Finding fault for an old earthquake

Scientists in Southern California believe they’ve found evidence that finally identifies the source of one of the largest quakes since the region was settled.

The clues also permit the researchers to better estimate the magnitude of the 1812 temblor–an assessment that could translate into higher seismic risk for the region.

The San Cayetano fault, northwest of Los Angeles, is about 45 kilometers long and extends to within 50 km of the city’s downtown. Strata exposed by the excavation of a 59-meter-long, 4.5-m-deep trench along the fault showed that the two sides of the fault slipped 4.3 m past each other during the last major quake there. That motion corresponds to a temblor with a magnitude between 7.15 and 7.45.

Carbon dating of the sediments suggests that the big quake occurred within the past 350 years, say earthquake geologists James F. Dolan of the University of Southern Californiain Los Angeles and Thomas K. Rockwell of San Diego State University. They contend that this finding pegs the fault as the probable source of the California temblor of Dec. 21, 1812, the only significant quake in the region in historical times that hasn’t been blamed on a specific fault. The analysis appears in the December 2001 Bulletin of the Seismological Society of America.

The researchers say that their estimated magnitude for the previously homeless quake suggests that the region’s seismic risk may need to be revised. Currently, State of California geologists estimate that the San Cayetano fault can generate quakes with a maximum magnitude of 6.8.

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