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GEO1011
Chap. 19 : Earthquakes
Chap 19: Earthquakes
• What is an earthquake and its relation to
plate tectonics
• The seismic waves
• How to locate an earthquake
• The sizes of an earthquake and how to
measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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Chap 19: Earthquakes
• What is an earthquake and its relation to
plate tectonics
• The seismic waves
• How to locate an earthquake
• The sizes of an earthquake and how to
measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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Earthquakes in subduction zones
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Earthquakes in continental regions
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• Earthquakes occur in the cold, brittle parts
of the Earth:
• the upper part (upper crust and upper part
of the upper mantle)
• the subducted lithosphere
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The theory of the elastic rebound
Forces associated with plate motion act on
plates, but friction inhibits motion until a given
stress is reached. Then, slip occurs suddenly.
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Friction in the fault plane
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Cycles of the elastic rebound
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Description of a fault plane
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Three angles to characterize a fault plane
and its slip
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• Normal faults in extension regions like on
mid-oceanic ridges, graben structures
• Reverse faults in regions under
compression, like subduction zones
• Strike-slip faults along transform faults or
in regions with shear
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Plate Boundaries
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Trace of the Fuyun earthquake (Mongolia)
Fault trace 60 years after an M=8 earthquake
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Lamia fault, Greece.
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Strike-slip earthquake in Landers (California)
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Surface traces of faults after erosion
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Most fault systems are complex
The North-Anatolian fault close to Istanbul
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The tectonic setting of the North-Anatolian
fault
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Focus: where the slip starts at depth
Epicenter: its projection on the
surface
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The rupture propagates along the fault plane
at a velocity of about 3km/s. The rupture lasts
a few seconds for moderate earthquakes.
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Dimensions of earthquake fault planes:
• largest dimensions: 1000km (Chile 1960)
• smallest: no lower limit. Any small crack is
an earthquake. Thrust Fault Example
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Thrust Fault Example
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Chap 19: Earthquakes
• What is an earthquake and its relation to
plate tectonics
• The seismic waves
• How to locate an earthquake
• The sizes of an earthquake and how to
measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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Seismic waves
Distinguish between the earthquake itself
(some motion on a fault) and the vibrations
that this sudden motion generates in the
surrounding media: the seismic waves.
Destruction come from the seismic waves
associated with the earthquake.
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• Seismic waves = vibrations
• Equivalent to sound waves in the air or
waves in the water.
The earthquake is the stone you throw in
the water.
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Seismic waves produced by earthquakes
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The waves propagate away from the
earthquake, also called source
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• Seismic waves propagate at velocities of a
few km/s: much faster than water waves
or sound waves in the air, for which the
velocity is 0.3km/s.
• At a few km from an explosion, the ground
vibration will arrive before the sound.
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• In the air or in fluids, we have pressure
waves only. In queues also.
• In solids, we have pressure and shear
waves:
http://www.whfreeman.com/understandingea
rth
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The periods of these waves:
from around 0.01s (local earthquakes)
to 53 mn (maximum on Earth)
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• How are these waves registered?
They are registered by seismographs.
You have different types of seismographs:
• Short-period: for rapid vibrations
• Long period: for slow vibrations
• Broadband: for all vibrations
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The principle of a seismograph:
a damped pendulum.
weight which
can oscillate
recording
system
+ clock
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Long period electromagnetic seismographs
at ATD (Djibouti)
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The entrance to the ATD station (Djibouti)
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The electronic equipment at ATD:
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The entrance of the tunnel to the KIP station
(Hawai)
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Seismological
stations in
Norway
+ one in the
basement of the
department
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Chap 19: Earthquakes
• What is an earthquake and its relation to
plate tectonics
• The seismic waves
• How to locate an earthquake
• The sizes of an earthquake and how to
measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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Seismic waves produced by earthquakes
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• Velocities of waves:
P waves: about 5.6 km/s in the crust (first
few tens of km in the Earth)
S waves: about 3.4 km/s in the crust
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We can read the arrival time of the P wave tp.
If we knew the origin time of the earthquake t0, we
could write:
tp = t0 + d / Vp
which implies for the distance:
d = Vp*(tp – t0)
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The arrival times of the P and S waves are:
tp = t0 + d / Vp
ts = t0 + d / Vs
which implies: ts – tp = d / Vs – d / Vp
= d ( 1/Vs -1/Vp )
= d (Vp-Vs)/(VsVp)
This gives:
d = (ts - tp) Vs Vp / (Vp – Vs)
or about d = 8 (ts-tp) for d in km and t in s
and local earthquakes
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Wave paths for regional earthquakes
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• Wave propagation for distant earthquakes
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Main layers in the Earth
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P
S
P
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Wavepaths for distant earthquakes
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Paths of S waves in the mantle and in the
core
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Note the time scale:
long-period instruments
are required to register
these waves.
