Earthquakes and tsunamis

Earthquakes and tsunamis
Szichuan (China)
Northridge (California, 1994)
Kobe earthquake (Japan 1995)
Loma Prieta (California, 1989)
soil liquefaction
Most devastating earthquakes
Date
Location
Deaths
Magnitude
1556
Shensi (China)
830,000
8
1976
Tangshan (China)
255,000
7.5
1138
Aleppo (Syria)
230,000
?
2004
Sumatra
228,000
8.4-9.1
1920
Ningshia (China)
200,000
7.8
856
Damghan (Iran)
200,000
?
893
Ardabil (Iran)
150,000
?
1920
Kanto (Japan)
140,000
7.9
Some numbers
Number/year
M>8
1
7 < M <8
17
6 < M <7
134
5<M<6
1,300
4<M<5
13,000
3<M<4
130,000
dMagnitude dmotion
dE
1
x 32
x 10
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Geographic distribution of seismicity
What is an earthquake?
Stress and rupture
Stereographic projections
Representation of earthquake focal mechanisms
Tectonic applications
Seismic risk
Sumatra earthquake & Tsunami
Ondes sismiques
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Ondes internes: P (de
compression ou
longitudinales) et S (de
cisaillement ou
transversales)
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Ondes de surface (Love,
Rayleigh)
Mouvement de la surface du
sol.
Longueur d’onde: λ
Periode: T
Vitesse de l’onde v = λ /T
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Mouvement est
enregistre par
seismometres.
3 composantes du
mouvement doivent
etre enregistrees pour
decrire les
deplacements du sol.
Mercalli scale
Richter scale
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ML = log10A(mm) +
(Distance correction
factor)
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logES = 4.8 + 1.5M
(radiated seismic energy
in J)
Definitions
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focus
Epicentre
Magnitude (Richter scale)
M= Log10(A/Aref)
Aamplitude du mouvement
mesure par seismometre
standard a 100km de
l’epicentre.
E ~ A2 ?
Moment ~ µ S d
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Aftershock
Foreshock
Energie totale /an = 1018 1019 J/an
Magnitude 9 ~ 1019 J
Earthquake focal mechanism
Mecanisme au foyer du tremblement de terre?
San Andreas fault
Imperial Fault, Ca
Crowley lake, California
1906 SF earthquake
Elastic rebound
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Deformation autour de
la zone de faille
Accumulation d’energie
elastique
Rupture de la faille
Modele simplifie
Description des contraintes
Tenseur des contraintes
Les contraintes ne sont pas normales aux
surfaces. Sur chaque surface, 3
composantes pour le vecteur contrainte.
Il faut connaitre la contrainte dans 3
directions differentes. Il y a donc 9
composantes distinctes au tenseur des
contraintes.
Contraintes principales
Parce que le tenseur est symetrique, il
existe des directions ou la contrainte
est normale a la surface. Ce sont les
directions principales. Ces directions
sont orthogonales entre elles..
Autrement dit, il est possible de
trouver un systeme de reference ou le
tenseur des contraintes est diagonal
Fracture = cisaillement maximum
Contraintes principales et fracturation
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Cisaillemment est
maximum dans les 2
plans definis par
contrainte intermediaire
et direction a 45 deg
entre contraintes max et
min. (cette direction est
celle du mouvement)
2 Plans nodaux
perpendiculaires et deux
vecteurs de mouvement.
Representation des plans nodaux sur la demi-sphere (inferieure)
centree au foyer du tremblement de terre.
Il n’est jamais possible de reconnaitre le
plan de faille du plan nodal conjugue
Observations du premier mouvement des ondes P sont
projetees sur la demi-sphere inferieure centree au foyer.
Objectif: determiner contraintes, plans nodaux et
deplacements possibles
Mecanismes simples
Rifting in Africa shown by focal mechanisms
Subduction
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In general, normal faulting events are the least
powerful
Thrust faulting are the most powerful
Strike-slip in between. Many damaging quakes
due to strike slip (e.g. California, Haiti)
New Madrid quakes 1811-1812
On the basis of the large area of
damage (600,000 square kilometers),
the widespread area of perceptibility
(5,000,000 square kilometers), and the
complex physiographic changes that
occurred, the Mississippi River valley
earthquakes of 1811-1812 rank as
some of the largest in the United
States since its settlement by
Europeans. The area of strong shaking
associated with these shocks is two to
three times larger than that of the
1964 Alaska earthquake and 10 times
larger than that of the 1906 San
Francisco earthquake.
