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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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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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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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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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R1R2

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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







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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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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.

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