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DECEMBER 2004 INDIAN OCEAN EARTHQUAKE AND TSUNAMI. 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)

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slide2

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

slide3

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

slide4

INDIAN PLATE MOVES NORTH

COLLIDING WITH EURASIA

Gordon & Stein, 1992

slide5

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

slide6

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

slide7

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 aren’t exactly periodic for reasons we don’t understand

slide8

MODELING SEISMOGRAMS shows how slip varied on fault plane

Maximum slip area ~400 km long

Maximum slip ~ 20 m

Stein & Wysession

slide9

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

slide10

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.

slide11

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

slide12

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”

slide13

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

slide14

EARTHQUAKE MAGNITUDE 9.3

Stein & Wysession after IRIS

One of the largest earthquakes since seismometer invented ~ 1900

slide15

SUCH GREAT EARTHQUAKES ARE RARE

Stein & Wysession, 2003

slide16

SOME MAJOR DAMAGE DONE BY EARTHQUAKE SHAKING ITSELF, BUT STRONG GROUND MOTION DECAYS RAPIDLY WITH DISTANCE

0.2 g

Stein & Wysession, 2003

slide17

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."

slide19

TSUNAMI GENERATED ALONG FAULT, WHERE SEA FLOOR DISPLACED, AND SPREADS OUTWARD

Hyndeman and Wang, 1993

Red - up motion, blue down

http://staff.aist.go.jp/kenji.satake/animation.gif

slide20

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

slide21

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

Stein & Wysession

Tsunami wave bends as water depth & thus speed changes

slide22

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

1 hour

Woods & Okal, 1987

slide24

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

slide25

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

slide26

HOWEVER, HARD TO PREDICT EARTHQUAKES

recurrence is highly variable

Sieh et al., 1989

Extend earthquake history with geologic records -paleoseismology

M>7 mean 132 yr s 105 yr Estimated probability in 30 yrs 7-51%

slide27

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

slide28

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.

“It’s hard to predict earthquakes, especially before they happen”

slide29

Mt Saint Helens

1980 eruption

PLATE TECTONICS IS DESTRUCTIVE TO HUMAN SOCIETY

USGS

  • 1989 Loma Prieta earthquake
slide30

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

slide31

“CIVILIZATION EXISTS BY GEOLOGICAL CONSENT”

The same geologic processes that make our planet habitable also make it dangerous