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Published March 2004 | Published
Journal Article Open

Preliminary Report on the 22 December 2003, M 6.5 San Simeon, California Earthquake


The M_w 6.5 San Simeon earthquake struck the central California coast on 22 December 2003 at 19:15:56 UTC (11:15:56 am local time.) The epicenter was located 11 km northeast of the town of San Simeon, and 39 km west-northwest of Paso Robles (Figure 1), as reported by the California Integrated Seismic Network (CISN, the California region of the Advanced National Seismic System [ANSS]). The mainshock nucleated at 35.702°N, 121.108°W and a depth of 7.1 km, and the rupture propagated unilaterally to the southeast. The strong directivity of the rupture resulted in a concentration of damage and aftershock activity to the southeast of the hypocenter. The worst earthquake damage occurred in Paso Robles, where two people died in the collapse of an unreinforced masonry building. The accurate and rapid earthquake information provided in near real-time by CISN/ANSS to the Governor's Office of Emergency Services made it possible to focus emergency response in the source area, although the earthquake was felt from San Francisco to Los Angeles.

Additional Information

© 2004 Seismological Society of America. The California Integrated Seismic Network is funded by the USGS internal and external programs, the USGS Advanced National Seismic System, the California Governor's Office of Emergency Services, and the California Geological Survey. We thank the staff of the CISN Northern California Management Center and the NCEDC for their efforts to make rapid reports such as this one possible. We also thank the staff of CGS (CISN Engineering Center) for making strong-motion data available and for dispatching field crews to retrieve strong-motion data and rapid processing on the day of the earthquake. We thank IRIS/DMC for making teleseismic data available in near real-time. We thank Jascha Polet for providing the Caltech/CISN CMT solution for the San Simeon mainshock, and Colin Williams for information about the Paso Robles hot springs. John Filson of USGS and Tim McCrink, Jerry Treimann, and Bill Bryant of CGS assisted with geological investigations. The following people assisted with the BARD/SCIGN GPS vectors: Duncan Agnew, Yehuda Bock, John Galetzka, Ken Hurst, Nancy King, Jessica Murray, Doug Neuhauser, Jerry Svarc, Chris Walls, Frank Webb. We thank Gary Puis, Andrew Michael, and William Ellsworth for helpful reviews of the manuscript. We thank Debi Kilb at the SIO Visualization Center for making the aftershock data available in 3D online.

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