Transcript Earthquake Hazard

Earthquake Hazard
Session 1
Mr. James Daniell
Risk Analysis
Earthquake Risk Analysis
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Learning Objectives
 Develop an understanding of basic earthquake
processes
 From the source, via a path, to the site
 Explore seismic principles such as waves and other
concepts in terms of hazard
 Know about ground motion, site effects
 Know how seismic hazard is integrated into loss models
 Know the difference between probabilistic and
deterministic models
 Apply hazard assessment to a real situation
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Plate Tectonics
Tectonic plates are moving relative to
each other via a process called plate
tectonics
Plate Tectonics, mantle, crust, subduction
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Seismic Hazards
 Most earthquakes occur on faults –
areas of broken and displaced rocks.
 There are 3 main types of faults:
Strike-Slip, Reverse (compression) and
Normal (tension).
 Earthquakes occur in both oceanic and
continental crust at varying depths.
Strike-Slip
Reverse
Normal
Faults, oceanic and continental crust, reverse, normal, strike-slip
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How do earthquakes occur?
 Built up energy, stress and strain
release on plate boundaries and
faults, via deformed rock vibrating.
 Initial earthquake rupture occurs
on the fault at the hypocentre (also
known as focus).
 Movement (slip) occurs along the
whole fault plane with rapid energy
release in all directions.
Hypocentre, epicentre, slip, fault plane, rupture, elastic rebound
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What happens when the fault slips?
 This energy release sends
seismic waves into the earth.
 These waves have a certain
wavelength (called period) and
a certain amplitude.
 From the source, the waves
travel along the path, and
arrive at the earth’s surface
(i.e. at the site). The amplitude
and period of these waves are
measured and called the
ground motion. This is
measured by a seismometer.
Seismic waves, period, amplitude, path, site, ground motion, seismometer
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Seismic Waves
 There are 2 types of waves – body and surface.
 Body waves travel through the earth (primary waves=P waves with a pushpull action, and secondary waves=S waves with an up-down action).
 Surface waves have both horizontal and vertical motion, have a high
amplitude and cause the most damage close to the epicenter.
 Waves are faster in rock and slower in loose soils. The amplitude of the
wave is increased in loose soils.
L. Braile, 2005.
SDSU, 2009.
Body waves, P and S waves, surface waves.
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Duration and EQ Records
 The longer the duration of earthquake shaking, in general the
greater the damage caused by an earthquake.
 Measurement can occur at seismic stations by printing out the
movement (ground motion) at a certain location – called an
earthquake record or an accelerogram.
Duration, earthquake record, accelerogram.
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Size and Distance of Earthquakes
 To measure an earthquake, we use magnitude or intensity.
Intensity
Magnitude
Measured from human damage
reports.
Measured from wave amplitude or
energy release.
Simple and easy measurement of
earthquake effects.
Logarithmic; for a 1.0 increase: 10x
ground motion & 32x energy release.
Not good for physical size of
earthquake – more useful for risk.
Does not always correlate with
damage and many different methods.
 The distance away from an earthquake
source reduces the ground
motion/shaking called attenuation.
 In areas where many earthquakes
occur, this attenuation is stronger than
in areas where few earthquakes occur.
Magnitude, logarithm, intensity, attenuation
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How do we measure ground motion?
 Ground motion is measured using
accelerometers/seismometers (spectra-based)
or as human impact indices (damage-based).
 Damage-based indices include MMI which is a
12 class system ranging from no damage to
complete destruction based on qualitative
measurement of people’s perception of
damage at a location. Other similar scales
include MSK (Russia), JMA (Japan), EMS
(Europe) and Ross-Forel.
damage-based scale, MMI, MSK, JMA, EMS, Ross-Forel.
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How do we measure ground motion?
 Spectra-based indices include
measurements of acceleration,
velocity and displacement.
 PGA (Peak Ground
Acceleration) is the maximum
amplitude of ground
acceleration (measured in
m/s2)
 Spectral Acceleration (Sa) is the
ground motion as measured at
different periods to find the
peaks in ground motion. Waves
travel at both short and long
periods.
