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Module: Seismicity and seismic risk
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Mário Lopes
([email protected])
Departamento de Engenharia Civil,
Arquitectura e Geo-Recursos do Instituto
Superior Técnico, Lisboa
Bolonha, 3-6 March 2014
3 – Characterization of seismic
actions
 Prediction of earthquakes
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 Seismicity
 Probabilistic definition of the seismic
action
 Definition of the seismic action for
structural applications
 Annual probability of ocurrence. Return
period
 Quantifications of other seismic effects
Prediction of earthquakes
It is not possible to predict well in advance the date of
occurrence of the next strong earthquake at a certain
location.
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It is extremely unlikely that before an earthquake
happens there is any sign that it is about to happen,
allowing to evacuate towns (short term prediction). There
is only one sucess case in all history of mankind, in
Haicheng, China, in 1975.
Even if the short term prediction was reliable, it would not
prevent the destruction of towns and of the economy.
It is possible to estimate the probability that earthquakes
of given characteristics occur at a given location during a
certain period of time (long term prediction).
Seismicity
studies for long term prediction
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It is used for the definition of the design seismic action
It is based on information from three components:
- Hystorical seismicity
- Instrumental seismicity
- Geological evidence
Hystorical seismicity
Information from past earthquakes, when there were no
records, that allows the characterization of those
earthquakes, as for instances:
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 Date
 Zones more affected by the earthquake
 Level of damage
 Occurrence of tsunami
Analysis of characteristics of earthquakes
based on hystorical information
- Epicentral location using isosseismals maps.
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Account for site
effects and
irregular
exposition, that
may distort the
isoseismals map
- Epicentral distance based on the tsunami
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The tsunami can be used to determine the epicentral
distance using information on the difference of time
between the earthquake and the arrival of tsunami,
knowing the speeds of the seismic waves and the
tsunami.
- Magnitude based on damage
Knowing the characteristics of the constructions at the
location and time of the earthquake, damage on the
constructions can be used to calculate the ground
accelerations that would induce that damage. Knowing
the attenuation laws of seismic waves it is possible to
evaluate the characteristics of those waves at the
epicenter and therefore the magnitude.
- Maximum credible earthquake at the fault
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Knowing the epicenter, by means of geological and
tectonic studies of the zone and and it is possible to
estimate the maximum credible earthquake that can
take place at the fault.
Shortcomings of hystorical information
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- Reliability and precision – part of the information
may not be accurate, given the circunstances in
which it is obtained: possible panic and lack of
knowledge of the witnesses, and errors in registering
the information
- It is available usually for a period of time much
smaller than the period of the large tectonic
movements and the return period between large
earthquakes
Instrumental seismicity
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It is the information of real earthquakes based on
records of seismographs or accelerometers. It dates
from the early 20th century, in a more systematic way
only from the middle of the XX century onwards.
Advantages
-
It is much more accurate than the hystorical
information.
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It
allows
better
determination
of
epicenters and magnitudes.
-
It allows the identification of occult faults, that do not
reach the Earth´s surface.
-
It allows to register earthquakes of small magnitude,
not felt by humans.
Disadvantage
It concerns an extremely short period, lower than the
period
associated
with
the
hystorical
seismicity,
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compared with the return period of large earthquakes
Seismographs , Accelerographs
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Lai, 2013
Seismographic network (2008)
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Accelerometric network (2008)
Magnitudes and epicenters of earthquakes recorded in Portugal
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Geological evidence
- Location of faults
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- Dimension of faults and characteristic earthquakes
of those faults
- Activity in those faults
- Past earthquakes in those faults
For instances paleossismological studies on adequate
trenches open on active faults, may allow the
identification periods of ocurrence and magnitudes of
earthquakes that took place thousands of years ago.
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Paleosseismological field works at the Vilariça Fault (north
of Portugal). It allowed the detection of two earthquakes of
magnitude above 7 during the last 18 000 years, much
more than the largest hystorical earthquake known, with
magnitude 5,8 and dated from 1858
Paleosseimological studies on the effects of tsunamis,
like movements of large stones and sand deposits,
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may also be used to characterize tsunamis and the
earthquakes that gave rise to them thousands of years
ago.
Earthquake catalogues
Based on all sources of information, the main
earthquakes and all the available information on their
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main characteristics are assembled in earthquake
catalogues.
These catalogues usually refer to the last centuries,
sometimes more in some zones. The degree of
knowledge and reliability of the information varies
depending on how long ago the earthquake took place
and the sources of information.
