seismic Wave Attenuation
(1.27)
Where
S(f) is the coda source factor at frequency f which is independent of time and radiation pattern,
f is the frequency,
Qc (f) is the quality factor of coda waves,
is the geometrical spreading parameter.
Body wave scattering has a value of 1, surface wave scattering has a value of 0.5 and diffusion
waves have a value of 0.75 (Sato and Fehler, 1998).
Studies by Aki (1981) show that the coda waves are S to S back scattered waves. This is
consistent with the observation that coda Q and Q of direct shear waves are often shown to be
identical (Aki, 1980: Kvamme, 1985). Since coda waves are body waves, in the analysis done
for the coda Q, a spreading parameter of
is assumed. It was found by Rautian and
Khalturin (1978), that equation (1) is valid only for lapse time t, greater than 2 times for S
travel time,
Taking the logarithm on both sides of equation (1.27) and arranging the
following equation is obtained:
(1.28)
(1.29)
22
Chapter 1: Seismic Wave Attenuation
The value of Q can be obtained by linear regression of
on t at a constant
f. If the slope of the graph is assumed to be b, then Q is determined using:
(1.30)
A(f,t) is usually found by bandpass-filtering the signal with a narrow passband around f
and fitting a time decay envelope to the filtered signal (Rautian and Khalturin,1978). Equation
1.30 is valid only for lapse times that are chosen to be greater than twice the S-wave travel
time. This is done so there is no interference by the data from the direct S-wave.
This method is used by the program SEISAN (Havskov and Ottermoller, 2003) to
calculate the value of the coda Q in our analysis for a number of earthquakes that are recorded
at the same station.
One Q value for the same region can be obtained after inverting simultaneously all the
data available from the decay curves that are available for the same region (Aki and Chouet,
1975; Phillips, 1985). The same result can be obtained by first obtaining one Q value for each
decay curve and then averaging the Q-1 values (Kvamme, 1985). This latter method had faster
computation and the equation can be checked for each individual case.
In all coda Q studies done it has been shown that Q increases as the lapse time increases.
As the start time for the coda window was increased and longer windows were used, the value
of coda Q also increased (Kvamme, 1985; Lee et al., 1986). The sampling volume for the
back-scattered coda waves at lapse time t is an ellipsoid with source and station at the focal
points and semi – major axis equal to
, where
is the S-wave velocity (Pulli, 1984;
Scherbaum and Kisslinger, 1985). As coda Q increased with lapse time it has been interpreted
that it is increasing with depth (Roecker et al., 1982; Pulli, 1984). Coda Q values may also
increase due to other factors. These are multiple scattering (Gao et al., 1983). Another factor
is coda model parameters example the geometrical spreading, v. Therefore to obtain best
results, coda wave time windows of a constant and fixed length that start at about the same
lapse times in order to be able to compare results from different areas .
23
Chapter 1: Seismic Wave Attenuation
1.4 Coda-Attenuation Measurements
In a study conducted by Rautian and Khalturin (1978), it was found that the S coda has a
common amplitude decay curve. This is true if the lapse time is greater than twice the S-wave
travel-time. For a given region the shape of the decay curve is the same and this decay curve is
quantified using the parameter of coda attenuation.
The coda attenuation Qc-1 is an exponential decaying function. It is independent of the
source and the location of the station but it depends on the frequency band. Qc-1 can be
measured from records observed at a single station. This makes it possible to take
measurements of the coda attenuation even at locations where there aren’t more than one
station located. This is the case of Malta where the only station available to monitor the
seismicity around the Maltese Islands is WDD station.
On a seismogram, the coda amplitude decay with lapse time is characterized by this coda
attenuation Qc-1. The larger the Qc-1 values means that the coda amplitude decay is more rapid.
This is schematically illustrated in the following figure 1.14.
Figure 1.14: Seismograms showing high and low coda attenuation
24
Chapter 1: Seismic Wave Attenuation
1.4.1 Tectonic dependence of Coda Attenuation
Values for the coda attenuation, Qc
have been obtained world wide. Since the
lithosphere is characteriszed by a heterogenety, studies have been conducted in the frequency
range between 1 to 30Hz. These measurements have been compared with seismtectonic
activity. The values of Qc vary by more than a factor of 10 from region to region. Different
regions have different tectonic activity. This is seen in the variation of measurements of Qc-1
with frequency from region to region. As a general trend in our study Qc is dependent on
frequency following the relationship
(Mitchell, 1981). The frequency-dependence
relationship obtained indicated that attenuation at higher frequencies is less pronounced than at
lower frequency. This is because high frequency waves interact with discontinuities and smallscale heterogeneities and then the main arrivals are drawn out into a coda. Low frequency
meaning long wavelength waves are unaffected by the small scale reflectors.
In studies done it has been shown that the values of Qc depend on the type of rocks
found in the region being studied. Regions which are characterized by hard, competent rocks
usually have high Qc values while areas characterized by soft, molten rocks such as volcanic
areas usually have low Qc values. The values of Qc also depend on the age of the rocks in the
area being studied. Sinn and Herrmann (1983) studied short period seismograms of local
earthquakes in the U.S.A. The highest values of Qc were found in central U.S.A. where the
type of rocks that are exposed are the oldest. This study shows that Qc is higher i.e. Qc-1 is
smaller in areas that are tectonically stable and the values are lower in areas that are active
where the lithosphere is highly heterogeneous. The frequency parameter α increases as the
tectonic activity of the region increase (Aki, 1981). An example is the Andaman Islands
(Parvez, Sutar, Mridula, Mishra & Rai, 2008). This is an active tectonic area where the
lithosphere is highly heterogeneous and so is characterized by low coda Q values.
Low-frequency dependence values have been obtained in seismically active areas
in different parts of the world (Japan, Yoshimoto et al. 1993; Northern Greece, Hatzidimitriou
1995; Turkey, Akinci & Eydogan 1996; and Horasan & Boztepe- Guney 2004). These low
25
Chapter 1: Seismic Wave Attenuation
values are the cause of processes such as faulting which are likely to introduce strong
heterogeneities. In general low-frequency-dependence of Q values are lower in volcanic
region and in the shallow crust. Keeping in mind that Q is inversely related to the strength of
attenuation, means that values of Qc-1 are higher in volcanic regions and so waves are more
attenuated in such regions (Tres Virgenes Volcanic Area Mexico: Wong, Cecilio, Munguia,
2001) and in Mt. Etna (Del Pezzo et al. 1995). This suggested that the presence of magma
under volcanic regions would contribute to the dominance of intrinsic attenuation due to
anelasticity over attenuation due to scattering losses. In particular in such tectonically active
areas the value of the frequency parameter α was found to increase up to a value of 1 (Rovelli,
1982; Kvamme and Havskov, 1989; Akinci et al., 1994; Gupta et al., 1998).
26
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
Chapter 2 Tectonics and Seismicity of the
Maltese Islands Region
2.1 History of the Mediterranean
In general it is accepted that the Earth was originally a hot gaseous mass. By time this
mass cooled down and changed from a gas state into a liquid state. It then formed a solid crust
on the surface. Evidence brought from studies show that the land masses are made of light
igneous rocks. These rocks by time have been covered by sedimentary and metamorphic
rocks. The Oceanic Crust consists mainly of rocks known as gabbro and basalt which is denser
than the igneous rock making up the Continental crust which is known as granite. The oceanic
crust areas are found under several kilometers of sea water and continental crust areas form
the main land areas but area also found in shallow seas.
As the depth and the temperature of the earth increases, the density of these rocks
increases as well. The core is the deepest part of the earth. The pressure found in the earth’s
core is enormous. Despite this pressure, the earth’s core is in a molten state. Therefore the
earth is made of a liquid core, a mantle and a crust. Oceans and continents have different crust
thickness and composition. A lot of convection currents are present in the mantle. These
currents force the crust to ride over the mantle since the crust tends to be lighter. It is thought
that in the past, these convection currents caused fractures in the crust. These fractures resulted
in a number of continental plates. These plates are moving with respect to each other and to
the Earth’s rotational axis.
They are continuously pushing together and pulling apart
depending on the direction of the currents (Pedley et al. 2002). The seismicity of the world
gives us an indication of the active regions of Earth and also roughly pictures the plate
boundaries.
27
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
Around 540 to 250 million years ago, the continental crust underlying the Maltese
Islands formed a projecting corner of the African continent. At this time Africa was part of a
large continent. South America, India, Australia and Antarctica were all part of the southern
half continent while North America was part of the northern half continent. An east-west
ocean known as the Tethys separated these two continents. This ocean used to lie on the
southern edge of the Maltese segment (Pedley et.al, 2002).
This large continent started to break up into the continents we know today around 150
million years ago. Africa and southern America separated and formed the South Atlantic
Ocean. This resulted in the eastwards movement of North Africa to southern Europe. Around
100 million years ago the Atlantic began to open. This resulted in Europe being split eastward
away from North Africa. Such movement is still going on. The continents were then drifted
apart by plate movements. This resulted in the development of a narrowing zone between
northern Africa and Southern Europe. This has now developed into an area which is known as
the Mediterranean. The movements that resulted due to the slitting of the northern Africa from
southern Europe resulted in the building up of many stresses. This gave rise to the formation
of many islands like Sardinia and Corsica. Mountainous islands some of which are volcanic
are the result of compression stresses that arose in the Mediterranean sea-bed. Malta and
southern Sicily were part of the Pelagian Spur. Around 10 million years ago this started to tear
away from the main African land. This resulted in the opening of deep sub-sea rift valleys.
The stresses in the crust between Africa and Europe lead to an opening of another new
ocean basin. This ocean basin is found between Sardinia and Calabria and which is known as
the Tyrrhenian Sea. This Sea continued to expand and Calabria was then forced to move
eastwards. As the Tyrrhenian Sea continued to expand, the Calabrian continental block
between southern Italy and the Pelagian Spur broke away from the African continent. This
resulted in a series of NW-SE fractures that then produced rift valleys in the continental crust
across the shallow Pelagian Platform (Reuther, Eisbacher, 1985).
28
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
2.2 Tectonics of the Mediterranean Region and the Maltese
Islands
The Mediterranean Sea divides the continent of Europe and the continent of Africa. The
Maltese Islands being 316 square kilometers lie in the central part of the Mediterranean
Sea between Sicily and North African coast. The Maltese Islands are found in the Sicily
Channel. They lie on a stable plateau of the African foreland, the Pelagian Platform, about 200
km south the Europe-Africa plate boundary which is part of Sicily.
Figure 2.1: Bathymetric Map of Central Mediterranean around the Maltese Islands.
(Limestone Isles in a Crystal Sea-The Geology of the Maltese Islands: Pedley, Hughes Clarke,
Galea, 2002).
29
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
The area of sea that lies between the southern Sicily and the North African coast is
shallow with sea depth not exceeding the 200 meters. In this relatively shallow sea one can
find several important deep valleys running from the northwest to southeast. These are known
as the Pantelleria Rifts. In this great mass of shallow water the only lands presents are the
Maltese Islands, the Pelagian Islands: Lampedusa, Linosa, Lampone and the Pantelleria
(Pedley et.al, 2002). This large area of mainly shallow sea separates the Eastern and Western
Mediterranean. This shallow sea area is called the Sicilian-Tunisian Platform, scientifically
known as the Pelagian Platform. The difference in sea depth between the shallow seas of the
Sicilian-Tunisian Platform and the deeper areas of the Western and Eastern Mediterranean is
visible to the east of the Maltese Islands.