PGA, Spectral Acceleration
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Some other effects on Ground Motion
 Site soil structure impacts on
ground motion. For soft soils, clays
etc., increase the shaking of the
ground (ground motion
amplification) during an
earthquake.
 Topography also impacts on the
ground motion; valleys with deep
soil layers increase ground motions.
 Direction of fault rupture can also
focus earthquake energy, resulting
in greater shaking at a certain
location (directivity).
 Shakemaps show differences in
ground motion on maps.
Amplification, topography, directivity, Shakemaps
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Secondary Hazards
There are 5 main sources of secondary hazard due to
earthquakes:
Fire
Liquefaction
Fault Rupture
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Landslides
Tsunami
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Earthquake Sequences
 An earthquake can be a single event or can contain a foreshock or aftershock
sequences if the fault needs to release more energy.
Note: tree is only to indicate ground level. NOT TO SCALE.
 In geologic time (millions of years), earthquakes occur periodically on a fault to
release energy. Some faults will have a major energy release every few hundred
years on a certain rupture length, whereas some may have earthquakes every
million years.
Foreshock, mainshock, aftershock, sequence, geologic time.
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Earthquake Catalogues
 Earthquakes appear to follow a pattern through time in terms of no.
of earthquakes vs. magnitude.
 More smaller magnitude earthquakes occur than larger magnitude
earthquakes worldwide.
 At any location, predictive
earthquake catalogues can be
produced to give people an idea
of how often a certain magnitude
earthquake or ground motion will
be exceeded (rate of exceedance).
magnitude-frequency, earthquake catalogue, rate of exceedance
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How is hazard shown in Building Codes?
 Generally, buildings should be
designed for a certain additional
earthquake loading based on this
hazard catalogue.
 Under low probability events, the
infrastructure should not collapse
and under frequent events, minor
damage.
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Zone V – 0.36g (Intensity IX)
Zone IV – 0.24g (Int. VIII)
Zone III – 0.16g (Int. VII)
Zone II – 0.10g (Int. VI or less)
Earthquake zone, building code
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Ground Motion Prediction Equations (GMPEs)
 GMPEs or attenuation relations predict the ground motion at sites of interest
for a scenario earthquake using existing or simulated earthquake data.
 A GMPE is of the following form:
log[Sa(T)] = median fn(M,R,T,V) + uncertainties,
where the spectral acceleration (Sa) can be calculated for given periods (T)
 Many models exist around the world
 When choosing a GMPE, check for:
1) Tectonic regime (stable or active)
2) Fault type (normal, subduction etc.)
3) Distance
4) Magnitude
5) Geology/Geotechnical Information
6) Location
7) Ground Motion Parameter.
GMPE, attenuation relation, tectonic region, uncertainty
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Deterministic Seismic Hazard Assessment (DSHA)
 A DSHA is undertaken to calculate the probability of ground motions for a
single scenario earthquake, (historical, worst case or otherwise).
 Useful for emergency planning, seismic risk awareness, simple assessment or
high-risk facilities.
 There are three main steps:
1) Define all the possible sources to cause
significant hazard at a site using historic data.
2) Choose a fixed distance, fixed magnitude
earthquake and place it on the closest position
to the site on each source.
3) Estimate ground motions via GMPEs to
determine the ground motions at the site in
terms of PGA, MMI, Sa or other measures.
 Variability in ground motions can be modeled
within a DSHA but not extensively.
Deterministic Seismic Hazard Assessment
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Probabilistic Seismic Hazard Assessment (PSHA)
 A PSHA calculates the probability of exceeding all levels of ground shaking
at a certain location (all different earthquake scenarios)
 This is useful for reinsurance/insurance purposes (annual premiums),
design code exceedance and government hazard.
 It consists of the following steps:
1) Collect data on tectonics & geology
2) Compile an earthquake catalogue for
the region
3) Define seismic sources zones
4) Determine magnitude-frequency
relationships
5) Select an appropriate set of GMPEs
6) Calculate probability of each level of
acceleration
7) Construct hazard curves and maps
Probabilistic Seismic Hazard Assessment
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