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Transcript
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
2
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
3
4
5
Earthquakes in subduction zones
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
Description of a fault plane
12
Three angles to characterize a fault plane
and its slip
13
14
• 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
15
Plate Boundaries
16
17
Trace of the Fuyun earthquake (Mongolia)
Fault trace 60 years after an M=8 earthquake
18
Lamia fault, Greece.
19
Strike-slip earthquake in Landers (California)
20
Surface traces of faults after erosion
21
Most fault systems are complex
The North-Anatolian fault close to Istanbul
22
The tectonic setting of the North-Anatolian
fault
23
Focus: where the slip starts at depth
Epicenter: its projection on the
surface
24
The rupture propagates along the fault plane
at a velocity of about 3km/s. The rupture lasts
a few seconds for moderate earthquakes.
25
Dimensions of earthquake fault planes:
• largest dimensions: 1000km (Chile 1960)
• smallest: no lower limit. Any small crack is
an earthquake. Thrust Fault Example
26
Thrust Fault Example
27
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
28
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.
29
• 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.
30
Seismic waves produced by earthquakes
31
The waves propagate away from the
earthquake, also called source
32
• 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.
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
35
The periods of these waves:
from around 0.01s (local earthquakes)
to 53 mn (maximum on Earth)
36
• 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
37
The principle of a seismograph:
a damped pendulum.
weight which
can oscillate
recording
system
+ clock
38
Long period electromagnetic seismographs
at ATD (Djibouti)
39
The entrance to the ATD station (Djibouti)
40
The electronic equipment at ATD:
41
The entrance of the tunnel to the KIP station
(Hawai)
42
Seismological
stations in
Norway
+ one in the
basement of the
department
43
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
44
Seismic waves produced by earthquakes
45
• 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
46
47
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)
48
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
49
50
Wave paths for regional earthquakes
51
• Wave propagation for distant earthquakes
52
Main layers in the Earth
53
P
S
P
54
Wavepaths for distant earthquakes
55
Paths of S waves in the mantle and in the
core
56
57
58
59
Note the time scale:
long-period instruments
are required to register
these waves.
60
Surface waves: late, long-period and large
amplitude waves
61
62
Surface waves: late, long-period and large
amplitude waves
63
R1
R2
64
Surface waves: late, long-period and large
amplitude waves
65
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
66
• 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.
67
The Richter magnitude of local earthquakes
68
• 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.
69
• 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
70
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.
71
Bigger Faults Make Bigger Earthquakes
Kilometers
1000
100
10
1
5.5
6
6.5
7
Magnitude
7.5
8
72
Bigger Earthquakes Last a Longer Time
Seconds
100
10
1
5.5
6
6.5
7
7.5
8
Magnitude
73
74
Earthquakes in Norway between the 4th and
11th of November 2004
75
76
• 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.
77
ShakeMaps
78
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
79
• 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.
80
Long-term prediction based on the theory of
the elastic rebound
81
Cycles of the elastic rebound
82
Seismic gaps at present time
83
• Intermediate-term prediction: based on
stress redistribution after an earthquake.
Which fault is the next one to break in a
complex fault system?
84
The North-Anatolian fault system close to
Istanbul
85
86
87
88
• Short-term prediction: not possible yet
Therefore, we have to take earthquake risk
into account when we build.
89
• 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
90
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.
91
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.
92
It is also possible to study the traces of
very old earthquakes in sediments.
93
Definition of seismic hazard:
10% probability of acceleration larger than …
within 50 years.
But the wave period is important also…
94
95
96
• 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.
97
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.
98
• The only way to prevent damage from
earthquakes at the present time is to build
according to special rules called the
seismic code.
99
Origin of damages by earthquakes
• Direct: ground shaking
• More indirect: landslides, sediment liquefaction,
tsumanis
• Indirect: fire, water contamination, disease
100
101
What an earth vibration does to a building?
102
103
104
Indirect effects:
• Landslides and avalanches
• Sediment liquefaction
• Tsunamis
105
106
107
108
Tsunamis
109
Tsunami propagation across the pacific
Ocean
110
Lisbon earthquake, Nov 1.,1755.
111
• Exercices on the web-page of the course
for next week.
• This presentation on the web-page also.
112