For the events to be time independent, remove
foreshocks and aftershocks
Probabilistic definition of the seismic action
A given zone on a territory may be subjected to
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earthquakes of different seismic sources. Therefore it is
usual to divide the territory in seismogenic zones of similar
seismicity, either spread in that zone or concentrated in
certain faults previously identified. The seismicity of each
zone is characterized based on the 3 previously referred
sources of information (earthquake catalogue and known
faults).
Example of seismogenic zones for seismicity studies in Portugal
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Main seismic sources (earthquakes M>5), that affect the
Italian territory
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INGV, 2010
Example of seismogenic zones for seismicity studies in Italy
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The seismic action potentially felt at a given
location results from 3 main processes.
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1. the occurrence of earthquakes
2. the propagation of seismic waves from
the source to the location being studied
3. site effects
Occurrence of earthquakes
The occurrence of earthquakes is characterized by their
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distribution along time, the location of the ruptures of the
faults and the magnitude.
Given an earthquake catalogue, the frequency of
occurrence of earthquakes of a given magnitude is
usually assumed to follow the Gutemberg-Richter law
(1944), as follows:
𝑙𝑜𝑔 𝑀 = 𝑎 − 𝑏. 𝑀
M is the annual probability of occurrence of
earthquakes of magnitude  M
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a, b – constants that depend on the seismicity of the
seismogenic region being studied and are
derived from the earthquake catalogue
Basic assumption: the process is memory less, this is,
the ocurrence of earthquakes is independent of time. It
does not accounts for how much time passed since the
last event of similar characteristics
If we consider the earthquakes of all the XX century in
the whole Earth we obtain the following relationship,
with b= 0,898 and a=8. Note that in average there is 1 to
2 earthquakes M>8
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Incertainties
The above is a process with very large uncertainties due to:
 the short period range of earthquake catalogues, specially for
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large magnitude earthquakes whose return period may be
much larger than the period spanned by the catalogue; in fact
the rate of occurrence of large earthquakes results from an
extrapolation
of
the
rate
of
occurrence
of
smaller
earthquakes
 The incompleteness of earthquake catalogues, as some
hystorical events may not have been registered
 The fact that some active faults may not have been identified,
specially faults on the oceanic crust (fault trace under the
sea) or faults that don´t reach the Earth surface
Propagation of seismic waves
Seismic waves propagate from the focus of the
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earthquake in a way similar to waves on a lake if we
throw a stone on the water: the amplitude of the waves
atenuates as the distance to the source increases.
There are two main sources of attenuation:
Geometric attenuation
Damping at the earth crust
Geometric attenuation
– as the distance
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to the epicenter increases, the energy of the waves is
distributed by a greater surface, therefore the amplitude
decreases.
Damping – as the waves travel by the Earth crust,
the friction between solid particles that displace relatively
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to each other, reduces the total energy of the seismic
waves, decreasing their amplitude. This attenuation
effect is stronger on the high frequencies. Therefore at
larger distances from the epicenter the frequency
contents of the seismic waves changes (becoming richer
in low frequencies) as compared with the frequency
contents near the epicenter (richer in high frequencies).
Attenuation relationships
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Relationship between a ground motion descriptor,
such as PGA (Peak Ground Acceleration), Spectral
acceleration at a given period, Intensity, Arias
Intensity, and the distance to the fault or epicenter
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Teves Costa et al, 2002
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Sabetta F, Pugliese A. Estimation of response spectra and simulation of nonstationary
earthquake ground motions. Bull Seismol Soc Am. 1996;86(2):337-52.
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INCERTAINTIES
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Attenuation relationships have very large uncertainties
of two types:
- Due to the inherent scatter of ground motion data
- Due to the simplified and empirical nature of the
models
Site effects
It refers to the effects of the most superficial soil layers
on the amplitudes and frequency contents of the seismic
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waves that propagate from the bed rock to the surface.
The soil behaves like a multidegree of freedom
oscillator, that increases or decreases the amplitudes of
some frequencies
Transform a function in a sum of synusoidal functions.
Fourier series
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Dynamic amplification.
