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Earthquake Hazards Program

Tectonic Summary

Magnitude 6.5 CENTRAL CALIFORNIA
2003 December 22 19:15:56 UTC

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

The earthquake likely occurred on the Oceanic fault zone in the Santa Lucia mountains of coastal Central California, north of the town of Cambria. The location of the earthquake is in a region with a history of frequent small and moderate shocks. A preliminary study of the elastic-waves radiated from the source indicates that today's earthquake was caused by reverse faulting. Rupture propagated to the southeast from the hypocenter over a distance of approximately 20 km. Previous shocks from the region have also been caused by reverse faulting or by oblique-reverse faulting. The largest historical earthquake in the vicinity of today's earthquake was the magnitude 6.2 Bryson earthquake of November 1952.

The broad-scale tectonics of coastal Central California are dominated by the northwestward motion of the Pacific plate with respect to the North American plate. Most of the relative plate motion is accommodated by slip on major strike-slip faults, such as the San Andreas fault, situated about 60 km northeast of the epicenter of today's earthquake, and the San Simeon - Hosgri fault system, about 10 km southwest of the epicenter of today's earthquake. The reverse faulting that generated the recent earthquake was caused by the release of compressive stress that was generated by the motion of crustal blocks within the overall strike-slip plate-boundary zone.


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