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) 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: 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 INDIAN PLATE MOVES NORTH COLLIDING WITH EURASIA Gordon & Stein, 1992 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: Stein & Wysession, 2003 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 MODELING SEISMOGRAMS shows how slip varied on fault plane Maximum slip area ~400 km long Maximum slip ~ 20 m Stein & Wysession 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 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 SumatraAndaman 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 Stein & Wysession after IRIS One of the largest earthquakes since seismometer invented ~ 1900 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 DAMAGE DEPENDS ON BUILDING TYPE RESISTANT CONSTRUCTION REDUCES EARTHQUAKE RISKS 0.2 g Damage onset for modern buildings “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 QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Hyndeman and Wang, 1993 Red - up motion, blue down http://staff.aist.go.jp/kenji.satake/animation.gif 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 QuickTime™ and a GIF decompressor are needed to see this picture. 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 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 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 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 TSUNAMI WARNING Deep ocean buoys can measure wave heights, verify tsunami and reduce false alarms 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 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% 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 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” PLATE TECTONICS IS DESTRUCTIVE TO HUMAN SOCIETY Mt Saint Helens 1980 eruption USGS 1989 Loma Prieta earthquake 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 “CIVILIZATION EXISTS BY GEOLOGICAL CONSENT” The same geologic processes that make our planet habitable also make it dangerous