Download Surface Waves

INTRODUCTION TO EARTHQUAKE
ENGINEERING
• What is an Earthquake:
Earthquake is the strong shaking of ground, which is typically a
result of some major disturbances deep within earth crust.
• What
Wh t causes the
th Earthquake:
E th
k
Earthquake can happen due to a variety of causes (& theories).
However there are four broad categories:
• Tectonic earthquakes which are most common and occur when
rocks in the earth's crust break due to geological forces created
by movement of tectonic plates.
• Volcanic earthquakes which occur in conjunction with volcanic
activity.
• Collapse earthquakes which are small earthquakes in
underground
g
caverns and mines.
• Explosion earthquakes which result from the explosion of nuclear
and chemical devices
A BRIEF ON EARTH GEOLOGY
StratigraphicCrossSection.mov
TECTONIC PLATES
•D
Depending
di
on h
how th
they are defined,
d fi d there
th
are usually
ll seven or
eight "major" plates: African, Antarctic, Eurasian, North American,
South American, Pacific, and Indo-Australian. The latter is
sometimes subdivided into the Indian and Australian plates
Plate tectonics.mov
TECTONIC MOVEMENT
• Plate movement is believed to occur so as to maintain thermomechanical equilibrium in the materials composing the earth
• This movement is in the form of either aseismic or seismic
deformations. Whereas the former type of deformations occur
slowly and continuously, the latter occur suddenly and are
associated with larger movements
• Plate boundaries:
1) Subduction zone
2)) Spreading
p
g Rifts
3) Transformation faults
SUBDUCTION ZONES
• At subduction zone boundaries, plates move towards each other
and one plate subducts underneath the other.
• Subducting plates heat up, fragment and sink into the mantle.
•S
Subduction
bd ti
zones are therefore
th
f
more closely
l
l associated
i t d with
ith
volcanic formation than seismic activity
• Subduction process.mov
• Boundary Types.mov
SPREADING RIFTS & TRANSFORM FAULTS
• The opposite form of movement takes place at spreading rift
boundaries. As plates move away from each other, molten rock is
allowed to rise from the mantle to the surface and cool down to
form part of plates.
• By moving apart, the plates therefore grow in size and the surface
becomes older away from the ridges
p
pass
p
each other without
• At transform faults boundaries,, plates
either enabling new crust to form or causing existing crust to sink
into the mantle.
• Transform faults are mainly associated with spreading rifts,
rifts as
they are usually formed by surface movement due to
perpendicular spreading ridges on either side
EARTHQUAKE MECHANISM(S)
• The dilatational source theory that earthquakes are caused by
sudden fracturing of material in the earth’s crust.
• The elastic rebound theory that earthquakes are caused by
sudden movement along faults and the release of strain energy.
BOTH THEORIES MAY BE TRUE
• Elastic Rebound.mov
• Normal Fault.mov
• Reverse Fault.mov
• Oblique Fault.mov
• Stike-Slip Fault.mov
BASIC EARTHQUAKE DEFINITIONS
• Hypocenter / Focus: The point were rupture initiates and seismic waves
start to propogate
• Epicenter: The projection of Hypocenter directly on ground above
TYPES OF SEISMIC WAVES
• Body waves
Travel through the earth's interior
• Surface
S f
W
Waves
Travel along the earth's surface - similar to ocean waves
Particle motion of surface waves is larger than that of body waves,
so surface waves tend to cause more damage
Body waves
P
S
Surface Waves
Love
Rayleigh
“Ground Roll”
• Body waves travel through the interior of the Earth. They create
raypaths refracted by the varying density and modulus (stiffness)
of the Earth's interior. The density and modulus, in turn, vary
according to temperature, composition, and phase. This effect
resembles the refraction of light waves.

The first kind of body
y wave is the P wave or p
primary
y wave.
This is the fastest kind of seismic wave.

The P wave can move through solid rock and fluids, like water
or the liquid layers of the earth.
•
The motion of an individual particle that a P-wave travels
through is parallel to the direction of propagation, hence called
longitudinal wave as well

It pushes and pulls the rock it moves through just like sound
waves push and pull the air.

