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Earthquake Hazards Program

Tectonic Summary

Friday, October 08, 2004 at 14:36:05 UTC

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

This earthquake occurred in the inclined seismic zone that dips east from the Manila Trench. The tectonics of the northern Philippines are determined by motion of microplates that lie between the converging Philippine Sea and Sunda plates. The Sunda plate moves to the east-southeast with respect to the Philippine Sea plate with a velocity of about 10 cm/year. This relative motion is accommodated by displacements on several major tectonic boundaries or fault zones and many minor faults as well. At the Manila Trench, the Sunda plate thrusts eastward beneath the lithosphere of Luzon and Mindoro. The earthquake occurred within the subducted Sunda plate, which in this region is seismically active to depths of almost 300 km.

Earthquakes, such as this one, that have focal-depths between 70 and 300 km are commonly termed "intermediate-depth" earthquakes. Intermediate-depth earthquakes typically cause less damage on the ground surface above their foci than is the case with similar magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters.


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