In a distance of only 15 kilometers from the Maltese Islands, the variation in depths is
large. Depths vary from 200 meters to over 3000 meters and even over 4000 meters across the
Ionian Abyssal Plain. Sea depths also vary widely to the northeast of the Maltese islands. The
depths vary similarly between the shallow Sicilian-Tunisian Platform and the Western
Mediterranean basin at the west end of the Sicily Channel. These changes in depths and
escarpments are the result of long-standing geological contrasts.
These wide variations in the topography and bathymetry around the Maltese islands are
the result of processes that have been occurring over millions of years. These processes
include sedimentary deposition and volcanism that are controlled by movements in the mantle
and in the crust. The massive tectonic movements tore the lithified sediments apart and lifted
the islands above sea level. Plate tectonics theory links the most large scale features of the
world’s geology to the effects of movements of the large plates of the earth’s crust across the
surface of the globe. This concept shows that the Maltese island form a long-standing conflict
between the crustal plates of Europe and North Africa.
30
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
2.3 Pelagian Platform and the Pantelleria Rift System
The Pelagian Platform, the platform on which the Maltese islands lie, forms a shallow
shelf that separates the deep Ionian Basin from the Western Mediterranean. The sea-bed
topography is characterized by the Sicily Channel Rift Zone (SCRZ). This rift zone is a young
tectonic feature made up of three grabens which are the Pantelleria Graben, Malta Graben and
the Linosa Graben in which the eater reaches a depth of 1700 meters (Reuther and Eisbacher,
1985). This is shown in figure 2.2 below. These grabens make up a fault system that extends
throughout the Sicily Chanel from Southern Sicily to Tunisia. It has been responsible for the
major tectonic development of the Maltese islands (Illies, 1981). The SCRZ was interpreted in
many different ways. It was thought to be a set of pull-apart grabens (Reuther and Eisbacher,
1985; Reuther, 1990). It was more simply thought to be the result of the N-S extension regime
related to Tyrrhenian back-arc spreading (Argani, 1990). The rift zone was also interpreted as
a part of the Medina Wrench, which is a dextral transform fault of more than 800 kilometers.
It extends from the Sicily Channel to the Eastern end of the Medina Ridge, which is located at
200 kilometers SE of Malta.
The North African Margin have been subjected to different stresses and resulted in a
complex horst and graben system. This shearing motion has been associated with the major
shearing between the African and Eurasian Plates (Dewey et al., 1973). At the moment this
system appears to be stable with minor vertical motion taking place along the sides of the horst
blocks. The most of the fault system occurs at latitude 35˚N in the region of the PantelleriaLinosa-Malta Troughs. The direction of these normal faults indicates a NE/SW directed
tensional stress. This was attributed to the early Miocene crustal extension (Illies, 1981).
The grabens are bounded by normal faults that extended NW-SE. The rift is extending and is
being controlled by the dextral transforms that are reactivated faults. The Malta
Escarpment separates the Hyblean-Malta plateau from the deep Ionian Basin. It exhibits
normal faulting with a minor sinistral strike slip component. (Grasso et.al., 1989; Reuther et
al., 1993).
31
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
Figure 2.2: Bathymetry of the Sicily Chanel and main tectonic features of the Sicily Chanel.
This shows the Sicily Chanel Rift Zone-bounding normal faults and strike-slip lineaments
(modified after Reuther and Eisbacher, 1985 and Reuther, 1990). Also shown are the
Calabrian Arc subduction zone and epicenter of the 11/01/1693 earthquake (Boschi et al.,
2000). Inset shows the Maltese islands.
2.4 Seismicity around the Maltese Islands
Seismicity in the Mediterranean region is caused by the Eurasian and African plates.
Such plate movement caused stress which in turn builds up energy. This energy must in some
way be released and is usually released through seismic activity. A seismic map of
earthquakes that occurs in the Mediterranean region between 1980 and 2000 is shown in figure
2.3. A number of active or dormant volcanoes are found in the area around the Maltese
islands. Mt. Etna (Sicily) is found to the North; Mt. Epomeo (Ischia, Bay of Naples); the
volcanic islands of Stromboli and the Lipari Islands; and Mt. Albani, Mt. Vesuvius and the
Phlegraean Camps (Italy). The submarine Graham volcano and the volcanic island of
32
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
Panteleria both lie to the northwest of the Maltese Islands; the volcanic islands of Linosa and
Lampione are found to the southwest and Santorin volcano much further away from the
Maltese Islands.
Figure 2.3: Seismicity in the Mediterranean region between 1980 and 2000 for events
magnitude greater than 4.0 (Limestone Isles in a Crystal Sea-The Geology of the Maltese
Islands: Pedley, Hughes Clarke, Galea, 2002).
These volcanoes in the vicinity of the Maltese Islands affect its seismicity. Many
earthquakes in the past which affected the Maltese Islands were accompanied by volcanic
eruptions. In January 1692 an earthquake was felt both in Sicily and Malta. This earthquake
was accompanied by an eruption from Mt. Etna. These are called volcanic quakes which are
due to the sudden release of steam or other volcanic gases which are under pressure. These
type of earthquakes lie at different depths under the sea. The earthquakes around the Maltese
islands are not usually due to the volcanoes as these volcanoes are situated at a fair distance
away from the Maltese Islands. It is more probable that earthquakes around the Maltese
Islands are tectonic in origin and the volcanic eruptions could have been the result of the
widespread earthquakes in the region. The disturbances arising from the earthquakes occurring
close to the Maltese Islands are more frequent than originally thought.
33
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
Figure 2.4: More reliably located seismicity, 1990-2003. (From Said, 1997; Zammit, 2003)
The Sicily-Tunisian platform is characterized by a small magnitude earthquake activity.
These earthquakes are being monitored by the seismic stations on Malta and on the Pelagian
islands of Pantelleria and Lampedusa. The Maltese Islands lie about 200m far away from the
Afro-Eurasian plate boundary and so are not affected by the large magnitude earthquakes that
occur near this boundary. But a large earthquake say magnitude 7 in Southern Sicily will be
strongly felt in Malta. A strong quake occurred in Eastern Sicily on 11th January 1693. This
earthquake left several victims in the country and also many damages occurred in Malta. This
means that the seismic potential of the faults in the Sicily Chanel can cause earthquakes that in
fact can be large in magnitude. Earthquakes being recorded by stations in the Sicily Chanel
including the station in Malta show that a large seismic activity is occurring near the Maltese
Islands. This seismic activity surrounding the Maltese Islands shows that the tectonic activity
that resulted in the formation of the Maltese Islands as known today is still going on but does
not cause any damage.
34
Chapter 2: Tectonics and Seismicity of the Maltese Islands Region
Figure 2.5: The seismogram of an earthquake located about 130 km SW of Malta on 6 June
2006, as recorded on the broadband digital seismograph WDD on Malta.
35
Chapter 3: SEISAN – Earthquake Analysis Software
Chapter 3 SEISAN - Earthquake Analysis Software
In this chapter, a brief description is given on how the Earthquake Analysis Software, Seisan
(Havskov and Ottermoller, 2003) works. The main properties of this program are explained
mainly the part used to determine Codaq values for the set of given earthquakes. A more
detailed description of how the program works is given in the manual of the program, Seisan.
3.1 Structure of Seisan-Directories
The Earthquake Analysis Software, Seisan 8.0 (Havskov and Ottermoller, 2003) is used
to obtain Codaq values for a number of chosen earthquakes. The whole Seisan system is
located in subdirectories residing under a main directory called Seismo. The system contains
many subdirectories containing information that the program needs to run. The following are
the main subdirectories:
REA: This contains earthquake readings and full epicenter solutions in the database.
WOR: The users work directory, initially empty.
PRO: Programs, source code and executables
INC: Include files for programs and subroutines in PRO and LIB
COM: Command procedures
DAT: Default and parameter files, e.g. station coordinates
WAV: Digital waveform data files
CAL: System calibration files
INF: Documentation and information
SUP: Supplementary files and programs
Table 3.1: The main subdirectories of SEISAN
36
Chapter 3: SEISAN – Earthquake Analysis Software
The database of Seisan contains two main directories REA and WAV. The REA
directory contains all the readings and information about the earthquakes that needs to be
analyzed while the WAV directory contains all the waveform data.
Figure 3.2: Structure of SEISAN
The REA directory contains all the phase reading and the derived source information
like hypocenters. The main directory REA is sub divided into a number of directories which
correspond to different databases. These sub directories are created by the user and are used to
store all the earthquake events that are going to be analyzed. The database names can have
between 3 to 5 characters. Each database has default storage of events. Each event is stored in
a single S file in yearly directories and monthly subdirectories. If new data is entered into the
database it is automatically saved as an individual event file. But when the interactive work
has finished, the single event flies are overwritten and stored in monthly files. The CAT-files
are these monthly files that serve as a backup data for the single files.
S file database structure
\REA\MALTA\2010\07\
This is a single S file representing one event. This S file corresponds to an event that happened
on the 9th of July, 2010. This event is being stored in the sub directory MALTA under the
main directory REA. Each event is given an ID. The ID line contains a unique ID and it also
37
Chapter 3: SEISAN – Earthquake Analysis Software
contains status information about the event like last action and the last time when it was
updated. An example of an S-file name is: 09-0019-24L.s201007.
The location program uses these S-files as an input and also as an output when a permanent
update is done to the event. The letter before the . indicates the event type. This can be L,R or
D for local, regional or distant event respectively. This is the same indicator as given in the
header line of the S-file.
3.2 Waveform Data
3.2.1 Data Format
SEED (Standard for the Exchange of Earthquake Data) is standard defined by the
FDSN6. A data format was needed since large amounts of seismic data needed to be
transferred within the network, analyzed and backed up. This type of data format contains a
header. This header contains the instrument name, location, sensitivity and the data selection
containing the waveform data.
SAC (Seismic Analysis Code) is a general program designed for studying time series
data. It was developed in ForTran and converted to C which was then renamed to SAC2000.
SAC2000 is the primary tool used by seismologists to analyze seismic signals.
Seisan
works
with
various
waveform
formats
including
SEISAN,
GSE,
SEED/MINISEED and SAC. The waveform data is usually kept in one format only mainly for
simplicity. There may be different arguments on which format to choose that depends on the
user’s requirements. SAC and GSE are widely used formats. SEISAN is a different format
which is a multi trace binary format. It makes it possible to access individual traces. GSE is a
multi-trace ASCII waveform format. The GSE format can keep a number of traces but it is
6
International Federation of Digital Seismograph Networks
38
Chapter 3: SEISAN – Earthquake Analysis Software
usually recommended not to include more than 3 traces in a single file. Data centers mostly
use SEED and MINISEED formats.
The events available at the Seismic unit at the University of Malta are in SAC format
and so this is the format chosen to store the events in Seisan. SAC is a single trace binary or
ASCII format with a large number of header parameters. SAC format is widely used in
programs that are research oriented. SAC format is also recommended when a single file
include more than 3 traces.
