San Simeon, California Earthquake

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Posting Date: December 22, 2003, 1:15:00 PM

A strong earthquake shook central California on Monday, December 22, 2003 at 11:15 a.m. local time (19:15 UTC). The United States Geological Survey (USGS) has assigned a preliminary magnitude (Mw) of 6.5 and a focal depth of 4.7 miles (7.6 km), making this a shallow event. The quake occurred about five miles (10 km) northeast of San Simeon, which is located about 200 miles north of Los Angeles. Today’s earthquake occurred about 60 km west of the San Andreas fault in an area that is dominated by both strike slip and thrust faults. Data available from the USGS clearly indicate a reverse rupture mechanism dipping either northeast or southwest. Initial reports, i.e. the aftershocks and ground motion distributions, support the northeast rupture of the Oceanic fault, which is a local inland thrust fault parallel to the San Andreas fault. Historically, this region has experienced large magnitude earthquakes. In 1927, a magnitude 7.1 earthquake occurred near the town of Lompoc about 60 miles (100 km) south of the current earthquake. The quake was felt in Los Angeles to the south and as far as San Francisco to the north. Close to the epicenter, there have been reports of significant damage to older buildings, most of unreinforced masonry. At least three people were killed when a commercial structure collapsed in Paso Robles, some 30 miles from the epicenter. Some highway and building damage, as well as power outages, were reported in cities along the Central Coast. More than 30 aftershocks have been reported, the strongest measuring 4.7. It is in the very early aftermath of this event and detailed damage reports are just beginning to come in. What is clear, however, is that the situation could have been much worse. While the magnitude and focal depth make this a significant event from a seismological point of view, the good news is that the earthquake occurred in a sparsely populated stretch of the California coast. The AIR earthquake team is monitoring the situation closely and has begun running scenarios based on the currently available information. The results of their analysis will be posted tomorrow.

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Posting Date: December 22, 2003, 1:15:00 PM