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)