The WAV directory contains the event files with digital waveform data. The analysis
system always uses the WAV directory to search for the files. Waveforms area automatically
transferred to WAV. The event files in the WAV directory are usually of the form: yyyy-mmdd-hhmm-ssT.NETWO_nnn with the abbreviations yyyy: year, mm: month, dd: day, hh: hour,
mm: minute, ss: second, T: file type indicator usually S, NETWO: maximum 5 letter network
code and nnn: number of channels. When storing events in the WAV database, it is required
that the waveform names start either yymm or yyyy-mm. Therefore the database consists of
single files with names corresponding to time down to second as well as the event type (L, R
or D). This means that two events can get the same name. A new event can therefore be over
written on an existing event. When using MULPLT to enter new events into the database, the
use will be prompted if a new event is to overwrite an existing one.
A directory MALTA is created under the REA directory. This is done using the program
MAKEREA. Under the MALTA directory, 4 sub directories one for each year from 2006 to
2010. Each year sub directory is again divided into months. Using another program called
MULPLT the data was accessed and the traces for each event where seen on screen and also
registered. Once registered, an S-file is created for the event and each event is saved in a sub
directory according to when it happened.
There are two ways to get digital data into the database. One method is by making the
individual S file directly in the REA directories using the editor. But this is rather slow. If the
original data available is a digital event waveform file another method is available. As already
explained the waveform can have different formats. These waveforms are stored in the WAV
directory and usually also in the WOR directory. The main aim is that the digital data is
39
Chapter 3: SEISAN – Earthquake Analysis Software
transferred from the field station, demultiplexed and converted to SEISAN waveform format.
This is done using the program MULPLT. This program plots channels form a single
waveform file. The user can than decide whether or not to keep this event. If the event is
chosen to be kept then an S file is created in the database and the event is now moved in the
WAV directory.
3.3 Programs
Seisan organize incoming data from different sources into different directories. This is
using a simple time ordered database and also using a set of programs that are installed in
seisan. Some of the most important programs include:
EEV: This program is used when working with single events. This is used to find a given
event in the database. When this event is selected a large number of options can be applied ton
it such as phase picking and earthquake location.
MULPLT: This is the main program used for signal analysis and plotting. This can be used to
pick phases and amplitudes.
HYP: This is the general program used for hypocenter location. It can use all global and
crustal phases and can use all types of input data whether from single stations or arrays.
EPIMAP: This is the hypocenter plotting program and is used to make epicenter maps and
hypocenter profiles.
CODAQ: This program is used to determine the attenuation of local earthquakes using the
coda Q. Another program, SPEC determines Q by calculating the spectral ratios or else
calculates the near surface attenuation using the spectral decay method.
Other programs are also available to create a database, to input and output a large amount of
data into the database and also to manipulate the waveform data.
40
Chapter 3: SEISAN – Earthquake Analysis Software
3.4 Calculation of coda q, CODAQ
The coda Q program calculates q for a series of events and stations at given frequencies.
Average values of q can then be calculated and a q versus f curve is then plotted using the
calculated values. This program plots the individual events and also plots the filtered coda
windows. The principle used to calculate the coda q is the standard coda q method where a
coda window is bandpass filtered. An envelope is then fitted. This envelop is a calculated
RMS value of the filtered signal using a 5 cycle window. Then the coda q at the corresponding
frequency is calculated (Havskov et al., 1989). This program can use all the waveform file
types that are accepted by seisan.
3.4.1 Input
The calculations are done using the parameter file codaq.par and the events lists to be used are
given in codaq.inp. An example of an input file and a parameter file are shown below:
Figure 3.3: Example of an input file
41
Chapter 3: SEISAN – Earthquake Analysis Software
Figure 3.4: Example of a parameter file
Start in s-times: the coda window usually starts at twice the lapse time which is the Stravel time from the origin. This factor can be chosen differently. The S-time is calculated
from the P-time and so the P-time is inputted in the parameter of each event.
Absolute start time: A 0.0 parameter is usually used. A time different from zero can be
chosen and the start of the coda window is put at an absolute time relative to the origin. This
would mean different lapse times and so different q-values may be produced. This parameter
must be chosen long enough.
Window length: This is the coda window measured in seconds and it must be chosen at
least to be 20secs for stable results.
Spreading parameter: It is the geometrical spreading parameter and the value of this
parameter is usually chosen to be 1.0.
42
Chapter 3: SEISAN – Earthquake Analysis Software
Constant v in q=q0*f* *v: For all values of q(f), q0 is calculated keeping the value of v
fixed.
Minimum signal to noise ratio: When calculating an average value of q, the signal to
noise ratio must be chosen to be above this value. The signal to noise ratio is calculated using
the last tRMS secs of the filtered window and the first tRMS secs of the data file window. If
the data file starts with noise then this ratio will not be accurate. Usually a reasonably value of
5.0 is chosen.
Maximum counts to use: this is the maximum count in a coda window above which the
window is not used.
Noise window in front of signal and length of noise window, tnoise and tRMS: The first
number is the number plotted in front of the signal and gives the number of seconds of noise.
This is the number of noise found before P. The second number is the length of the noise
window that is then used to calculate the single to noise ratio.
Minimum correlation coefficient: In order that the average value of q calculated is
correct, the correlation coefficient must be larger than or equal to this value. This value
depends on the data being analyzed and a value higher than 0.5 is chosen. In reality the values
of this coefficient is negative.
Number of frequencies: The number of frequencies at which each value of q is
calculated. Maximum number of frequencies is usually 10.
Frequency and bands: These are the frequencies and each corresponding band. The
frequency band should increase as the frequency increase. E.g. 8, 3 mean that the signal is to
be filtered between 6.5 and 9.5Hz. It is important that each band has the same amount of
energy. This is done by using a constant relative bandwidth filtering. RBW is the relative
bandwidth and is defined by (fu-fl) / f0 where fu and fl are the upper and lower frequencies.
Such a filter would then be for example: 4±1. The energy in each filter band is represented by
the frequency. This frequency is the geometric center frequency and is given by
When calculating the coda Q at the given frequency, fu and fl are calculated such that the given
bandwidth is used. The actual values of fu and fl give the specified central frequency.
43
Chapter 3: SEISAN – Earthquake Analysis Software
Figure 3.5: Calculating Codaq
Default stations: Stations be used are specified here or else in the codaq.inp file. In the
following line the components are specified. Then the event station information is obtained
from the codaq.inp file. In this case only one station is specified since many events recorded at
the same station MN_WDD are being analyzed.
The codaq.inp file will consist of al list of events. Each event has its own identity with
which it is identified. The program used default stations that are given in the codaq.par. An
example is given below:
\seismo\REA\MALTA\2010\07\13-2330-01L.s201007
\seismo\REA\MALTA\2010\07\13-1318-53L.s201007
\seismo\REA\MALTA\2010\07\13-1119-31L.s201007
3.4.2 Operating CODAQ
The program read the parameter file, codaq.par and also the input file, codaq.inp
containing the events to analyze. These files are both found in the current directory. In the S
file the name of the waveform is given. The program then searches for the station and the
components being used. The program searches in the WAV database and so the program can
work without moving the data from the database. The data header was adjusted for the correct
44
Chapter 3: SEISAN – Earthquake Analysis Software
origin time of all events since the program uses the origin time and P arrival times from the S
files to calculate the S arrival time.
If no plot is chosen, one line will appear on the screen for each frequency. Each event is
recorded in a new page by the program. If the program is plotting the events on screen, the
next plot is obtained by hitting the return button.
A summary is given at the end. This information if found in the output file codaq.out. The
program has some abbreviations that are given below.
H: Focal depth
M: Magnitude
TP: P travel time
TC: Start time of coda window relative to origin time
F:
Frequency
Q: Corresponding coda q, if 0 value is >1000 or negative
S/N: Signal to noise ratio AV
Q:
Average q
SD: Standard deviation for average
NT: Total number of q values at all frequencies
N:
Number of q values at all frequencies
q:
Average of q values
1/q: q is calculated at 1/q averages
f:1/q: Q is calculated using the relation derived from the 1/q averages
cq0: Constant q0 obtained using the fixes user selected v
v:
Constant v determined corr: Correlation coeffieicent in determining q vs f
Table 3.6: Abbrevaiations used in SEISAN
45
Chapter 3: SEISAN – Earthquake Analysis Software
The coda q value is calculated by program by reading the P arrival time from the S file.
This P arrival time is written in the S file manually for each event or else using a progam. The
S file for each event is in Nordic format. This format uses free columns to obtain a readable
format. There are many ways by which data can be written in an S file using different line
commands. The line command chosen depends on the type of information to be inputted in the
S file. The line 4 command has been chosen since different events recorded at the same station
are being analyzed. The line 4 :
Figure 3.7: Type line 4 using Nordic Format
46
Chapter 3: SEISAN – Earthquake Analysis Software
3.4.3 Output
When running the codaq program an output file codaq.output is generated. This is the
parameter file consisting of all the events each generated in a separate line. These are the list
of events that have been accepted by the program. The program accepted these events after
calculating the correlation and the signal to noise ratio of each event separately. Each line
event also has its average q value. The q values are averaged directly and the 1/q are averaged
separately. In this output file, there will also be the fits to the relation q=q0*f* *v.\
Another output file is generated codaq1.out that contains the same output as codaq.out but no
print for each event is generated. Below is an example of the codaq.out file:
Figure 3.8: An example of an output file
The following figure shows an example of a codaq plot. No options are available for the codaq
plots and the length of the window is always the first 200 secs from the original trace. If the
origin time or coda window is outside the 200 secs window, the coda window is not plotted.
47
Chapter 3: SEISAN – Earthquake Analysis Software
Figure 3.9: A codaq plot for an earthquake recorded on 7th August 2009 by WDD station.
(SEISAN, Havskov and Ottermoller, 2003)
Shown here is an example of a coda Q plot. The trace shown on top is the original trace
and the coda windows shown below are the filtered ones. The selected filtered coda window
has 15 secs of noise in front. S/N ratio is calculated from the first 5 secs of noise shown. On
each filtered plot is given F: Center frequency, Q: Q-value, zero value means no Q-value
could be calculated, S/N: Signal to noise ratio.
48
Chapter 4: Data Processing
Chapter 4 Data Processing
In this chapter an overview of the seismic recording in Malta is given. A brief description on
the website where the seismic data is available online is also given. Further in this chapter the
data set chosen for the study of codaq is given.
4.1 Seismic Recording in Malta
The main aim of a seismic instrument is to record ground motion. This ground motion is
the result of both natural and man-made disturbances. Such seismic instrument is the
seismograph which is an instrument capable of making a seismic disturbance visible by
writing it as a continuous record of ground motion which is known as the seismogram. The
visible seismogram is the actual conversion that occurred between the signal that arrived at the
seismometer and a time record of the seismic event. The seismic ground motion that arrives at
a seismograph has the form of analogue data. This is then converted into electrical signals,
amplified, filtered and finally registered in a chart recorder. This is the acquisition of a
seismogram.
The first seismograph in Malta was installed at the beginning of the 20th century at the
University of Malta. The seismograph that was installed was a Milne-Shaw horizontal
pendulum seismograph. At this time this was the main seismograph used world-wide due to its
high reliability. This instrument operated in Malta till around the 1950. Such recordings of this
seismograph are still available at the University of Malta.
The Milne-Shaw seismograph was replaced by a vertical component long period
Sprengnether seismograph in 1977. This was again installed at the University of Malta. Such
instrument having photographic recordings had a main disadvantage.
This was that the
49
Chapter 4: Data Processing
seismograms had to be developed each time and so were not available instantly after the
seismic disturbance occurs. Such instrument was capable of recording events having
frequencies of 0.01-0.1Hz and was only capable of recording teleseismic earthquakes only and
so few seismic recordings are available of this time.
