Slide 1
DECEMBER 2004 INDIAN OCEAN EARTHQUAKE AND TSUNAMI
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
BASIC CONCEPTS: RIGID PLATES
BASIC CONCEPTS: THERMAL EVOLUTION OF OCEANIC LITHOSPHERE
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
Gordon & Stein, 1992
INDIAN PLATE MOVES NORTHCOLLIDING WITH EURASIA
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
India subducts beneath Burma microplate at about 50 mm/yr
Earthquakes occur at plate interface along the Sumatra arc (Sunda
trench) These are spectacular & destructive results of many
years of accumulated motion
INTERSEISMIC:
India subducts beneath Burma microplate at 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
HOW OFTEN:
Fault slipped ~ 10 m = 10000 mm / 50 mm/yr
10000 mm / 50 mm/yr = 200 yrLonger if some slip is aseismic
Faults arent exactly periodic for reasons we dont
understand
Stein & Wysession, 2003
MODELING SEISMOGRAMS shows how slip varied on fault planeMaximum
slip area ~400 km longMaximum slip ~ 20 m
Stein & Wysession
TWO VIEWS OF THE PART OF THE SUMATRA SUBDUCTION ZONE THAT
SLIPPED
Seismogram analysis shows most slip in southern 400 km
Aftershocks show slip extended almost 1200 km
C. Ji
ERI
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.
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
0S2 YIELDS SEISMIC MOMENT Mo = 1 x 1030 dyn-cm
2.5 TIMES BIGGER THAN INFERRED FROM 300-s SURFACE WAVESCORRESPONDING 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
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
EARTHQUAKE MAGNITUDE 9.3
One of the largest earthquakes since seismometer invented ~
1900
Stein & Wysession after IRIS
SUCH GREAT EARTHQUAKES ARE RARE
Stein & Wysession, 2003
SOME MAJOR DAMAGE DONE BY EARTHQUAKE SHAKING ITSELF, BUT STRONG
GROUND MOTION DECAYS RAPIDLY WITH DISTANCE
0.2 g
Stein & Wysession, 2003
0.2 g Damage onset for modern buildings
DAMAGE DEPENDS ON BUILDING TYPERESISTANT CONSTRUCTION REDUCES
EARTHQUAKE RISKS
Earthquakes don't kill people; buildings kill people."
Coburn & Spence 1992
TSUNAMI - water wave generated by earthquake
NY Times
TSUNAMI GENERATED ALONG FAULT, WHERE SEA FLOOR DISPLACED, AND
SPREADS OUTWARD
Red - up motion, blue down
Hyndeman and Wang, 1993
TSUNAMI SPEED IN DEEP WATER of depth d c = (gd)1/2g = 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
WAVE PATH GIVEN BY SNELLS LAWGoing from material with speed v1
to speed v2Angle of incidence I changes bysin i1 / v1 = sin i2 /
v2
SLOWFAST
Tsunami wave bends as water depth & thus speed changes
Stein & Wysession
TRACE RAY PATHS USING SNELLS LAWRAYS BEND AS WATER DEPTH
CHANGESFIND WHEN WAVES ARRIVE AT DIFFERENT PLACESDENSITY OF WAVES
SHOWS FOCUSING & DEFOCUSING
Woods & Okal, 1987
1 hour
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
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
TSUNAMI WARNING
Deep ocean buoys can measure wave heights, verify tsunami and
reduce false alarms
HOWEVER, HARD TO PREDICT EARTHQUAKES recurrence is highly
variable
M>7 mean 132 yr s 105 yr Estimated probability in 30 yrs
7-51%
Sieh et al., 1989
Extend earthquake history with geologic records
-paleoseismology
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
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 togrow 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
PLATE TECTONICS IS DESTRUCTIVE TO HUMAN SOCIETY
Mt Saint Helens1980 eruption
USGS
1989 Loma Prieta earthquake
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
BUT PLATE TECTONICS IS ALSO CRUCIAL FOR HUMAN LIFE
Press & Siever
CIVILIZATION EXISTS BY GEOLOGICAL CONSENT
The same geologic processes that make our planet habitable also
make it dangerous
*
1906 SF 4 m of slip on 450-km long fault 3 x 10**16 Joules of
elastic energy equivalent to a 7 Megaton bomb (Hiroshima was 0.012
Mt)1960 Chile 21 m of slip on a 800 km long fault 10**19 J of
elastic energy (more than a 2000 Mt bomb larger than all nuclear
bombs ever exploded largest was a Soviet atmospheric test of 58
Mt)
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