Iotsunami
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Transcript of Iotsunami
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
Stein & Wysession 2003
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
microplateat 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 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
HOW OFTEN:
Fault slipped ~ 10 m = 10000 mm / 50 mm/yr
10000 mm / 50 mm/yr = 200 yrLonger if some slip is aseismic
Faults aren’t exactly periodic for reasons we don’t understand
Stein & Wysession, 2003
MODELING SEISMOGRAMS shows how slip varied on fault plane
Maximum slip area ~400 km long
Maximum slip ~ 20 mStein & 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. JiERI
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 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”
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
http://staff.aist.go.jp/kenji.satake/animation.gif
Red - up motion, blue downHyndeman and Wang, 1993
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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
QuickTime™ and aGIF decompressor
are needed to see this picture.
http://staff.aist.go.jp/kenji.satake/animation.gif
WAVE PATH GIVEN BY SNELL’S LAW
Going from material with speed v1 to speed v2
Angle of incidence I changes by
sin i1 / v1 = sin i2 / v2
SLOW
FAST
Tsunami wave bends as water depth & thus speed changes
Stein & Wysession
TRACE RAY PATHS USING SNELL’S LAW
RAYS BEND AS WATER DEPTH CHANGES
FIND WHEN WAVES ARRIVE AT DIFFERENT PLACES
DENSITY OF WAVES SHOWS FOCUSING & DEFOCUSING
Woods & Okal, 1987
1 hour
NOAA
IN DEEP OCEAN tsunami has long wavelength, travels fast, small amplitude - doesn’t 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 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 it’s 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.
“It’s 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