In 1982, a 3-component, short period analogue seismograph was installed at the
University of Malta. This short period seismometer has a very short natural period and a high
resonant frequency. It is capable of responding to a seismic frequency of 1 to 10Hz and a
period range of 0.1 to 1s (Lowrie, 1997). It records 3 components of ground motion. These are
the vertical component (Z component), the North - South component (N component) and the
East – West Component (E component). This seismometer produced visible recordings unlike
the previous seismometers that used photographic recording. This was a huge advance is the
seismology of the Maltese Islands since now it was possible for local events to be detected.
All the events between 1983 and 1992 that were detected by this seismometer are chart
recorded and found at the University of Malta.
4.1.1 The Wied Dalam Station, WDD
In the early 1990’s the need to replace the existing analogue seismographs bye digital
one emerged. Digital broadband seismographs increase the dynamic range and the frequency
band of each event being recorded by the seismogram. A search started for a place having
minimum disturbances both natural and artificial that was appropriate for this seismograph to
be installed. In 1995 this digital seismograph was installed in Wied Dalam in the south of
Malta. The station is known as WDD. This seismic observatory station is located about 20m
below ground at 35.8374N, 14.5245E. The situation of this station in Malta is shown in figure
4.1.
50
Chapter 4: Data Processing
Figure 4.1: Location of the WDD station. (Digital Seismic recording in Malta – 13 years on,
Galea, P; Aguis, M, 2008)
The seismometer at Wied Dalam is a digital broadband seismograph. This seismometer
is a Streckeiser Model STS-2 sensor triaxial component connected to a QUANTERRA 24-bit
integer data acquisition system. This is shown in figure 4.2.
Figure 4.2: Wied Dalam Station WDD in the south of Malta. (Digital Seismic recording in
Malta – 13 years on, Galea, P; Aguis, M, 2008)
51
Chapter 4: Data Processing
It operates many channels

HH at 80 samples per second

BH at 20 samples per second

LH at 1 sample per second

VH at 0.1 samples per second

UH at 0.01 samples per second
In our case the HH component was chosen since seismic waves of local earthquakes are
predominant in the higher frequencies and more important is that they are attenuated at long
distances.
SeisComp is responsible for data transmission. It is a concept used within the
MEREDIAN7 project. This software is responsible for the acquisition, recording, monitoring
and controlling of seismic data. Once data is recorded it is transferred via the internet to
another computer at the University of Malta where another copy of the data is kept.
The WDD station forms part of the MEDNET, Mediterranean Network. This is a
network of the broadband seismographic stations that are installed in the countries of the
Mediterranean region shown in figure 4.3. This network having 14 stations is maintained by
the Instituto Nazionale Di Geofisica in Rome, the INGV with the help of other geophysical
institutes. Such network gives an instrumental coverage of the Mediterranean area which is an
area of high seismicity and a complex tectonic activity. Its aim is to improve the knowledge of
the structure of the Mediterranean region and so this will help to minimize earthquake losses.
Data can only be accessed from our station. The disadvantage of having one station is that the
epicentral point of a seismic event cannot be determined. This is because using the arrival
times of many seismic phases recorded at different stations, the earthquake hypocenter and
origin time can be determined.
7
An EU-funded project coordinated by the ORFEUS Data Centre in de Bilt, the Netherlands
(www.orfeus-eu.org).
52
Chapter 4: Data Processing
.
Figure 4.3: The MedNet Network (Mediterranean Very Broadband Seismographic Network,
I.N.G.V, 2011)
4.1.2 Aims of the Malta seismograph station
The primary aim of the seismograph station in Malta is to continuously monitor and analyze
the seismic activity in central Mediterranean. It focuses its analysis mainly in the seismicity
around the Maltese Island. Using such information seismologists can identify active faults in
the sea bed of the Sicily Channel. Such information will be useful in providing an assessment
of the seismic hazard in the Maltese Islands. This seismograph also improves the epicentral
location capability of the Mediterranean. Another aim is to contribute to the world-wide
gathering of seismic data done by the network of digital seismographs. This will provide
accurate information about the structure of the Earth.
53
Chapter 4: Data Processing
4.2 Seismic Monitoring and Research Unit at the
University of Malta
All the seismic data that has been recorded at WDD since 2006 have been uploaded into
an
online
database.
Such
database
can
be
accessed
by
the
following
link:
http://www.phys.um.edu.mt/seismic/. The main page of this webpage is shown in figure 4.4. This
website has many features and provides much information about the seismic events recorded by at the
WDD. Each event can be analyzed individually and each plot can be viewed using the program
Seisgram2k.
4.2.1 Earthquake locations
Various steps are carried out to generated earthquakes locations. Analysis is carried
daily at 4am local time and the earthquake is then verified within the next 24 hours.
Earthquakes recorded on WDD are located especially those occurring in the Sicily Channel.
Location of regional earthquakes is also done by the stations CEL and IDI. Single-Station
earthquake location is done. A list of events for a particular day is produced by LESSLA
(Aguis, 2006). Then events are added to the central database and grouped as either an
Earthquake or a Blast. P and S arrival times are checked manually. Each event is then
classified by the SMRU8 according to its quality. When the information available is reliable an
earthquake is verified and marked in red and placed in the database.
8
Seismic Monitoring and Research Unit
54
Chapter 4: Data Processing
Figure 4.4: The main page of the Seismic Monitoring and Research Unit website
(http://www.phys.um.edu.mt/seismic/)
Chapter 4: Data Processing
4.2.2 The Website
The website includes several press releases. These give details about seismic data and
maps of major earthquakes that have been recorded by the WDD and are here released to the
public. There are also many other links available on the website. These links give information
about the Seismic Monitoring and Research Unit and also projects, papers, posters and
presentations are available here. Apart from these links a real-time plot of seismic activity is
available. Such plot of 13 February 20011 is seen in figure 4.5. This displays all the data
being recorded during that day. Using such a plot the seismic activity of that day can be
analyzed in detail. Each day this data is stored into the database and a new active plot begins.
4.2.3 The Seismic Database, Online
All events recorded at the WDD since 2006 are stored in this database. A Google map is
available. This map displays the epicenters of all the events that have been verified by the
SMRU. A query is also available. This make it possible to the user to chose earthquakes
depending on a certain criteria. Such criteria can be the date when the earthquake occurred or
the latitude and the longitude of an earthquake. Such list of events is shown in figure 4.6.
The row in the table accounts for a single event, displaying all the relative information
about the event. Manual Attributes are displayed at the end of each row. These are labels used
reveling information about the event. Each event can be viewed separately. The window
displays a Google map showing the final location of the event. A blue circle is shown. This
has its center at the WDD station. The red line is equal to the radius of the circle. It extends to
the circumference and has an angle equal to the azimuth. This angle is calculated relative to
the geographical north of the map. The location of the earthquake is calculated by LESSLA
(Aguis, 2006). This is shown by the point where the red line intersects with the circumference
of the circle. Nine seismographs are displayed for each event, one for each of the nine
channels. The event can be seen by using the program Seisgram2k. A single event display is
shown in figure 4.7.
56
Chapter 4: Data Processing
Figure 4.5: The real-time plot for 13 February 2011
(http://www.phys.um.edu.mt/seismic/)
Chapter 4: Data Processing
Figure 4.6: The online database of seismic events. It is displaying the events that occurred
between 1st January 2006 and 31st January 2011 having latitude between 32 and 36 and
longitude between 12 and 1 . (http://www.phys.um.edu.mt/seismic/)
58
Chapter 4: Data Processing
Figure 4.7: Single event displayed online .Its displaying the main page of the January 1 earthquake located to the SW of the Maltese
Islands. This page displays all the relative information about the event. (http://www.phys.um.edu.mt/seismic/)
Chapter 4: Data Processing
4.3 The Data Set
4.3.1 South of Malta Events
The earthquakes were chosen from the database available at the Seismic Monitoring and
Research Unit at the University of Malta. Out of 185 events, a total of 43 events are finally
selected for the determination of the Q factor. These are given in Table 1.1 in Appendix 1.
These occurred during 1st January, 2006 and 31st January, 2011 in the latitude area from 32 to
36
and longitude area 12
to 1 . This area chosen for analysis of coda Q is shown in the
hybrid map in figure 4.8.
Figure 4.8: A hybrid map showing the South of Malta earthquakes between latitude area 32
and 36 and longitude 12 and 1
(http://www.phys.um.edu.mt/seismic/)
60
Chapter 4: Data Processing
4.3.2 North-West of Malta Events
Another area of study was chosen for determination of the coda Q factor and so that an
analysis of the variation of codaq Q from one region to another could be done. These events
were chosen in the North-West of Malta area near Pantelleria. Out of 22 events, 6 events were
finally selected for the determination of the Q factor. These are given in Table 1.2 in Appendix
1. Such events again occurred between 1st January, 2006 and 31st January, 2011 in the latitude
area from 36 to 37.5 and longitude area 8 to 1 . This area of study is shown in the hybrid
map in figure 4.9.
Figure 4.9: A hybrid map showing the North-West of Malta earthquakes between latitude area
36° and 37.5° and longitude 8° and 14°. (http://www.phys.um.edu.mt/seismic/)
61
Chapter 4: Data Processing
4.3.3 Crete Events
For further comparison another area of study was chosen for determination of the coda
Q factor. This is the Subduction Zone near Crete. This will allow us to analyze the difference
between the Q values which were obtained around the Maltese Islands and the Q values
obtained in this highly active tectonic region. This could be done since real-time data is
received at the Seismic Monitoring and Research Unit at the University of Malta from the IDI
station on Crete. The Institute of Geodynamics’ database consisted of over 2000 events that
were recorded by IDI in this subduction zone. Out of these, the events that were available at
the Seismic Monitoring and Research Unit could be chosen. 42 events were finally selected
for the determination of the Q factor. These are given in Table 1.2 in Appendix 1. Such events
again occurred between 1st January, 2006 and 28th March, 2011 in the latitude area from 33.5
to 35.5 and longitude area 23 to 27 . This area of study is shown in the hybrid map in figure
4.10.
Figure 4.10: A hybrid map showing the Crete earthquakes in the latitude area 33.5 and 35.5
and longitude 23 and 27 Shown also here is IDI station on Crete.
(http://www.phys.um.edu.mt/seismic/)
62
Chapter 4: Data Processing
Each plot was checked manually for data quality. This includes duration, distortion,
spikes, saturation and signal-to-noise ratio. More than half the original data were rejected by
visual inspections mostly due to low signal-to-noise ratio. Other events were discarded due to
the fact that length of the coda wasn’t long enough for a time window of 20 seconds to be
taken after twice the lapse time. The final data set for this area consisted of 43 events in the
south of Malta area, 6 events in the North-West of Malta area near Pantelleria and 42 events in
the Subduction Zone near Crete.
4.3 Calculating the Qc - values
The Q values were calculated through the CODAQ in the seismic analysis package
SEISAN 8.0 (Havskov and Ottemoller, 2003). The lapse time portion of the coda wave used
in this is selected at
where
is the S- wave travel time. This is calculated using
the P-wave arrival time using the equation
The
is used in this way so that direct and forward scattering waves are avoided
(Rautian and Khalturin, 1978). All the selected seismograms are then bandpass-filtered at
central frequencies of 2.0, 5.0, 7.0, 9.0 and 12.0 Hz with bandwidths of 1, 2.5, 3.5, 4.5 and 6,
respectively. An increasing frequency band is used for increasing central frequency. This is
done to avoid the ringing effect and to take constant relative bandwidths for getting an equal
amount of energy into each band (Havskov and Ottemoller, 2003). One window length was
taken at 13 sec.
