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Earthquake Hazards Program

Tectonic Summary

Tuesday, September 28, 2004 at 17:15:24 UTC

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

Tectonics Plates
This earthquake occurred on a relatively straight section of the San Andreas Fault near Parkfield, California. The well-known San Andreas fault is a right-lateral strike-slip fault that stretches 1300 km from the Gulf of California to Cape Mendocino in Northern California. The fault is part of the principal plate boundary between the North American plate and the Pacific plate. The Pacific plate moves northwest with respect to the North American plate at a velocity of 46 mm/year. At the latitude of Parkfield, approximately seventy percent of the overall relative plate motion is accommodated by slip on the San Andreas.

In the Parkfield section of the San Andreas (Parkfield to Goldhill), the fault transitions from a creeping zone in the north to a locked zone in the south. The creeping section stretches north of the Parkfield section to San Juan Bautista. Along the creeping section, the motion between the Pacific and North American plates is accommodated by continuous non-seismic slip along the fault and numerous small earthquakes with magnitudes generally less than 5. To the south of the Parkfield section, the San Andreas has been locked since the magnitude 7.9 Fort Tejon earthquake of 1857. In this section of the San Andreas, relative plate motions have caused elastic strain to accumulate for almost a century and a half. When the strength of the fault is finally exceeded, the ensuing earthquake will be much larger than the shock of September 28, 2004.

The Parkfield section has been characterized for a century and a half by frequent minor and moderate shocks. Earthquakes of comparable size to this recent quake (magnitude 6) occurred in 1857, 1881, 1901, 1922, 1934, and 1966. There is substantial overlap between the zone of the aftershocks of this recent earthquake and the aftershock zone of the 1966 earthquake. The fault segment that ruptured in the 1966 earthquake had a length of about 25 km and a width of about 5 km; the average displacement of the fault rupture was about 50 cm.


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