Geological evidence suggests 1906 San Francisco quake one of series

EUGENE, Ore., Feb. 13 (UPI) -- Geologic evidence suggests San Francisco's devastating 1906 disaster was the third in a series of ground-rupturing events in the region, researchers say.

A study led by a University of Oregon doctoral student in California's Santa Cruz Mountains has found evidence of two earthquakes, in 1838 and 1890, on the San Andreas Fault, the university reported Thursday.

Earlier tremors before 1906 had been theorized by many researchers based on written accounts about damage to Spanish-built missions in the Monterey and San Francisco bay areas.

"We found the first geologic evidence of surface rupture by what looks like the 1838 and 1890 earthquakes, as well as 1906," geology doctoral student Ashley R. Streig said.

Unlike the 1906 quake that ruptured 296 miles of the fault, the 1838 and 1890 quakes ruptured shorter portions of the fault, possibly limited to the 39-mile stretch of the Santa Cruz Mountains, she said.

"This is the first time we have had good, clear geologic evidence of these historic 19th century earthquakes," she said. "It's important because it tells us that we had three surface ruptures, really closely spaced in time that all had fairly large displacements of at least half a meter and probably larger."

More studies will dig deeper into the region's geological record to determine if the ensuing seismically quiet years since 1906 make up a normal pattern -- or not -- of quake frequency along the fault, Streig said.