The RMS amplitude of the last 5 sec cycle length of the lapse-time window is divided by
the noise data of the same length before the onset of the P wave to calculate the signal-to-noise
ratio. The Qc were accepted only when the correlation coefficient, C for the best-fit line for
coda decay slope with respect to lapse time were greater than 0.5. Initially the signal-to-noise
ratio was chosen to be 2. But the number of data reduces considerably and so events having
63
Chapter 4: Data Processing
signal-to-noise ratio, S/N greater than 1.2 were chosen. Other events were rejected for reliable
values of Qc.
A description of the single back scattering model is given previously. The coda wave
amplitude at central frequency f and elapsed time t from the origin
is found by band-
pass filtering the coda window trace data using a 6-pole Butterworth filter centered at
frequency f and calculating rms values using a sliding window of length 5/f sec. Then a time
decay envelope is fitted to this filtered signal. This is seen in figure 4.10.
Figure 4.11:
found by band-pass filtering the signal and then fitting a
time-decay envelope to this filtered signal.
Finally, values of Qc are calculated using the slope of the linear regression of the
logarithm of product of RMS amplitude and lapse time
slope of such graph, b is given by
against lapse time t. The
and so values of Qc can be calculated. Figure
4.11 shows the steps involved in the computation of the values of Qc with time. This is
procedure is carried out the program CODAQ in the seismic analysis package, SEISAN. The
Qc - values for such data sets were then averaged for each central frequency. Also the standard
deviation for each central frequency was calculated.
64
Chapter 4: Data Processing
Figure 4.12: Procedure in calculating Qc (a) Unfiltered data trace with coda window, (b) and
(c) bandpass-filtered amplitudes of coda window at 1.5-2.5 Hz and 9.0-15.0 Hz respectively,
(d) and (e) the RMS Amplitude values multiplied with the lapse time along with the best
square fits of selected coda window at central frequencies 2 and 12 Hz respectively. The Qc is
determined from the slope of best square line.
(Coda Q Estimates in the Andaman Islands using Local Earthquakes: Parvez, A,. et al, 2008).
65
Chapter 5: Results
Chapter 5 Results
In this chapter the results of the Coda Q values calculated by SEISAN are given. Also given in
this chapter are the calculations done.
5.1 The frequency dependence of Q relationship
The Q factor increases with frequency (Mitchell, 1981) and it follows the following
relation
(5.1)
where
is the quality factor at the reference frequency f0 (generally 1Hz) and
is the
frequency parameter. This power law is fitted for Qc at each frequency.
This law is arranged in logarithmic form and is given by the following equation:
(5.2)
Therefore the value of the frequency parameter is obtained from the slope of a graph of
against
and the value of
is obtained from the intercept of such graph.
66
Chapter 5: Results
5.2 Results for the South of Malta Earthquakes
Table 5.1 shows the mean values of Qc at different central frequencies. Given also in this table
are the standard deviation and the number of observations for each central frequency. These
are used for the calculation of Qc. In Figure 5.1 all the values of Qc are plotted against
frequency. These values are given in Table 2.1 in Appendix 2. In figure 5.2 the mean Qcvalues against central frequencies are plotted. . A log is then plotted as shown in figure 5.3 so
that the relationship for the South of Malta events could then be obtained. It is observed that
Qc increases as the frequency increases.
Table 5.1
Average Quality factor, Qc and Estimated Standard Deviation at different frequencies
Frequency (Hz)
Qc
S.D
N
ln f
ln Q
2
91
41.012
36
0.693
4.512
5
7
9
12
591
912
1349
1984
356.381
281.428
911.461
808.223
40
33
36
23
1.609
1.946
2.197
2.485
6.382
6.816
7.207
7.593
In the column heading, S.D. indicates the standard deviation and N is the number of observations made
for each central frequencies.
67
Chapter 5: Results
12000
10000
Qc
8000
6000
4000
2000
0
0
2
4
6
8
10
Frequancy (Hz)
Figure 5.1: A graph of Qc against frequency for the South of Malta events
12
14
Chapter 5: Results
3000
2500
Average Q
2000
1500
1000
500
0
0
2
4
6
8
10
12
14
Frequency (Hz)
Figure 5.2: A graph of the Average Q values against frequency for the South of Malta events. Vertical error bars are shown.
Chapter 5: Results
8
7.5
7
Ln (Q)
6.5
6
5.5
5
4.5
4
0
0.5
1
1.5
2
Ln (f)
Figure 5.3: A Graph of ln (Qc) against ln (f) for the South of Malta events
2.5
3
Chapter 5: Results
5.2.1 The frequency dependence of Q relationship for the South of Malta
Events
The graph of
against
is plotted and the following calculations are made. This is
shown in figure 5.3.
and
Therefore
71
Chapter 5: Results
5.3 Results for the North-West of Malta Earthquakes
Table 5.2 shows the mean values of Qc at different central frequencies. Given also in this table
are the standard deviation and the number of observations for each central frequency. These
are used for the calculation of Qc. In figure 5.3 all the values of Qc are plotted against
frequency. These values are given in Table 2.2 in Appendix 2. In figure 5.4 the mean Qcvalues against central frequencies are plotted. . A log is then plotted as shown in figure 5.6 so
that the relationship for the North-West of Malta events could then be obtained. It is observed
that Qc increases as the frequency increases.
Table 5.2
Average Quality factor, Qc and Estimated Standard Deviation at different frequencies
Frequency (Hz)
2
5
7
9
12
Qc
117
519.5
1489.5
3453.5
4028
S.D
11.313
41.719
1175.918
2901.259
3073.086
N
5
5
5
5
3
ln f
0.693
1.609
1.946
2.197
2.485
ln Q
4.762
6.253
7.306
8.147
8.301
In the column heading, S.D. indicates the standard deviation and N is the number of observations made
for each central frequencies.
72
Chapter 5: Results
2000
1800
1600
1400
Qc
1200
1000
800
600
400
200
0
0
2
4
6
8
10
12
14
Frequency (Hz)
Figure 5.4: A graph of Qc against frequency for the North-West of Malta events
73
Chapter 5: Results
6000
5000
Average Q
4000
3000
2000
1000
0
0
2
4
6
8
10
12
14
Frequency (Hz)
Figure 5.5: A graph of the Average Q values against frequency for the North- West of Malta events. Vertical error bars are shown.
74
Chapter 5: Results
9
8.5
8
ln (Qc)
7.5
7
6.5
6
5.5
5
4.5
0
0.5
1
1.5
2
2.5
3
ln (f)
Figure 5.6: A Graph of ln (Qc) against ln (f) for the North-West of Malta events
75
Chapter 5: Results
5.3.1 The frequency dependence of Q relationship for the North-West of
Malta Events
The graph of
against
is plotted and the following calculations are made. This is
shown in figure 5.6.
and
Therefore
76
Chapter 5: Results
5.4 Results for the Subduction Zone near Crete
Earthquakes
Table 5.3 shows the mean values of Qc at different central frequencies. Given also in this table
are the standard deviation and the number of observations for each central frequency. These
are used for the calculation of Qc. In figure 5.7 all the values of Qc are plotted against
frequency. These values are given in Table 2.3 in Appendix 2. In figure 5.8 the mean Qcvalues against central frequencies are plotted. A log is then plotted as shown in figure 5.9 so
that the relationship for the Crete events could then be obtained. It is observed that Qc
increases as the frequency increases.
Table 5.3
Average Quality factor, Qc and Estimated Standard Deviation at different frequencies
Frequency (Hz)
2
5
7
9
12
Qc
204.5
470.0
938.0
1520.0
2720.5
S.D
180.312
176.777
661.852
185.212
647.003
N
34
37
36
32
33
ln f
0.693
1.609
1.946
2.197
2.485
ln Q
5.321
6.153
6.844
7.326
7.909
In the column heading, S.D. indicates the standard deviation and N is the number of observations made
for each central frequencies.
77
Chapter 5: Results
4500
4000
3500
3000
Qc
2500
2000
1500
1000
500
0
0
2
4
6
8
10
12
14
Frequency (Hz)
Figure 5.7: A graph of Qc against frequency for the Crete events
78
Chapter 5: Results
3500.0
3000.0
Average Q
2500.0
2000.0
1500.0
1000.0
500.0
0.0
0
2
4
6
8
10
12
14
Frequency (Hz)
Figure 5.8: A graph of the Average Q values against frequency for the Crete events. Vertical error bars are shown.
79
Chapter 5: Results
8.5
8.0
7.5
Ln Q
7.0
6.5
6.0
5.5
5.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ln f
Figure 5.9: A Graph of ln (Qc) against ln (f) for the Crete events
80
Chapter 5: Results
5.4.1 The frequency dependence of Q relationship for the Crete Events
The graph of
against
is plotted and the following calculations are made. This is
shown in figure 5.9.
and
Therefore
81
Chapter 5: Results
5.4.2 The variation of Q with depth for the Crete Events
The 42 events for the Crete Subduction Zone were divided into 5 groups having 15km,
25km, 35km, 45km and 53km as their maximum depth respectively. Table 5.4 shows the Q0
value and the frequency dependent α values calculated using the frequency dependence of Q
relationship (Mitchell, 1981) for each group of events.
Table 5.4
Estimated Q0 and Frequency Dependence α values at Different Depths
Depth Range(km)
4-15
17-25
26-35
36-45
46-53
Q0-value
143.88 ± 1.63
118.98 ± 1.15
104.38 ± 1.06
86.74 ± 1.36
65.17 ± 1.49
α - value
0.93 ± 0.26
0.71 ± 0.08
0.76 ± 0.03
1.25 ± 0.16
1.33 ± 0.21
N
10
9
14
6
3
N is the number of observations made for each group having different maximum depth.
82
Chapter 5: Results
Depth = 4 - 15km
8
7.8
7.6
7.4
7.2
7
Ln Q
6.8
6.6
6.4
6.2
6
5.8
5.6
5.4
5.2
5
0
0.5
1
1.5
2
2.5
3
3.5
Ln f
Figure 5.10: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range 4-15 km.
83
Chapter 5: Results
Depth = 17 - 25km
6.8
6.6
6.4
6.2
Ln Q
6
5.8
5.6
5.4
5.2
5
0
0.5
1
1.5
2
2.5
3
Ln f
Figure 5.11: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range 14-25 km.
84
Chapter 5: Results
Depth = 26 - 35km
6.8
6.6
6.4
Ln Q
6.2
6
5.8
5.6
5.4
5.2
5
0
0.5
1
1.5
2
2.5
3
Ln f
Figure 5.12: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range 26-35 km.
85
Chapter 5: Results
Depth = 36 - 45km
7.8
7.6
7.4
7.2
7
6.8
ln Q
6.6
6.4
6.2
6
5.8
5.6
5.4
5.2
5
0
0.5
1
1.5
2
2.5
3
ln f
Figure 5.13: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range 36-45 km.
