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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.

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