Five earthquakes of magnitude
MSn 8.0 or higher occurred in the
period December 16, 1811
through February 7, 1812.
Est du Canada
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Most earthquakes occur under the St.
Lawrence River, between Charlevoix
County on the north shore and
Kamouraska County on the south shore.
To consider this aspect, the Charlevoix
Seismic Zone (CSZ) is often referred to as
the Charlevoix-Kamouraska Seismic
Zone.
Historically, the zone has been subject to
five earthquakes of magnitude 6 or larger:
in 1663 (Mag. 7); 1791 (Mag. 6); 1860
(Mag. 6); 1870 (Mag. 6 1/2); and 1925
(magnitude MS 6.2 ± 0.3). Since the 1925
was the only event recorded by
seismographs, the previous events have
approximate magnitudes evaluated on felt
areas and damage. Similarly, preinstrumental locations of events are less
precise. Overall, the distribution of
historical and recent events shows an
earthquake concentration between La
Malbaie and Rivière-du-Loup.
Few quakes in the Cascadia subduction zone?
Evidence for M>9 events in the Cascadia
subduction zone
A trench cut through a coastal intertidal marsh
exposes a peat layer, the remains of a former, now
buried, marsh. The marsh abruptly subsided 1/2-1
m in a great earthquake about 300 years ago. The
sand above the buried peat layer was swept into
the subsided coastal region by the waves of the
resulting great tsunami (after Clague and
Bobrowsky, 1994a).
An earthquake of magnitude 9 rocked the Pacific Northwest
on January 26, 1700 around 9:00 PM. A series of
independent discoveries over the past two decades have
narrowed the window of occurrence of this "great quake" to
a precise time. Preliminary evidence consistent with this
date came from carbon dating, tree ring studies, and soil
deposits. Although these lines of evidence enabled
scientists to verify that a great subduction zone earthquake
struck the Pacific Northwest within one or two decades of
1700, the exact date date remained unknown.
Kenji Satake, a researcher from the Geological Survey of
Japan, along with a team of scientists from the University
of Tokyo found Japanese records to tsunami occurrences
along the country's eastern coastline between January 27
and 28, 1700. Careful analysis of these historic tsunami
records indicated that several coastal villages were
damaged. Accounts were recorded in different villages
along Japan's coastline. The following is a chronicle by the
head of Miho village, 145 km southwest of Tokyo. This
account tells of sea water covering land as if it were high
tide.
Induced seismicity
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Reservoir impoundment (most frequent)
Usually, activity decreases with time
Oil pumping
Water pumping
What is to be done?
(Lenin, 1902)
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Construction zoning: Is it enough?
Earthquake prediction?
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•
•
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So far,it is only probabilistic
Many methods proposed
Monitoring of deformation (satellite geodesy)
Changes in physical properties b4 earthquake (Vp
/ Vs, conductivity, porosity -> gaz emission)
Earthquake control? Induced seismicity. Can
we trigger small quakes to avoid the big one?
Tsunamis
The 2004 Sumatra-Andaman
earthquake and tsunami
Fréquence des grands séismes dans la région:
8,4 en 1797
8,5 en 1861
8,7 en 1833
7,9 en 2000
Période de répétition de l’ordre de 250 ans
Vertical-component ground displacements for periods <1000 s observed for the three largest earthquakes of the
past 40 years. The upper trace shows the seismogram from the 26 December 2004 Sumatra-Andaman earthquake
observed 130° away in Pasadena, California, USA; the middle trace is for the 28 March 2005 Sumatra earthquake
observed 131° away in Pasadena, California, USA; the lower trace shows a seismogram for the 23 June 2001 Mw
8.4 earthquake off the coast of Peru, observed 126° away in Charters Towers, Australia.
Map showing aftershock locations
for the first 13 weeks after the 26
December 2004 earthquake from
the NEIC (yellow dots, with radii
proportional to seismic magnitude).