86
Chapter 5: Results
Depth = 46 - 53km
8
7.8
7.6
7.4
7.2
7
Ln Q
6.8
6.6
6.4
6.2
6
5.8
5.6
5.4
5.2
5
0
0.5
1
1.5
2
2.5
3
ln f
Figure 5.14: A Graph of ln (Qc) against ln (f) for the Crete events having depth in the range 46-53 km.
87
Chapter 5: Results
150
140
130
Q0
120
110
100
90
80
70
60
0
10
20
30
40
50
60
Depth (km)
Figure 5.15: A Graph of Q0 against depth for the Crete events
88
Chapter 6: Discussion
Chapter 6 Discussion
In this chapter the values obtained for the South and the North-West of Malta events are
discussed and compared together. These Qc values obtained for these two areas are then
compared to values obtained for the Subduction Zone near Crete in this study and to other Qc
obtained in other different regions of the world.
6.1 Analyzing the results
The relationship obtained for the North-West of Malta Region is 24.62f
2.1
. Such a low
value is normally associated with areas that are tectonically active. A low value in this area is
associated with the NW trending Pantelleria Rift or Sicily Channel Rift Zone (SCRZ) which is
a system that features three grabens of Miocene Pliocene age. These are the Pantelleria
Graben, Malta Graben and the Linosa Graben. Waves from the North-West of Malta travel
along the grabens and are usually more attenuated. In this region the crust is thinner and so
these waves could be sampling the mantle. As mentioned earlier, areas characterized by soft,
molten rocks usually have Qc values. Since the crust in a graben system is thinner, the waves
usually travel through the mantle which is characterized by soft and molten rocks. So a low
value of attenuation in this area could be attributed to this fact. In this area strong frequency
dependence was found. This was as expected since the frequency parameter increase as the
tectonic activity of the region increase. This could be related to the size of heterogeneities.
This active fault system is clearly studied using the accurate plotting of earthquakes.
Qc values were also obtained for the South of Malta region. The relationship obtained for
this region is 30.26f 1.73. Again in this area the earthquakes recorded were small in magnitude.
This shows that energy along these faults is being released gradually but in small amounts.
89
Chapter 6: Discussion
The Qc values obtained in this area were higher than those obtained for the North-West of
Malta region and the frequency dependence is slightly higher in the North- West of Malta
region. A scattered activity in the Sicily Chanel is obtained when plotting the epicenters of
earthquakes correctly. This scattered activity roughly coincides with the trend of the
Pantelleria Rift. These earthquakes are shallow earthquakes since they occur within the upper
25km of the earth’s crust.
The relationship obtained for the Subduction Zone near Crete is 63.83f
1.43
. Such a low
value is associated with active tectonic areas and this is as was expected since this is a
subduction zone which is a highly active tectonic region. The variation of Q with depth was
also investigated in this region. This was done by plotting values of Q0 and investigated how
the attenuation characteristics vary with depth. It was found that Q0 decreases linearly with
depth. This means that attenuation of waves of 1Hz frequency increases with depth. This result
can be interpreted in many ways. One explanation is based on the observation that shear waves
are strongly attenuated as they travel through partially molten regions of the mantle. This
could be attributed to waves propagating across the mantle wedge. Such waves propagate
efficiently as they travel within the cold, high-strength lithosphere slab. Such variations with
depth at subduction zones have been used in past studies to identify the geometry of
subduction zones (Oliver and Isacks, 1967; Barazangi and Isacks, 1971; Barazangi et al.,
1972; Barazangi et al., 1973; Mele, 1998). The decrease of Q0 with depth can also be due to an
increase in the heterogeneity of the medium with depth beneath the study region.
6.2 Comparing the South of Malta, North-West of Malta
and Crete Results
The frequency relationship obtained for the North-West of Malta is 24.62f2.1 compared
to a 30.26f1.73 relationship obtained for the South of Malta and the 63.83f
1.43
relationship
obtained in the subduction zone near Crete. The value of Q0, the quality factor at 1Hz in the
North-West of Malta is lower than the value obtained in the South of Malta. This was the
90
Chapter 6: Discussion
expected case as the waves in the North-West of Malta were expected to be more attenuating
than those in the South of Malta. The frequency parameter also reflects this. The frequency
parameter increases as the tectonic activity of the region increase. This results shows that the
North-West of Malta is more tectonically active than the South of Malta.
Comparing values at the same frequency from the South of Malta and Crete one can
observe that Q values around Crete are higher. Taking for example the 2Hz frequency, the Q
value in the South of Malta are 91 compared to a Q value of 204.5 in the Crete region. This is
also observed at higher frequencies. Taking the 12 Hz frequency, the Q value for the South of
Malta region is 194 compared to a Q value of 2720.5 in the Crete region. This shows that
seismic waves from Crete are less attenuated as they travel towards Malta. Comparing the
values from to the South of Malta to those obtained from Crete one should keep in mind that
the crust around the Maltese Islands is different from that in the Crete area. There is evidence
that the low-frequency band dips down towards the edge of the Malta Escarpment, where
landward-dipping reflectors separate continental and oceanic crust lie in the central tract of the
Malta Escarpment. The crust in the Ionian region is Oceanic crust while that around the
Maltese Islands is Continental crust. This difference in crust type around the two regions is
shown in figure 6.1. Oceanic crust is composed of basalt, a dense rock while continental crust
consists of granite which is a less dense igneous rock. This explains why seismic waves
travelling from Crete are less attenuated since waves travelling through hard competent rock
are less attenuated than waves travelling trough soft, molten rocks. This could explain why
seismic waves travelling from Crete to Malta, a distance of around 814km are felt in Malta
while earthquakes occurring at the same distance in Italy or even at less distance are not felt in
Malta.
91
Chapter 6: Discussion
Figure 6.1: A Google Map showing different types of crust between the Malta Escarpment
and Ionian Region (Google Maps, 2011)
The results for the South, the North West of Malta together with those of Crete are plotted
together against frequency as shown in figure 6.2. In the three areas it can be shown that the
values of Q increases as the frequency increases. The Q values for the South of Malta events
vary linearly with frequency while the Q values for the North-West of Malta do not follow any
linearity. The value of frequency parameter, α is different for the three different regions
studied here. This will affect the Q values to vary differently from one region since they
variation of Q with frequency depend on the variation of α. For low frequencies up to 5Hz the
values of the North-West are linear but as frequency increases the Q values do not follow any
linearly but increase rapidly than the values at low frequencies. Taking for example Q values
at 5Hz, the values obtained for the North-West and Crete are lower than the values obtained
for the South of Malta. This means that at low frequencies, seismic waves at the North-West
of Malta and at the Subduction Zone in Crete are more attenuated than those in the South of
Malta. But as frequency increase, the Q values for the North-West and Crete are higher than
those for the South of Malta. The values for the North-West of Malta increase drastically. This
could be attributed to the number of problems that were encountered when analyzing the
North-West of Malta events.
92
Chapter 6: Discussion
4000
3500
3000
Average Q
2500
2000
1500
1000
500
0
0
2
4
6
8
10
12
14
Frequency (Hz)
South of Malta
Crete
North - West of Malta
Figure 6.2: A graph of the Average Q values against frequency for the North-West of Malta, the South of Malta and Crete evens
93
Chapter 6: Discussion
Inaccuracy in such a value could be due to the fact that very few events where recorded
in this study area. 22 events were available in this area compared to the 185 events that were
available in the South of Malta. These were then again down listed since not all events could
be used for the calculation of codaq. In fact only 6 events were finally chosen for the study of
codaq in this area. Another problem could also be that since the earthquakes recorded are
usually small in magnitude, the epicenter plotting is usually inaccurate. If the accuracy of
location is increased, then a correlation can be obtained between these events and the
individual active faults of the graben systems. The small size of the earthquakes shows that
energy along these faults is being released gradually but in small amounts. Many other small
earthquakes are occurring but having small magnitude does not allow an estimate for the
epicenter location to be made.
Another problem could be that the events in the North-West of Malta have a lower
signal to noise ratio than those in the South of Malta as seen in figure 6.2. Shown also here is
that the events from the South of Malta have a nicer waveform that the events from the NorthWest of Malta. Both in both areas it can be shown that coda wave amplitude exponentially
decreases as the lapse time increases. The study for the North-West of Malta becomes
unreliable at high frequencies and so the study should be carried out again when a more
reliable dataset is available.
Figure 6.3: Comparing the South of Malta seismograms to the North-West of Malta
seismograms.
94
Chapter 6: Discussion
6.3 Comparison with other Areas
Table 6.3
Frequency dependence of Qc for different tectonic and volcanic areas in the world.
Zone
NW of Malta
Relation Qc= Q0 f α
25 f 2.1
0.9
Authors
This Study (2011)
Mt. Etna
29 f
Del Pezzo et al. (1995)
South of Malta
30 f 1.73
This Study (2011)
Tres Virgenes Volcanic Area (Mexico)
50 f 0.65
Wong, Rebollar, Munguia (2001)
51 f
1.01
Akinci et al. (1994)
State of Washington
63 f
0.97
Havskov, Malone, Mcclurg, Crosson (1989)
Crete Subduction Zone
63f1.43
This Study (2011)
Dead Sea Region
65 f 1.05
Van Eck (1988)
Anaolian Highlands
0.87
Charleviox Region
75 f
Friuli
80 f 1.1
Rovelli (1982)
South-eastern Canada
91 f 0.95
Woodgold (1990)
Konya Region (India)
1.09
96 f
Woodgold (1994)
Grupta et al. (1998)
Cerro Prieto Geothermal Field
(Mexico)
111.5 f 0.41
Minguez, Rebollar, Fabriol (1997)
Granada Basin (Spain)
126 f 0.95
Ibanez et al. (1990)
Table 6.3 shows the frequency dependence of Qc for different tectonic and volcanic
areas in the world. The relationships for the South of Malta, the North-West of Malta and
Crete found in this study are also inputted in this table.
Similar Qc like those found in the North West of Malta were obtained in the neighboring
volcanic region that of Mt. Etna (Del Pezzo et al., 1995). Values obtained in the neighboring
area that of South-Eastern Sicily show higher Qc than those obtained for the North West of
Malta. This could be explained with a higher degree of heterogeneity in the study area. On the
other hand, the presence of molten materials under the volcanic area of Mt. Etna may produce
95
Chapter 6: Discussion
higher frequency independent intrinsic attenuation (Dainty, 1981) respect to the North-West of
Malta area.
The Qc values obtained in the South of Malta area are similar to those obtained in the
South-Eastern Sicily with a frequency relationship being 49f 0.88. The frequency parameter for
the South of Malta is higher than that for South-Eastern Sicily. This means that the tectonic
activity in the South of Malta is higher than that in South-Eastern Sicily. Higher Qc values
were obtained in Western Anatolia (Akinci et al., 1994), Konya Region (Grupta et al., 1998)
and those obtained in Eastern Canada (Woodgold, 1994). In the State of Washington
(Havskov; Malone; Mcclurg; Crosson, 1989), in the Dead Sea Region (Van Eck, 1988) and in
the Charleviox Region (Woodgold, 1994) all show a lower frequency dependence than that
obtained for the South of Malta Region in this study.
