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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. Chap 19: Earthquakes. - PowerPoint PPT PresentationTRANSCRIPT
GEO1011
Chap. 19 : Earthquakes
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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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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 onplates, but friction inhibits motion until a givenstress 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 planeat a velocity of about 3km/s. The rupture lastsa 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/understandingearth
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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.
+ clock
weight whichcan oscillate
recording system
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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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+ one in thebasement of thedepartment
Seismologicalstations in Norway
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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
P
S
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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 instrumentsare required to registerthese 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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R1R2
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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 EarthquakesBigger Faults Make Bigger Earthquakes
1
10
100
1000
5.5 6 6.5 7 7.5Magnitude
Kilo
me
ters
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Bigger Earthquakes Last a Longer TimeBigger Earthquakes Last a Longer Time
1
10
100
5.5 6 6.5 7 7.5 8
Magnitude
Sec
onds
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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 largeone.
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It is also possible to study the traces ofvery 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.