DECEMBER 2004 INDIAN OCEAN EARTHQUAKE AND TSUNAMI

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DECEMBER 2004 INDIAN OCEAN EARTHQUAKE AND TSUNAMI
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BASIC CONCEPTS RIGID PLATES
Earth's outer shell made up of 15 major rigid
plates 100 km thick Plates move relative to
each other at speeds of a few cm/ yr (about the
speed at which fingernails grow) Plates are
rigid in the sense that little (ideally no)
deformation occurs within them, Most (ideally
all) deformation occurs at their boundaries,
giving rise to earthquakes, mountain building,
volcanism, and other spectacular
phenomena. Style of boundary and intraplate
deformation depends on direction rate of
motion, together with thermo-mechanical
structure

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BASIC CONCEPTS THERMAL EVOLUTION OF OCEANIC
LITHOSPHERE

Stein Wysession 2003
Warm mantle material upwells at spreading centers
and then cools Because rock strength decreases
with temperature, cooling material forms strong
plates of lithosphere Cooling oceanic
lithosphere moves away from the ridges,
eventually reaches subduction zones and
descends in downgoing slabs back into the mantle,
reheating as it goes Lithosphere is cold outer
boundary layer of thermal convection system
involving mantle and core that removes heat from
Earth's interior, controlling its evolution
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INDIAN PLATE MOVES NORTH COLLIDING WITH EURASIA