A similar study to that of Crete Subduction Zone was conducted in the Source Region of
the 1999 Chamoli Earthquake by Mukhopadhyay, S et al. (2008). In this study the values of
Q0 increased linearly with depth while in our investigation of Crete Subduction Zone the
values of Q0 decreased linearly with depth. This could be attributed to different tectonic
setting. The values obtained from different areas of the world are plotted with the values
obtained for the North-West of Malta, South of Malta and Crete as seen in figure 6.4. The
values plotted here for different areas of the world are brought from previous studies that have
been conducted worldwide. The study of attenuation for the Charleviox Quebec Region and
that for Southeastern Canada was conducted by Woodgold (1994). The frequency dependence
of Qc in the Mt.Etna region was conducted by Del Pezzo et al. (1995). Taking the 5-6 Hz one
can observe that the lowest values were obtained in the tectonically active region of Mt.Etna
(Del Pezzo et al., 1995). Following are the values obtained for the Crete subduction Zone in
this study and the Charleviox Region in Quebec (Woodgold, 1994). The values of this study
follow after these values with the values obtained for the North-West of Malta being lower
than the values obtained for the South of Malta. Highest values are obtained in South-Eastern
Canada (Woodgold, 1994). This means that the most attenuated region is that of Mt.Etna. The
values obtained in this study are not the lowest or the largest that have been obtained
worldwide. This means that the region around the Maltese Islands is not the most tectonically
active region in the world but is not a stable tectonic region either.
96
Chapter 6: Discussion
4000
3500
3000
Average Q
2500
2000
1500
1000
500
0
0
2
4
6
8
10
12
14
Frequency (Hz)
South of Malta
Crete
North - West of Malta
Charleviox
Mt.Etna
South-East Canada
Figure 6.4: Comparison of the Qc relations obtained in different tectonic and volcanic areas in the world.
97
Chapter 6: Discussion
6.4 Further Work
This should be carried out again using a larger amount of events if time permits and
should be carried out for more areas of Study. This could now be done since the same
procedure adopted here could be used. Also the Study carried out for the North-West of Malta
area should be carried out again when a more reliable data set is available
98
References

Abela, M. (1969). Earthquakes in Malta, History Thesis (University of Malta).

Aguis, J. (2003). Discrimination between Quarry Blasts and Micro Earthquakes around
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
Aguis, M; Galea, P. (2008). Automated Single- Station Earthquake Location in the
Sicily Channel using WDD broadband station in the Maltese Islands. Available online:
http://193.188.45.245/downloads/Automated_SingleStation_Earthquake_Location_in_the_Sicily_Channel_using_WDD_Broadband_Statio
n_on_the_Maltese_Islands.pdf, last accessed on 15 April 2011.

Aguis, M; Galea, P. (2008). Digital Seismic Recording in Malta. Available online:
http://193.188.45.245/downloads/Digital_Seismic_Recording_in_Malta__13_Years_On.pdf, last accesses on 15 April 2011.

Akamatsu, J. (1991). Coda attenuation in the Lützow-Holm Bay, East Antarctica,
Phys. Earth Planet. Interiors, 67, 65-75.
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Appendix 1: Recorded Events
Appendix 1
Recorded Events
Table A: The south of Malta earthquakes that were used for the calculation of coda Q. These
were recorded by the WDD station and obtained from the Seismic Monitoring and Research
Unit database at the University of Malta. In this table the date, origin time, P and S arrival
times, the SP time, the calculated lapse times, magnitude, distance, latitude and longitude for
each event are given
Event
No.
date
Origin Time
GMT
station
P Time
GMT
S Time
GMT
SP Time
GMT
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
05/01/2011
15/01/2011
03/12/2010
01/12/2010
18/11/2010
12/11/2010
21/09/2010
03/08/2010
15/07/2010
09/07/2010
15/06/2010
30/08/2009
25/08/2009
16/08/2009
07/08/2009
30/07/2009
05/07/2009
26/05/2009
16/05/2009
28/04/2009
25/04/2009
12:22:19
05:14:47
02:02:20
11:41:52
08:20:09
04:43:35
04:39:44
08:06:49
17:38:45
00:19:24
23:29:24
02:10:57
03:21:05
07:49:42
09:06:22
02:58:36
18:52:14
21:37:21
13:08:42
02:56:40
01:36:25
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
12:22:25
05:14:50
02:02:28
11:42:20
08:20:24
04:43:50
04:39:55
08:06:56
17:39:14
00:19:44
23:29:31
02:11:12
03:21:22
07:49:59
09:06:30
02:58:54
18:52:35
21:37:38
13:08:45
02:56:45
01:36:53
12:22:30
05:14:53
02:02:34
11:42:40
08:20:35
04:44:01
04:40:03
08:07:01
17:39:36
00:19:58
23:29:36
02:11:23
03:21:34
07:50:12
09:06:35
02:59:07
18:52:51
21:37:51
13:08:48
02:56:49
01:37:14
21:15:50
07:35:02
01:40:48
11:15:22
23:09:36
04:22:05
09:50:24
01:16:19
15:11:31
12:12:58
05:08:10
01:03:22
11:19:41
16:16:19
13:13:26
06:20:10
15:01:26
00:10:05
07:12:00
04:03:22
15:30:14
Appendix 1: Recorded Events
Event
No.
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
date
Origin Time
23/03/2009
23/12/2008
30/11/2008
05/07/2008
15/10/2007
03/10/2007
05/09/2007
05/09/2007
05/09/2007
05/09/2007
16/08/2007
11/08/2007
24/06/2007
20/05/2007
11/05/2007
31/03/2007
31/03/2007
30/03/2007
22/03/2007
15/02/2007
26/01/2007
27/01/2007
GMT
18:44:35
23:47:24
16:28:27
00:59:41
17:36:44
02:15:08
09:21:53
08:02:48
07:58:12
06:36:03
01:51:28
02:19:05
16:13:04
22:09:31
03:29:58
13:08:01
12:22:32
13:59:47
12:18:57
04:26:22
23:35:46
23:03:16
station
P Time
S Time
SP Time
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
WDD
GMT
18:45:02
23:47:31
16:28:33
00:59:45
17:36:57
02:15:14
09:21:58
08:02:54
07:58:18
06:36:09
01:51:33
02:19:12
16:13:12
22:09:39
03:30:05
13:08:25
12:22:58
13:59:54
12:19:05
04:26:30
23:36:04
23:03:33
GMT
18:45:21
23:47:36
16:28:38
00:59:48
17:37:07
02:15:18
09:22:02
08:02:59
07:58:22
06:36:13
01:51:37
02:19:17
16:13:18
22:09:44
03:30:11
13:08:43
12:23:16
13:59:59
12:19:10
04:26:36
23:36:17
23:03:45
GMT
19:03:22
05:03:50
00:43:12
22:27:50
14:13:55
08:34:05
00:00:00
07:32:10
07:20:38
08:26:53
13:43:41
19:06:14
19:01:55
14:54:14
16:49:26
18:33:07
20:42:43
01:30:43
19:00:29
23:05:17
22:07:41
10:40:48
Appendix 1: Recorded Events
Event
No.
Lapse Time: S
Seconds
2S
Seconds
new time ( original +2S)
GMT
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
11
6
14
48
26
26
19
12
51
51
12
36
29
49
13
30
37
30
49
13
49
46
12
11
7
30
13
12
27
88
40
30
31
43
13
13
42
44
22
12
28
96
52
52
38
24
102
68
24
72
58
98
26
62
74
60
98
26
98
92
24
22
14
60
26
24
54
176
80
60
62
86
26
26
84
88
12.22.41
05.14.59
02.02.48
11.42.28
08.20.59
04.44.27
04:40:22
08:07:13
17:40:27
00:20:32
23:29:48
02:11:49
03:22:03
07:50:42
09:06:48
02:59:38
18:53:28
21:38:21
13:09:14
02:56:58
01:38:03
18:46:07
23:47:48
16:28:49
00:59:55
17:37:30
02:15:28
09:22:21
08:03:10
07:58:32
06:36:23
01:51:46
02:19:29
16:13:32
22:09:57
03:30:24
13:09:25
12:24:00
Appendix 1: Recorded Events
39
39
39
39
40
41
12
12
12
12
13
14
24
24
24
24
26
28
14:00:01
14:00:01
14:00:01
14:00:01
12:19:23
04:26:50
42
31
62
23:36:48
43
29
58
23:04:14
Appendix 1: Recorded Events
Event
No.
distance
(km)
Ml
Md
latitude
(deg)
longitude
(deg)
1
2
3
4
5
6
7
38.093
17.811
47.633
173.169
88.325
90.175
66.831
2.379
1.914
2.919
4.612
3.800
3.800
2.759
1.683
1.315
2.024
3.064
2.380
2.476
2.027
35.921
35.741
35.411
34.350
35.065
35.037
35.259
14.115
14.682
14.477
15.083
14.749
14.674
14.720
8
39.431
2.669
1.885
35.533
14.300
9
10
184.092
119.062
2.800
3.558
2.799
2.690
34.194
35.407
14.293
13.320
11
12
13
40.723
89.000
101.266
1.800
1.661
2.477
1.860
1.941
2.188
35.517
35.111
35.835
14.307
14.934
15.646
14
15
16
103.048
43.433
108.142
3.241
3.431
2.824
5.034
1.845
2.472
34.923
35.894
35.101
14.705
15.001
15.302
17
18
128.975
105.907
3.617
2.510
2.805
2.440
35.048
34.989
13.452
15.053
19
20
17.678
32.375
2.517
3.024
1.441
1.764
35.988
35.616
14.584
14.756
21
174.820
4.600
3.142
34.307
14.092
22
166.892
4.627
3.396
34.475
13.759
23
40.699
1.715
1.715
35.583
14.848
24
39.246
2.802
1.886
35.665
14.903
26
76.714
3.500
2.321
35.152
14.613
27
28
33.874
31.034
1.816
2.005
2.354
1.154
35.542
35.624
14.616
14.304
29
33.532
3.057
1.668
35.617
14.271
30
31
32
33.465
33.831
27.656
2.712
2.924
1.991
1.698
1.754
1.378
35.560
35.561
35.911
14.382
14.679
14.817
33
34
37.373
45.388
3.687
2.160
2.091
1.846
35.578
35.468
14.262
14.737
35
43.999
1.528
1.790
35.448
14.437
36
37
44.645
148.326
1.779
3.655
2.138
4.400
35.591
34.522
14.914
14.265
Appendix 1: Recorded Events
Event
distance
Ml
No.
(km)
38
158.296
4.546
39
39.507
40
Md
latitude
longitude
(deg)
(deg)
3.041
34.472
14.039
2.630
1.865
35.739
14.945
45.380
2.940
2.940
35.976
14.051
41
46.754
3.432
2.191
35.586
14.938
42
105.167
3.787
2.594
34.909
14.741
43
101.036
3.546
2.311
34.944852
14.726923
Appendix 1: Recorded Events
Table B: The events at the North-West of Malta area near Pantelleria that were used for the
calculation of coda Q. These were recorded by WDD station and obtained from the Seismic
Monitoring and Research Unit at the University of Malta. In this table the date, origin time, P
and S arrival times, the SP time, the calculated lapse times, magnitude, distance, latitude and
longitude for each event are given.
Event
No.
date
1
2
3
4
5
6
7
19/03/2009
02/07/2008
11/02/2008
10/04/2007
11/02/2007
15/03/2006
13/03/2006
Origin
Time
GMT
station
10:25:59
09:17:52
08:05:41
19:17:30
20:30:59
20:33:13
18:05:00
WDD
WDD
WDD
WDD
WDD
WDD
WDD
P Time
GMT
S Time
GMT
SP Time
GMT
Lapse
Time: S
Seconds
10:26:26
09:18:19
08:05:51
19:18:01
20:31:18
20:33:35
18:05:26
10:26:47
09:18:38
08:05:59
19:18:24
20:31:32
20:33:51
18:05:45
07:07:41
14:19:41
11:54:14
21:33:07
07:48:00
05:03:50
06:15:50
48
45
18
54
33
38
45
2S
Seconds
96
90
36
108
66
76
90
Event
No.
