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GEO1011 Chap. 19 : Earthquakes
GEO1011
Chap. 19 : Earthquakes
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Chap 19: Earthquakes
• What is an earthquake and its relation to plate tectonics
• The seismic waves
• Location and focal mechanism
• 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
• Location and focal mechanism
• The sizes of an earthquake and how to measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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Plategrenser og kildedyp
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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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JordskjelvsyklusTektoniske krefter deformasjon spenningskrefter jordskjelv
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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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Izmit, Tyrkia
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Strike-slip earthquake in Landers (California)
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Surface traces of faults after erosion
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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
• Location and focal mechanism
• The sizes of an earthquake and how to measure them
• Earthquake prediction
• Seismic hazard and seismic risk
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The waves propagate away from the earthquake, also called source
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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 comes 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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• 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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VolumbølgerP-bølger (øverst) og S-bølger (nederst)
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OverflatebølgerRayleigh øverst, Love nederst
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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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• 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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• 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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Prinsippet for registrering av jordskjelvbølgerVertikal- (øverst) og horisontalbevegelse
+ clock
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Long period electromagnetic seismographs at ATD (Djibouti)
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The electronic equipment at ATD:
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The entrance to the ATD station (Djibouti)
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Globalt nettverk av seismologiske målestasjoner
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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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Strålebaner og gangtider for P-bølger
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Jordskjelv øst for Borneo Ufiltrerte data hyposenterdyp: 30km, styrke: 7,1 Registrert på Kerguelen-øyene i det Indiske hav
vertikal
N - S
Ø - V
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Filtrerte data
vertikal
N - S
Ø - V
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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
• Location and focal mechanism
• 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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Hvordan bestemme kildemekanismen:
Blå piler indikerer sammenpressing av materialet mot kilden.Røde piler indikerer at materialet ”strekkes” bort fra kilden (tensjon).
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Relation between tectonic stress and P-wave first motion
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Compression Rupture
P-wavesemitted
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Relation between displacement on the fault and P-wave first motion
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Outward first motionInward first motion
Outward first motion Inward first motion
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P-wave first motion at a distant station
The first motion around the source is
transmitted to the stations:
We have upward first motion at some
stations, and downward first motion at
some other stations
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Forkastningstyper og spenningsforhold
Hvite felt = sammenpressing, svarte felt = tensjon
Maks. kompresjon
Min. kompresjon
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The North-Anatolian fault system close to Istanbul
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Chap 19: Earthquakes
• What is an earthquake and its relation to plate tectonics
• The seismic waves
• Location and focal mechanism
• 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
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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
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10
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5.5 6 6.5 7 7.5 8
Magnitude
Sec
onds
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Totalt antall jordskjelv pr. år som funksjonav Richter-styrke og energiinnhold
Sumatra-skjelv
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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
• Location and focal mechanism
• 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 tectonic setting of the North-Anatolian fault
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Jordskjelvsyklus langs det Nord-Anatoliske forkastningssytem
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Intermediate term prediction
• Animations showing how the deformation concentrates at the tip of the fault planes, and how this works in Turkey.
http://quake.wr.usgs.gov/research/deformation/modeling/animations
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Teoretisk beregnet spenningsregime(publisert i 1998)
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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
• Location and focal mechanism
• 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
Earthquakes have been recorded for
one century. This is a too short time period to give a good idea 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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Relation between number of small earthquakes and large earthquakes
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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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Formation of 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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Tsunami of 1755.
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