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Tsunami Sunday, December 26, 2004


Sunday, December 26, 2004 at 00:58:53 UTC

Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver

The devastating megathrust earthquake of December 26, 2004, occurred on the interface of the India and Burma plates and was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. The India plate begins its descent into the mantle at the Sunda trench, which lies to the west of the earthquake's epicenter. The trench is the surface expression of the plate interface between the Australia and India plates, situated to the southwest of the trench, and the Burma and Sunda plates, situated to the northeast.

In the region of the earthquake, the India plate moves toward the northeast at a rate of about 6 cm/year relative to the Burma plate. This results in oblique convergence at the Sunda trench. The oblique motion is partitioned into thrust-faulting, which occurs on the plate-interface and which involves slip directed perpendicular to the trench, and strike-slip faulting, which occurs several hundred kilometers to the east of the trench and involves slip directed parallel to the trench. The December 26 earthquake occurred as the result of thrust-faulting.

Preliminary locations of larger aftershocks following the megathrust earthquake show that approximately 1200 km of the plate boundary slipped as a result of the earthquake. By comparison with other large megathrust earthquakes, the width of the causative fault-rupture was likely over one-hundred km. From the size of the earthquake, it is likely that the average displacement on the fault plane was about fifteen meters. The sea floor overlying the thrust fault would have been uplifted by several meters as a result of the earthquake. The above estimates of fault-dimensions and displacement will be refined in the near future as the result of detailed analyses of the earthquake waves.

The world's largest recorded earthquakes have all been megathrust events, occurring where one tectonic plate subducts beneath another. These include:

the magnitude 9.5 1960 Chile earthquake, the magnitude 9.2 1964 Prince William Sound, Alaska, earthquake, the magnitude 9.1 1957 Andreanof Islands, Alaska, earthquake, and the magnitude 9.0 1952 Kamchatka earthquake. As with the recent event, megathrust earthquakes often generate large tsunamis that cause damage over a much wider area than is directly affected by ground shaking near the earthquake's rupture

The great earthquake (Mw 9.0) of December 26 off Sumatra generated the tsunami in the Indian Ocean .  According to U.S.G.S., the aftershocks extended as far as 1,000 km toward north (the red circles indicate aftershocks occurred within 24 hours).  Assuming that the aftershock area represents the tsunami source, the tsunami propagation was computed.

The tsunami propagation is also animated (up to 5 hours) from a 1200 km fault. The red color means that the water surface is higher than normal, while the blue means lower. It indicates that initial tsunami to the east (e.g., Phuket) began with receding wave, while to the west (e.g., Sri Lanka ) large wave suddenly reached. The darker the color, the larger the amplitude.  The tsunamis were larger in the east and west directions.

Tsunami Animation: Courtesy: National Institute of Advanced Industrial Science and Technology, Japan



Banda Aceh, Indonesia

Banda Aceh Shore

Banda Aceh Shore (Before Tsunami)

Images from “DigitalGlobe”

Credit: The BBC, Office of Naval Research, USGS, The University of Washington, The Guardian