Gordon Stein, 1992
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COMPLEX PLATE BOUNDARY ZONE IN SOUTHEAST
ASIA Northward motion of India deforms all of
the region Many small plates (microplates) and
blocks
Molnar Tapponier, 1977
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India subducts beneath Burma microplateat about
50 mm/yrEarthquakes occur at plate interface
along the Sumatra arc (Sunda trench)These are
spectacular destructive results of many years
of accumulated motion
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INTERSEISMIC India subducts beneath Burma
microplateat about 50 mm/yr (precise rate hard
to infer given complex geometry) Fault interface
is locked EARTHQUAKE (COSEISMIC) Fault
interface slips, overriding plate rebounds,
releasing accumulated motion
Stein Wysession, 2003
HOW OFTEN Fault slipped 10 m 10000 mm / 50
mm/yr 10000 mm / 50 mm/yr 200 yr Longer if
some slip is aseismic Faults arent exactly
periodic for reasons we dont understand
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MODELING SEISMOGRAMS shows how slip varied on
fault plane Maximum slip area 400 km
long Maximum slip 20 m
Stein Wysession
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TWO VIEWS OF THE PART OF THE SUMATRA SUBDUCTION
ZONE THAT SLIPPED
ERI
C. Ji
Seismogram analysis shows most slip in southern
400 km
Aftershocks show slip extended almost 1200 km
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Earthquakes rupture a patch along fault's
surface. Generally speaking, the larger the
rupture patch, the larger the earthquake
magnitude. Initial estimates from the aftershock
distribution show the magnitude 9.3
Sumatra-Andaman Islands Earthquake ruptured a
patch of fault roughly the size of California
For comparison, a magnitude 5 earthquake would
rupture a patch roughly the size of New York
City's Central Park.
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NORMAL MODES (ULTRA-LONG PERIOD WAVES) SHOW
SEISMIC MOMENT 3 TIMES THAT INFERRED FROM SURFACE
WAVES IMPLIES SLIP ON AREA 3 TIMES
LARGER Entire 1200-km long aftershock zone
likely slipped
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0S2 YIELDS SEISMIC MOMENT Mo 1
x 1030 dyn-cm 2.5 TIMES BIGGER THAN INFERRED
FROM 300-s SURFACE WAVES CORRESPONDING MOMENT
MAGNITUDE Mw IS 9.3, COMPARED TO 9.0 FROM SURFACE
WAVES Comparison of fault areas, moments,
magnitudes, amount of slip shows this was a
gigantic earthquake
the big one
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IF ENTIRE ZONE SLIPPED, STRAIN BUILT UP HAS BEEN
RELEASED, LEAVING LITTLE DANGER OF COMPARABLE
TSUNAMI Risk of local tsunami from large
aftershocks or oceanwide tsunami from boundary
segments to south remains
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EARTHQUAKE MAGNITUDE 9.3
Stein Wysession after IRIS
One of the largest earthquakes since seismometer
invented 1900
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SUCH GREAT EARTHQUAKES ARE RARE
Stein Wysession, 2003
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SOME MAJOR DAMAGE DONE BY EARTHQUAKE SHAKING
ITSELF, BUT STRONG GROUND MOTION DECAYS RAPIDLY
WITH DISTANCE
0.2 g
Stein Wysession, 2003
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DAMAGE DEPENDS ON BUILDING TYPE RESISTANT
CONSTRUCTION REDUCES EARTHQUAKE RISKS
0.2 g Damage onset for modern buildings
Coburn Spence 1992
Earthquakes don't kill people buildings kill
people."
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TSUNAMI - water wave generated by earthquake
NY Times
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TSUNAMI GENERATED ALONG FAULT, WHERE SEA FLOOR
DISPLACED, AND SPREADS OUTWARD
Red - up motion, blue down
Hyndeman and Wang, 1993
http//staff.aist.go.jp/kenji.satake/animation.gif
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TSUNAMI SPEED IN DEEP WATER of depth d c
(gd)1/2 g 9.8 m/s2 d 4000 m c 200 m/s
720 km/hr 450 m/hr Tsunami generated along
fault, where sea floor displaced, and spreads
outward Reached Sri Lanka in 2 hrs, India in 2-3
http//staff.aist.go.jp/kenji.satake/animation.gif
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WAVE PATH GIVEN BY SNELLS LAW Going from
material with speed v1 to speed v2 Angle of
incidence I changes by sin i1 / v1 sin i2 / v2
SLOW FAST
Stein Wysession
Tsunami wave bends as water depth thus speed
changes
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TRACE RAY PATHS USING SNELLS LAW RAYS BEND AS
WATER DEPTH CHANGES FIND WHEN WAVES ARRIVE AT
DIFFERENT PLACES DENSITY OF WAVES SHOWS FOCUSING
DEFOCUSING
1 hour
Woods Okal, 1987
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NOAA
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IN DEEP OCEAN tsunami has long wavelength,
travels fast, small amplitude - doesnt affect
ships AS IT APPROACHES SHORE, it slows. Since
energy is conserved, amplitude builds up - very
damaging
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TSUNAMI WARNING
Because seismic waves travel much faster (km/s)
than tsunamis, rapid analysis of seismograms can
identify earthquakes likely to cause major
tsunamis and predict when waves will arrive
Deep ocean buoys can measure wave heights, verify
tsunami and reduce false alarms
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HOWEVER, HARD TO PREDICT EARTHQUAKES recurrence
is highly variable
Sieh et al., 1989
Extend earthquake history with geologic records
-paleoseismology
Mgt7 mean 132 yr s 105 yr Estimated
probability in 30 yrs 7-51
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EARTHQUAKE RECURRENCE AT SUBDUCTION ZONES IS COM
PLICATED In many subduction zones, thrust
earthquakes have patterns in space and time.
Large earthquakes occurred in the Nankai trough
area of Japan approximately every 125 years since
1498 with similar fault areas In some cases
entire region seems to have slipped at once in
others slip was divided into several events over
a few years. Repeatability suggests that a
segment that has not slipped for some time is a
gap due for an earthquake, but its hard to use
this concept well because of variability
GAP?
NOTHING YET
Ando, 1975
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EARTHQUAKE PREDICTION? Because little is known
about the fundamental physics of faulting, many
attempts to predict earthquakes searched for
precursors, observable behavior that precedes
earthquakes. To date, search has proved
generally unsuccessful In one hypothesis, all
earthquakes start off as tiny earthquakes, which
happen frequently, but only a few cascade via
random failure process into large
earthquakes This hypothesis draws on ideas from
nonlinear dynamics or chaos theory, in which
small perturbations can grow to have
unpredictable large consequences. These ideas
were posed in terms of the possibility that the
flap of a butterfly's wings in Brazil might set
off a tornado in Texas, or in general that
minuscule disturbances do not affect the overall
frequency of storms but can modify when they
occur If so, there is nothing special about
those tiny earthquakes that happen to grow into
large ones, the interval between large
earthquakes is highly variable and no observable
precursors should occur before them. Thus
earthquake prediction is either impossible or
nearly so. Its hard to predict earthquakes,
especially before they happen


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Mt Saint Helens 1980 eruption
PLATE TECTONICS IS DESTRUCTIVE TO HUMAN SOCIETY

USGS
1989 Loma Prieta earthquake
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BUT PLATE TECTONICS IS ALSO CRUCIAL FOR HUMAN
LIFE

Plate boundary volcanism produces atmospheric
gases (carbon dioxide CO2 water H2O) needed to
support life and keep planet warm enough for life
("greenhouse" ) May explain how life evolved on
earth (at midocean ridge hot springs) Plate
tectonics raises continents above sea
level Plate tectonics produces mineral resources
including fossil fuels
Press Siever
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CIVILIZATION EXISTS BY GEOLOGICAL CONSENT
The same geologic processes that make our planet
habitable also make it dangerous