Ressonance
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Changes in
seismic waves
due to site
effects, recorded
in Ashigara
Valley (Japan)
during an M=5.1
earthquake
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Reiter, 1991
Map of seismic
hazard in Italy
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Maximum horizontal
acceleration in soil
type A, with a
probability of
exceedence of 10%
in 50 years
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http://www.seismo.ethz.ch/GSHAP/
Definition of the seismic action
for structural applications
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Probability of exceedance in 50 years for current
buildings: 10% (EC 8)
Most common formats: response spectra or set of
accelerograms
Se(T)= ag . S.2,5. 
Sd(T)= ag . S.2,5/q
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𝑎𝑔 = 𝑎𝑔𝑅 × γ𝐼
𝑎𝑔𝑅 - peak horizontal acceleration on the bed rock (soil type A)
γ𝐼 - Importance factor
𝑆 - soil factor
 - Factor that accounts for the fact that the damping coefficient is
different of 5%
q - behaviour factor (q factor in EC 8 terminology)
Soil types
in EC 8
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The soil factor S
depends on the
stiffness of the
soil. The strength
of the soil and
topographic
effects may also
be accounted for
Vs is preferable to NSPT
Response spectra for Lisbon, for far and near field events, and two types of
soil, A and D
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The choice of the probability of collapse of a structure, that
can not be zero, is a political choice, not a technical one. It
depends on how much risk society is prepared to accept and
how much is it willing to pay to reduce it. It depends on
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cultural, hystorical and economic factors.
It is generally accepted that some structures are more
important than average (hospitals, scools, government
buildings, lifeline facilities) and others are less important (for
instances agricultural facilities whose collapse is unlikely to
cause human casualties). Therefore for these structures
different probabilities of collapse are required/accepted.
Alternatively, different lifetimes for these structures can be
considered.
The above is reflected in EC 8, in the Importance
factors I, that multiply the reference seismic action,
applicable to common housing and office buildings
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Portuguese National Annex of EC 8, Part 1, 2009
Seismicity
descriptor,
studies
for
often
express
an
instances
the
peak
earthquake
horizontal
acceleration on stiff soil, associated to a given type of
earthquake, as a function of the
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return period,
(average period of time between earthquakes).
TR,
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It is therefore usefull to relate the return period with the
probability of exceedance of earthquakes with certain
characteristics, for instances PGA>a, during given
periods of time (usually the lifetime, n years)
TR=1/P1  P1=1/TR
P1 – anual probability of occurrence of earthquakes of
certain characteristics (for instances PGA > a)
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1-P1=(1-1/TR) – anual probability of not occurring
earthquakes with certain
characteristics
(1-1/TR)n – probability of not occuring earthquakes with
certain characteristics in n years
Pn=1- (1-1/TR)n – probability of occuring earthquakes
with certain characteristics in n
years
Probabilidade anual de excedência (%)
10.00%
1.00%
0.10%
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0.01%
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
PGA (g) 0.8
Therefore, structures of higher Class of Importance
can be assigned lower probabilities of collapse (Pn) or
higher lifetimes (n years), to which will correspond
higher values of PGA on stiff soil, agR. The ratio of this
value by the value of agR of the standard Class of
Importance (II) is the respective Importance factor.
Quantification of other seismic effects
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Displacements between fault faces
at or near the Earth surface
(i)
the identification of faults traces can be done by
identification
of
epicenters
of
previous
earthquakes and/or direct observation
(ii)
the potencial displacement between fault faces
can be done using geological evidence and
information on the effects of previous earthquakes
Landslides – stability of slopes can be assessed by
analysing models with information on the topography, soil
characteristics and water contents. At a large scale it is
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also possible to estimate the potencial for landslides from
information
of
past
occurrences,
photogrametric techniques
namely
using
Liquefaction
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certain
– it may occur in saturated soil with
percentages
of
sand.
The
potencial
for
liquefaction can be analysed by means of dynamic tests
of the soils, involving cyclic changes in the loading
pattern at certain rates, leading to the evaluation of
stiffness and strength degradation
Testing equipment for dynamic soil testing, and results
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Tsunamis –
detection of tsunamis is done
essencially by means of instrumentation on solid
ground, that detects others consequences (vibrations)
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of
the
same
event
that
triggers
the
tsunami
(earthquake, the more frequent cause of tsunami, or
underwater landslide). Equipment on the sea is more
expensive, has high maintenance costs and very little
durability. The amplitude of a movement at the sea bed
necessary to trigger a tsunami can be evaluated based
on the location of the epicenter, local geology,
magnitude of the earthquake and models of tsunami
propagation.
Model of tsunami propagation for the repetition of the 1755 Lisbon earthquake
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