Highest velocity (6 km/sec in the crust)
K= bulk modulus or
modulu of
incompressibilty
m=G ((shear Modulus))
m
• P-Wave propagation
The second type of body wave is the S wave or secondary wave,
which is the second wave you feel in an earthquake.
An S wave is slower than a P wave and can only move through
solid rock. (3.6 km/sec in the crust)
This wave moves rock up
p and down,, or side-to-side.
• They are also called transverse waves because particles move in a
direction perpendicular to that of the wave propagation
• Depending on whether this direction is along a horizontal or
vertical plane, S-waves are subdivided into SV and SH-waves,
respectively
• S-wave propagation
SURFACE WAVES
• The first kind of surface wave is called a Love wave, named after
AEH L
A.E.H.
Love, a British
B iti h mathematician
th
ti i
who
h worked
k d outt the
th
mathematical model for this kind of wave in 1911.
• It's the fastest surface wave and moves the ground from side-toside
• Particle motion consists of alternating transverse motions
• Particle motion is horizontal and p
perpendicular
p
to the direction of
propagation (transverse)
•Particle motion is purely horizontal, focus on the Y axis (black
lines) as the wave propagates through it
•Amplitude decreases with depth (yellow lines).
•Material returns to its original shape after wave passes
• Love-wave propagation
• The other kind of surface wave is the Rayleigh wave, named for
J h William
John
Willi
St tt L
Strutt,
Lord
d Rayleigh,
R l i h who
h mathematically
th
ti ll predicted
di t d
the existence of this kind of wave in 1885.
• A Rayleigh wave rolls along the ground just like a wave rolls
across a lake or an ocean.
ocean
• Because it rolls, it moves the ground up and down, and side-toside in the same direction that the wave is moving.
• Most of the shaking felt from an earthquake is due to the
Rayleigh wave, which can be much larger than the other waves.
• Particle motion consists of elliptical motions (generally retrograde
elliptical) in the vertical plane and parallel to the direction of
propagation. Amplitude decreases with depth. Material returns
to its
i original
i i l shape
h
after
f
wave passes.
• Rayleigh-wave propagation
CHARACTERISTICS OF SEISMIC WAVES
Material
P wave Velocity (m/s)
S wave Velocity (m/s)
Air
332
Water
1400-1500
Petroleum
1300-1400
Steel
6100
3500
Concrete
3600
2000
Granite
5500-5900
2800-3000
Basalt
6400
3200
Sandstone
1400-4300
700-2800
Limestone
5900-6100
2800-3000
Sand (Unsaturated)
200-1000
80-400
Sand (Saturated)
800-2200
320-880
Clay
1000-2500
400-1000
Glacial Till (Saturated)
1500-2500
600-1000
RECORDING EARTHQUAKES
• Around 132 AD, Chinese scientist Chang Heng
invented the first seismoscope, an instrument
that could register the occurrence of an
earthquake.
• Seismographs record a zigzag trace that shows
the varying amplitude of ground oscillations
beneath the instrument.
• Sensitive seismographs, which greatly magnify
these
h
ground
d motions,
i
can detect
d
strong
earthquakes from sources anywhere in the
world.
• The time, location and magnitude of an
earthquake
th
k can be
b determined
d t
i d from
f
the
th data
d t
recorded by seismograph stations.
DEFINITIONS
• Seismometers are instruments for detecting ground motions
• Seismographs are instruments for recording seismic waves from
earthquakes.
• Recordings are called seismograms
HOW DOES A SEISMOGRAPH WORK?
• Vertical seismograph
• Vertical / Horizontal Seismograph
• Horizontal Seismograph
CHRONOLOGY OF SEISMIC WAVES
• P-waves travel faster, at
speeds between 1.5 and
8km/s while S
8km/s,
S-waves
waves are
slower, at 50% to 60% of
the speed of P-waves
• The velocity of R-waves
R waves is
approximately 0.92cS
• The L-wave velocity
generally
ll obeys
b
cS1
S1 < cL
L<
cS2,
ATTENUATION OF SEISMIC WAVES
• Attentuation is the “amplification” or “de-amplification” of seismic
waves from the source to the surface.
• Geometrical spreading leads to a reduction in amplitude of body
waves proportional to 1/r, where r is the distance form the source
• surface wave reduce in amplitude according to r-0.5.
• Attenuation tends to damp body waves faster than surface waves
• Frictional attenuation also leads to an exponential decay of
amplitude with time, with the higher frequency damping out
faster
ATTENUATION LAWS
• Attenuation laws are empirical formulae obtained using databases
of strong motion data.
• They are aimed at predicting the likely level of ground shaking at
given locations
• Whereas the output
p variable may
y be a strong
g motion parameter
p
such as peak ground acceleration (PGA), the input variables
should be the most influential parameters and should take into
account distance effects
• The larger the strong motion database, the more successful the
model is expected to be
• The model uncertainty is usually described by specifying the
expected standard deviation σ of the output parameter
ATTENUATION LAWS
• where A is the PGA usually in a horizontal direction, M is a magnitude, R is a distance
from the epicentre or ruptured fault and βi are the regression factors
• Campbell (1981) derived the following relationship from a database of strong ground
motion data from all over the world, for 5.0 < M < 7.7 and R < 50 km.
• where Ah is the peak horizontal ground acceleration in g, R is the shortest distance
to the ruptured fault in km and M is the magnitude given in terms of the local
(Richter) magnitude ML for M<6.0 or the surface wave magnitude MS for M>6.0.
• Ambraseys and Bommer (1991) derived the following relationship from a database
covering
i
E
Europe and
d neighboring
i hb i
regions.
i
•
where r = (d2+62)0.5
(d2+62)0 5 and d (in km) is the shortest distance to the
projection of the ruptured fault onto the ground surface.
QUANTIFYING EARTHQUAKES
• Magnitude: The magnitude of an earthquake is intended as an
absolute measure of its size.
• Richter Scale Definition:
• M iis th
the magnitude,
it d
• A is the maximum seismic wave amplitude recorded by a WoodAnderson seismograph (with a magnification of 2800, a natural
period of 0.8 seconds and a damping coefficient of 80%) at a
distance of 100 km from the disturbance and
• A0 is an amplitude
p
of 0.001 mm
• The original Richter definition (also call Local Richter Magnitude,
ML) is an old and somehow outdated one, however it is still
commonly used. There are other definitions available, the most
notable of them are:
• Body wave magnitude (mb),
• the surface wave magnitude (MS) and
• The moment magnitude (MW).
• The Richter magnitude is a tool that basically measures the
“amplitude” of an earthquake.
• For more practical purposes, this can be related to the energy
released during an earthquake through empirical expressions:
M shall be expressed as “surface wave magnitude; Ms”.
QUALITATIVE DESCRIPTION OF
EARTHQUAKES
• Intensity : The intensity of an earthquake is, a subjective
measure of the effect of an earthquake at a given place.
• A single earthquake would have different intensities at different
locations, tending to reduce with distance from the centre of the
disturbance.
• Intensity is typically measured by the Modified Mercalli intensity
Number.
• There are other intensity scales such as the Mercalli-CancaniSeiberg (MCS) used in southern Europe, the European
Macroseismic Scale (EMS) adopted since 1998 and the Japanese
Meteorological Agency (JMA) used in Japan
THANKS