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Surface waves: late, long-period and large
amplitude waves
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Surface waves: late, long-period and large
amplitude waves
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R1
R2
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Surface waves: late, long-period and large
amplitude waves
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Chap 19: Earthquakes
• What is an earthquake and its relation to
plate tectonics
• The seismic waves
• How to locate an earthquake
• The sizes of an earthquake and how to
measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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• The magnitude(s) measure the amplitude
of the seismic waves and the energy of the
earthquake.
• The intensity measures the destructions
related to the earthquake.
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The Richter magnitude of local earthquakes
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• The amplitude of the ground displacement
increases by a factor of 10 each time the
magnitude increases by 1.
• The energy increases by a factor of about
33 for a step of 1 in magnitude.
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• ML for local earthquakes (Richter
magnitude adapted to local structure)
• Mb, Ms: measured on P waves or surface
waves for distant earthquakes
• Moment magnitude Mw related to the
seismic moment M0: a more accurate
measurement which tells something about
the total energy of the earthquake
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The seismic moment M0
M0 = μ S d
μ is the rigidity around the fault zone
S is the surface of rupture
d is the length of slip along the fault plane
We make a magnitude Mw out of it.
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Bigger Faults Make Bigger Earthquakes
Kilometers
1000
100
10
1
5.5
6
6.5
7
Magnitude
7.5
8
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Bigger Earthquakes Last a Longer Time
Seconds
100
10
1
5.5
6
6.5
7
7.5
8
Magnitude
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Earthquakes in Norway between the 4th and
11th of November 2004
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• The intensity: a location dependent
measurement of the destructions caused
by an earthquake.
• From I (not felt) to XII (total destruction).
• Based on field observations and
questionnaires.
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ShakeMaps
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Chap19: Earthquakes
• What is an earthquake and its relation to
plate tectonics
• The seismic waves
• How to locate an earthquake
• The sizes of an earthquake and how to
measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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• Can we predict earthquakes?
At long term: partly, at least along plate
margins.
At intermediate term: some recent results
based on stress measurements and
calculations
At short term: no.
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Long-term prediction based on the theory of
the elastic rebound
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Cycles of the elastic rebound
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Seismic gaps at present time
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• Intermediate-term prediction: based on
stress redistribution after an earthquake.
Which fault is the next one to break in a
complex fault system?
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The North-Anatolian fault system close to
Istanbul
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• Short-term prediction: not possible yet
Therefore, we have to take earthquake risk
into account when we build.
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• What is an earthquake and its relation to
plate tectonics
• The seismic waves
• How to locate an earthquake
• The sizes of an earthquake and how to
measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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The seismic hazard
• Measure how frequent and how strong are
earthquakes in a given region
The earthquakes have been recorded for
only one century. Too short time period to
give a good image of the frequency of
large earthquakes in many regions.
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For regions without strong recent earthquakes,
it is possible to use the number of small
earthquakes to evaluate how often we get a large
one.
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It is also possible to study the traces of
very old earthquakes in sediments.
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Definition of seismic hazard:
10% probability of acceleration larger than …
within 50 years.
But the wave period is important also…
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• Then you need to take into account local effects
like amplification in sediments to get more
detailed maps which can be used for city
planning for example.
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The seismic risk
In a deserted area, it doesn’t matter if there are
strong earthquakes.
In a region with a dam or a nuclear power plant,
even a small earthquake can be a catastrophe.
The seismic risk takes into account the type of
building etc in the area in addition to the
vibrations caused by earthquakes.
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• The only way to prevent damage from
earthquakes at the present time is to build
according to special rules called the
seismic code.
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Origin of damages by earthquakes
• Direct: ground shaking
• More indirect: landslides, sediment liquefaction,
tsumanis
• Indirect: fire, water contamination, disease
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What an earth vibration does to a building?
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Indirect effects:
• Landslides and avalanches
• Sediment liquefaction
• Tsunamis
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Tsunamis
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Tsunami propagation across the pacific
Ocean
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Lisbon earthquake, Nov 1.,1755.
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• Exercices on the web-page of the course
for next week.
• This presentation on the web-page also.
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