Unstudied California fault poses hazard.

Unstudied California fault poses hazard

Northern Californians can now worry about another deadly fault in their midst. Preliminary studies suggest the next major quake to hit San Francisco could come from a relatively unstudied fault that scientists have not considered in any hazard assessments to date.

Called the Rodgers Creek fault, the structure runs northward from San Pablo Bay to Santa Rosa and represents a continuation of the well-known Hayward fault, which also threatens Bay area residents (SN: 10/21/89, p.261).

If a major quake struck along the Rodgers Creek fault, "the whole north Bay area would really suffer a lot of damage," says geologist David P. Schwartz. The fault is half as far from San Francisco as the branch of the San Andreas that spawned the magnitude 7.1 Loma Prieta earthquake on Oct. 17.

Schwartz, Karin E. Budding and their colleagues at the U.S. Geological Survey (USGS) in Menlo Park, Calif., raised the warning flag over Rodgers Creek on the basis of studies this summer showing the fault has produced several major earthquakes in the past and appears to be nearing another rupture, possibly within the next few decades.

The geologists dug trenches near the fault, looking for broken sediment layers, which signify past earthquakes. By dating charcoal in the sediments, they found the fault has generated at least three or four major earthquakes in the last 1,270 years. These and other findings suggest the quakes occurred roughly 256 to 620 years apart. At last week's meeting of the American Geophysical Union in San Francisco, Budding said this estimate is a maximum and warned that the time between jolts might be shorter.

The researchers remain unsure of when the fault last broke. While records indicate an earthquake his the north Bay area in 1808, geologists cannot determine whether it originated on Rodgers Creek or a nearby fault. Schwartz concludes Rodgers Creek has remained quiet for at least 181 years, perhaps much longer, and thus could break soon.

Other evidence supports the idea that the fault is storing energy for a quake. Seismologic records reveal it has not produced many "microearthquakes" in the past two decades. In addition, the opposite sides of the fault are not creeping past each other. This suggests they have locked together and strain will continue to build until an earthquake breaks the fault.

Scientists have long known of the Rodgers Creek fault, but the trenching analysis is the first to reveal its earthquake history, says Schwartz. He and his colleagues recommend that future hazard assessments include this fault. A 1988 USGS report did not consider Rodgers Creek when it forecast that the Bay area faces even odds of a magnitude 7 quake in the next three decades. Loma Prieta does not lower those odds.

A committee of earth scientists will meet next year to reassess the 1988 analysis. Schwartz says recognition of Rodgers Creek's quake potential will lead the group to increase its predicted odds of a destructive earthquake hitting the Bay area by 2020.