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Earthquake Hazards Program

Tectonic Summary

2004 May 03 04:36:50 UTC

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

This earthquake resulted from the release of stresses that were generated by subduction of the Nazca plate beneath the South American plate. Ongoing subduction occurs at a rate of about 8 cm per year, generates numerous earthquakes and volcanism, and actively builds the Andes mountains. Given the earthquake's fault orientation and depth, it likely occurred on the interface between the plates.

The earthquake occurred near the northern end of the rupture zone of the magnitude 9.5 1960 Chilean earthquake. The 1960 Chilean earthquake was the largest in the 20th century and ruptured a 900-km long swath along the Chilean coast. In contrast, this magnitude 6.6 earthquake likely had a rupture length of about 30 km.


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