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Earthquake Hazards Program

Tectonic Summary

Sunday, April 10, 2005 at 10:29:11 UTC

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

The earthquake of April 10, 2005, occurred as reverse-faulting in the Sumatra subduction zone, the plate boundary between the Australia plate and the Sunda plate. In the region of the earthquake, the Australia plate moves toward the north-northeast at a rate of about 5 cm/year relative to the Sunda plate. From the regional tectonic situation of the earthquake, we think that it probably occurred as thrust-faulting on the plate interface between the Australia and Sunda plates, although preliminary determinations of the earthquake’s focal-mechanism have some characteristics that are more consistent with reverse faulting in the interior of either the Australia plate or the Sunda plate.

This earthquake may have been triggered in part by stress changes caused by the December 26, 2004, (M 9.0) and March 28, 2005 (M 8.7) earthquakes to the northwest along the Sumatra subduction zone. It occurred about 125 miles (200 km) southeast of the causative fault of the March 28 earthquake and 400 miles (650 km) southeast of the causative fault of the December 26 earthquake. The epicentral region of the earthquake has been a persistent source of moderately large shocks in recent decades: since 1970, five shocks of M 6 and larger have occurred within one-hundred kilometers of the epicenter of the April 10, 2005 earthquake. This earthquake and other shocks of recent decades took place near the margins of the fault rupture that has been inferred for the magnitude 8+ Sumatra earthquake of 1833.


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