Geology-

The tsunami was caused by a “mega thrust” earthquake that occurred when stresses were released as the India plate was subducted beneath the overriding Burma plate. The India plate begins to descend into the mantle at the Sunda Trench, which lies to the west of the earthquake’s epicenter. The depth of the ocean is around 4,500 meters.

The India plate moves toward the northeast at a rate of 6 cm/year, which results in an oblique convergence at the Sunda Trench.

An estimated 600-mile-long rupture occurred along the seafloor as one fault slid past the other, much like seismologists expect the San Andreas Fault to do in California. 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.

According to the press spokesman of the Meteorological and Geophysical Office in Jakarta, Indonesia, the epicenter was about 41 miles off the coast of Sumatra and about 16 miles below the surface of the ocean and well beneath the ocean floor. Several aftershocks followed the main earthquake, as of 1:00PM, MST, December 29; sixty-eight aftershocks had been cataloged. The largest occurred about three hours after the main shock and was assigned a magnitude of 7.1. Thirteen of the aftershocks have magnitudes of 6.0 or larger. There have been no reports of tsunamis being generated from the aftershocks.

The energy released by the earthquake was described as 475,000 kilotons of TNT by the USGS, a force equivalent to 23,000 Hiroshima bombs.

A comparison of rupture area affected by the equivalent power if released in the Pacific Northwest region

 

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click on county's name to view aerial photo (source: BBC World News)