Moment-tensor solutions from the
Harvard CMT catalog (21) are
shown for the 26 December 2004
and 28 March 2005 mainshocks
(large solutions at bottom, with
associated centroid locations) and
aftershocks. Star indicates the
epicenter for the 2004 rupture
obtained by the NEIC. Dashed line
shows the boundary between the
aftershock zones for the two events
A 5 cm/an, 15 mètres représentent la
déformation cumulée sur trois siècles
Energie du séisme M=9.0 soit 20 .1017 Joules
(ou encore 475 Mt TNT
= 23000 fois la bombe d’Hiroshima)
Durée de la rupture 3 à 4 minutes
Fault slip 168s after rupture initiation estimated by using 20 azimuthally distributed teleseismic SH waveforms ( 45° to 85°). The rupture
models consists of two faults, the first having a strike of 329° and a dip of 8° and the second having a strike of 333° and a dip of 7° (based on
the mechanism of the 29 December 2004 MW = 6.0 aftershock). (B) Slip distribution from method II. The reliance on intermediate-period
surface waves and long-period seismograms reduces the detail imaged in the rupture but provides a first-order view of the slip distribution. (C)
Slip distribution of finite fault model III using teleseismic body waves (5 to 200 s), intermediate-period three-component regional waves (50 to
500 s), and long-period teleseismic waves (250 to 2000 s). The surface projections of three fault segments are colored on the basis of the slip
amplitude. The black thick and thin lines delineate the trench mapped from the ETOPO2 and 50-km iso-depth slab contour. The aftershocks
(Ml > 5) downloaded from the National Earthquake Information Center are indicated by black dots. Waveform fits for each model can be
Fig. 2. Gravity changes (in {micro}Gal) after the Sumatra-Andaman earthquake, computed from
averaging and filtering the two gravity changes between two different time periods (Fig
S.-C. Han et al., Science 313, 658 -662 (2006)
Published by AAAS
Tide gauges record after the
earthquake
Plage de Kalutara
(Sri Lanka)
Tsunami wave is a gravity wave
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Velocity depends on g
and on length scale
If λ >>h, h depth of sea
floor determines length
scale
For h=4000m, v =
200m/s =720km/hr
m 1/ 2 m
gh  ( 2 m) 
s
s
Tsunami model at a time of 1 hour 55
min after earthquake initiation,
computed for a composite slip model
with fast slip (50-s rise time) in the
southern portion of the rupture and
slow slip (3500-s rise time) in the
north. The northward propagating
rupture velocity is about 2 km/s for the
first 745 km, then slows to 750 m/s.
The amplitude of fast and slow slip on
the six fault segments are indicated by
white numbers and outlined numbers,
respectively. The overall seismic
moment of 8.8 x 1022 Nm (µ = 3.0 x
1010 N/m2) is divided fairly evenly
between slow and fast contributions.
Red colors in the map indicate
positive ocean wave height, blue
colors negative. The numbers along
the wavefront give wave amplitudes in
meters. Diagonal line is the track of
the Jason satellite that passed over the
region at about this time (10 min of
actual transit time along the profile).
The predicted (blue) and observed
(red) tsunami wave are shown in the
inset. The tsunami generated by the
fast component of slip alone cannot
explain the trough in the central Bay
of Bengal
La vague détectée par les satellites altimétriques
(amplitudes anormales le long du profil du 26/12/04)
Global chart showing energy propagation of the 2004 Sumatra tsunami calculated from MOST. Filled colors
show maximum computed tsunami heights during 44 hours of wave propagation simulation. Contours show
computed arrival time of tsunami waves. Circles denote the locations and amplitudes of tsunami waves in three
range categories for selected tide-gauge stations. Inset shows fault geometry of the model source and close-up of
the computed wave heights in the Bay of Bengal. Distribution of the slip among four subfaults (from south to
north: 21 m, 13 m, 17 m, 2 m) provides best fit for satellite altimetry data and correlates well with seismic and
geodetic data inversions.
Le tenseur des contraintes est symetrique
Si les 3 contraintes principales
sont egales, la contrainte est
toujours normale a toute surface
quelle que soit son orientation.
Le regime est dit“lithostatique”.
Il n’y a pas de cisaillement et il
n’y a pas de fracturation.
Si les contraintes pricipales ne
sont pas egales, le cisaillement
est maximum a 45 degre entre
les contraintes principales (min
et max).
Convention en seismologie: l’axe P definit la direction de la
contrainte principale compressive, l’axe T la contrainte
tensile, et l’axe neutre la contrainte intermediaire.
Avant rupture, contraintes de cisaillement maximum le long des deux
plans nodaux. Ces plans divisent espace en regions de compression et
de tension. A la rupture, le relachement de la tension cause une onde
de compression et le relachement de la compression une onde de
dilatation
La direction des contraintes est bien
determinee, mais les deux plans
nodaux ne peuvent pas etre distingues
CONVENTION: Les regions ou l’on
observe un premier mouvement de
compression sont ombragees.