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GEO1011
Chap. 19 : Earthquakes
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
2
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
3
4
5
Plategrenser og kildedyp
6
Earthquakes in continental regions
7
• 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
8
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.
9
Friction in the fault plane
10
Cycles of the elastic rebound
11
Jordskjelvsyklus
Tektoniske krefter
deformasjon
spenningskrefter
jordskjelv
12
Description of a fault plane
13
Three angles to characterize a fault plane
and its slip
14
15
• 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
16
Plate Boundaries
17
18
Trace of the Fuyun earthquake (Mongolia)
Fault trace 60 years after an M=8 earthquake
19
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
23
The tectonic setting of the North-Anatolian
fault
24
Focus: where the slip starts at depth
Epicenter: its projection on the
surface
25
The rupture propagates along the fault plane
at a velocity of about 3km/s. The rupture lasts
a few seconds for moderate earthquakes.
26
Dimensions of earthquake fault planes:
• largest dimensions: 1000km (Chile 1960)
• smallest: no lower limit. Any small crack is
an earthquake. Thrust Fault Example
27
Thrust Fault Example
28
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
29
30
The waves propagate away from the
earthquake, also called source
31
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.
32
• 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.
33
• 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
34
Volumbølger
P-bølger (øverst) og S-bølger (nederst)
35
Overflatebølger
Rayleigh øverst, Love nederst
36
The periods of these waves:
from around 0.01s (local earthquakes)
to 53 mn (maximum on Earth)
37
• 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.
38
• 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
39
Prinsippet for registrering av jordskjelvbølger
Vertikal- (øverst) og horisontalbevegelse
+ clock
40
Long period electromagnetic seismographs
at ATD (Djibouti)
41
The electronic equipment at ATD:
42
The entrance to the ATD station (Djibouti)
43
Globalt nettverk av seismologiske målestasjoner
44
45
Wave paths for regional earthquakes
46
• Wave propagation for distant earthquakes
47
Main layers in the Earth
48
P
S
P
49
Wavepaths for distant earthquakes
50
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
54
Filtrerte data
vertikal
N-S
Ø-V
55
56
Surface waves: late, long-period and large
amplitude waves
57
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Surface waves: late, long-period and large
amplitude waves
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R1
R2
60
Surface waves: late, long-period and large
amplitude waves
61
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
62
Seismic waves produced by earthquakes
63
• 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
64
65
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)
66
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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68
Hvordan bestemme kildemekanismen:
Blå piler indikerer sammenpressing av materialet mot kilden.
Røde piler indikerer at materialet ”strekkes” bort fra kilden (tensjon).
69
Relation between tectonic stress
and P-wave first motion
Compression
Rupture
P-waves
. emitted
70
Relation between displacement on the fault
and P-wave first motion
Inward first motion
Outward 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
79
• 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.
80
The Richter magnitude of local earthquakes
81
• 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.
82
• 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
83
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.
84
Bigger Faults Make Bigger Earthquakes
Kilometers
1000
100
10
1
5.5
6
6.5
7
Magnitude
7.5
8
85
Bigger Earthquakes Last a Longer Time
Seconds
100
10
1
5.5
6
6.5
7
7.5
8
Magnitude
86
Totalt antall jordskjelv pr. år som funksjon
av 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.
90
ShakeMaps
91
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
92
• 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.
93
Long-term prediction based on the theory of
the elastic rebound
94
Cycles of the elastic rebound
95
Seismic gaps at present time
96
• Intermediate-term prediction: based on
stress redistribution after an earthquake.
Which fault is the next one to break in a
complex fault system?
97
The tectonic setting of the North-Anatolian
fault
98
Jordskjelvsyklus langs det Nord-Anatoliske
forkastningssytem
99
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/deformati
on/modeling/animations
100
Teoretisk beregnet spenningsregime
(publisert i 1998)
101
• Short-term prediction: not possible yet
Therefore, we have to take earthquake risk
into account when we build.
102
• 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
103
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.
104
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.
105
Relation between number of small earthquakes
and large earthquakes
106
It is also possible to study the traces of
very old earthquakes in sediments.
107
Definition of seismic hazard:
10% probability of acceleration larger than …
within 50 years.
But the wave period is important also…
108
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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.
111
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.
112
• The only way to prevent damage from
earthquakes at the present time is to build
according to special rules called the
seismic code.
113
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
123
Tsunamis
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Tsunami propagation across the pacific
Ocean
126
Lisbon earthquake, Nov 1.,1755.
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Tsunami of 1755.
128
• Presentasjonen ligger på web-siden.
• Oppgavene ligger på web-siden.
• Ta med linjal, passer og lommekalkulator
.
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