New time
GMT
distance
(km)
Ml
Md
latitude
(deg)
longitude
(deg)
1
2
3
4
5
6
7
10:27:35
09:19:22
08:06:17
19:19:18
20:32:05
20:34:29
18:06:30
171.56638
165.06616
59.27484
196.09474
117.44272
134.20744
161.96033
3.478
4.498
2.069
4.411
3.448
3.365
3.735
3.286
3.096
2.125
3.578
2.336
2.954
3.133
36.863
36.456
36.004
37.028
36.776
36.505
36.490
13.096
12.856
13.900
12.911
13.925
13.281
12.914
Appendix 1: Recorded Events
Table C: The earthquakes from the Subduction Zone near Crete that were used for the
calculation of coda Q. These were recorded by IDI station, Crete and obtained from the
Institute of Geodynamics Athens’s database that is available online. In this table the Origin
time, P time, magnitude, depth, latitude and longitude for each event are given.
Event
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
Date
28/02/2011
1/11/2010
1/11/2010
31/10/2010
25/10/2010
21/10/2010
21/10/2010
14/10/2010
11/10/2010
11/10/2010
11/10/2010
19/08/2010
17/08/2010
13/08/2010
05/01/2010
31/12/2009
24/12/2009
17/12/2009
03/11/2009
23/10/2009
06/10/2009
26/09/2009
22/09/2009
17/08/2009
08/08/2009
02/08/2009
27/07/2009
23/07/2009
14/07/2009
10/07/2009
05/06/2009
30/05/2009
22/05/2009
Origin Time
GMT
07:49:07
16:23:11
14:47:58
21:29:47
02:18:57
09:48:44
10:06:45
09:08:22
22:24:03
19:49:09
14:11:53
21:34:03
14:04:29
15:37:54
20:31:19
00:12:41
17:04:42
14:53:44
12:33:55
17:59:48
16:19:08
02:52:01
21:49:02
08:36:36
16:47:16
08:49:42
17:46:06
03:03:45
23:06:12
07:29:02
07:46:01
13:29:32
19:39:39
P time Station
GMT
16:23:32
IDI
14:48:16
IDI
14:48:16
IDI
21:30:08
IDI
02:19:32
IDI
09:49:41
IDI
10:06:55
IDI
09:09:21
IDI
22:24:21
IDI
19:49:29
IDI
14:12:06
IDI
21:34:33
IDI
14:04:40
IDI
15:38:03
IDI
20:31:33
IDI
00:13:02
IDI
17:05:01
IDI
14:53:49
IDI
12:35:06
IDI
18:00:29
IDI
16:20:06
IDI
02:52:30
IDI
21:49:17
IDI
08:37:02
IDI
16:47:28
IDI
08:49:53
IDI
17:46:18
IDI
22:03:57
IDI
23:06:22
IDI
07:29:12
IDI
07:46:29
IDI
13:29:44
IDI
19:39:58
IDI
Latitude
(deg)
34.980
34.260
35.150
36.000
36.880
38.870
35.070
36.270
24.480
34.170
34.760
34.210
34.840
34.840
35.000
34.200
35.680
35.040
34.850
37.480
34.900
33.750
34.690
33.950
35.000
35.290
35.740
34.760
34.210
34.890
34.610
34.930
34.870
Longitude
(deg)
25.420
24.500
23.710
23.820
26.740
26.010
24.310
29.650
25.580
25.060
24.370
26.340
24.470
24.470
24.120
25.180
25.930
24.980
24.110
26.690
25.320
25.490
24.650
25.360
24.790
24.370
24.630
24.970
25.270
24.910
23.760
24.770
24.790
Depth
(km)
53
5
20
35
46
20
23
49
15
4
20
21
30
15
45
36
30
17
24
26
15
41
30
7
32
43
44
29
22
30
11
35
26
Magnitude
5.2
3.6
3.1
3.5
3.2
3.5
2.6
4.3
3.3
2.9
2.8
3.6
2.9
3.0
2.8
3.6
2.9
2.7
3.0
3.7
3.2
4.1
3.3
3.3
2.9
3.3
3.2
3.1
3.2
3.2
3.6
3.2
2.8
Appendix 1: Recorded Events
Event
No.
34
35
36
37
38
39
40
41
42
Date
01/05/2009
27/04/2009
19/03/2009
17/03/2009
29/11/2008
18/11/2008
04/08/2008
12/06/2008
12/04/2008
Origin time
GMT
22:42:25
03:10:48
14:15:13
11:14:21
21:18:07
01:00:46
19:38:23
00:20:43
07:58:31
P time station
GMT
22:42:32
IDI
03:11:13
IDI
14:15:41
IDI
11:14:41
IDI
21:18:18
IDI
01:00:59
IDI
19:38:56
IDI
00:21:04
IDI
07:58:56
IDI
Latitude
(deg)
35.600
35.590
35.100
35.830
34.81
35.460
33.890
35.110
34.070
Longitude
(deg)
24.770
26.450
23.440
23.710
25.02
25.710
26.560
26.190
25.310
Depth
(km)
33
30
37
12
10
25
32
29
9
Magnitude
2.8
3.6
4.8
3.2
3.3
3.0
5.0
5.0
4.1
Appendix 2- Codaq Q Values
Appendix 2
Coda Q Values - Data Tables
Table D: The values of the coda q, Qc that were calculated by SEISAN for the south of Malta
earthquakes.
Event
No.
1
date
05/01/2011
2
15/01/2011
3
03/12/2010
4
01/12/2010
5
18/11/2010
frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
120
993
4518
9920
91
545
223
261
477
53
331
870
860
165
389
497
361
946
226
566
576
673
-
Appendix 2- Codaq Q Values
Event
date
No.
6
7
8
9
10
11
12
frequency
Qc value
(Hz)
12/11/2010
21/09/2010
03/08/2010
15/07/2010
09/07/2010
15/06/2010
30/08/2009
2
61
5
147
7
323
9
476
12
1036
2
163
5
-
7
1111
9
705
12
-
2
106
5
843
7
-
9
2414
12
1413
2
-
5
780
7
-
9
-
12
-
2
92
5
6747
7
-
9
-
12
-
2
-
5
355
7
743
9
885
12
984
2
187
5
429
7
-
9
-
12
-
Appendix 2- Codaq Q Values
Event
No.
13
date
25/08/2009
14
16/08/2009
15
07/08/2009
16
30/07/2009
17
05/07/2009
18
26/05/2009
19
16/05/2009
frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
142
995
984
172
1232
973
121
192
337
468
1016
2272
7895
67
403
587
3313
42
282
374
187
231
251
222
-
Appendix 2- Codaq Q Values
Event
No.
20
date
28/04/2009
21
25/04/2009
22
23/03/2009
23
23/12/2008
24
30/11/2008
25
05/07/2008
26
15/10/2007
frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
74
153
281
386
554
95
898
570
831
1203
305
422
744
1210
132
261
320
376
603
105
138
549
1109
872
62
139
254
207
267
144
265
502
866
1124
Appendix 2- Codaq Q Values
Event
No.
27
date
03/10/2007
28
05/09/2007
29
05/09/2007
30
05/09/2007
31
05/09/2007
32
16/08/2007
33
11/08/2007
frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
78
269
248
929
90
339
713
2156
2556
50
171
402
777
899
41
564
988
54
266
622
3405
70
298
881
6964
1282
255
509
433
560
Appendix 2- Codaq Q Values
Event
No.
34
date
24/06/2007
35
20/05/2007
36
11/05/2007
37
31/03/2007
38
31/03/2007
39
30/03/2007
40
22/03/2007
frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
239
659
2346
5296
7549
1854
424
1978
799
208
952
1235
2032
3553
328
1043
1430
1368
1328
6897
59
345
348
694
1354
409
258
756
2281
-
Appendix 2- Codaq Q Values
Event
No.
41
date
15/02/2007
42
26/01/2007
43
27/01/2007
frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
193
357
366
606
190
320
263
519
911
62
494
1994
-
Appendix 2- Codaq Q Values
Table E: The values of the coda q, Qc that were calculated by SEISAN for the North-West of
Malta earthquakes.
Event
No.
1
2
date
19/03/2009
02/07/2008
3
11/02/2008
4
10/04/2007
5
11/02/2007
6
13/03/2006
frequency
(Hz)
2
5
7
9
Qc value
12
1855
2
5
7
9
102
276
658
5505
12
-
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
370
652
1656
83
769
735
4238
201
490
608
773
125
125
5637
2321
1402
6204
109
549
890
-
Appendix 2- Codaq Q Values
Table F: The values of the coda q, Qc that were calculated by SEISAN for the Crete
earthquakes.
Event
No.
1
date
28/02/2011
2
01/11/2010
3
01/11/2010
4
31/10/2010
5
25/10/2010
6
21/10/2010
7
21/10/2010
frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
77
595
3178
184
1406
2501
123
389
568
283
341
428
574
742
276
432
604
1065
1212
327
281
366
434
689
175
1909
1297
Appendix 2- Codaq Q Values
Event
No.
8
Date
14/10/2010
9
11/10/2010
10
11/10/2010
11
11/10/2010
12
19/08/2010
13
17/08/2010
14
13/08/2010
Frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
139
1099
435
858
1341
148
601
605
1225
315
401
1131
117
528
618
981
545
332
729
2420
286
180
261
403
565
Appendix 2- Codaq Q Values
Event
No.
15
Date
05/01/2010
16
31/12/2009
17
24/12/2009
18
17/12/2009
19
03/11/2009
20
23/10/2009
21
06/10/2009
Frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
174
615
1072
142
975
1108
1856
2390
125
330
1035
774
950
369
288
400
626
791
105
664
857
946
2831
343
1504
3931
173
495
775
575
847
Appendix 2- Codaq Q Values
Event
No.
22
Date
26/09/2009
23
22/09/2009
24
17/08/2009
25
08/08/2009
26
02/08/2009
27
27/07/2009
28
23/07/2009
Frequency
Hz
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
81
191
295
697
521
700
209
1576
314
538
1302
4301
225
614
427
827
1173
300
390
486
633
680
123
416
349
565
1925
Appendix 2- Codaq Q Values
Event
No.
29
Date
14/07/2009
30
10/07/2009
31
05/06/2009
32
30/05/2009
33
22/05/2009
34
01/05/2009
35
27/04/2009
Frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
89
243
358
538
911
403
534
459
431
577
468
847
1495
1684
141
533
537
555
2606
143
327
410
496
969
112
563
449
512
560
465
385
912
1052
1536
Appendix 2- Codaq Q Values
Event
No.
36
Date
19/03/2009
37
17/03/2009
38
29/11/2008
39
18/11/2008
40
04/08/2008
41
12/06/2008
42
12/04/2008
Frequency
(Hz)
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
2
5
7
9
12
Qc value
298
1007
937
1820
775
794
729
3700
478
448
546
623
169
436
466
584
679
289
643
916
740
81
333
470
539
685
332
345
2263
Department of Physics
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