3.9 Quake Was in Heart of Dangerous Fault Segment

The magnitude 3.9 earthquake centered in the East Bay on Thursday night occurred along a segment of the Hayward fault that has the potential to produce a destructive temblor, scientists said Friday.

The reason is that the fault passes directly through the thickly populated cities of Oakland and Berkeley. Moreover, the recurrence of big quakes along it is considered by scientists to be shorter than the best-known faults that pass through the Los Angeles area, Sierra Madre and Newport-Inglewood.

Still, quake intervals can be long, and the most recent big quake on the Hayward fault occurred 135 years ago, on Oct. 21, 1868, when a magnitude 7.0 temblor killed 30 people before the area was heavily built up.

Thursday’s quake did most of its damage along the south end of San Francisco Bay, but it may have ruptured as far north as Berkeley, which is in what U.S. Geological Survey scientist David Schwartz calls “a fuzzy, transitional zone” between fault segments.

North of the 1868 rupture, no big quake has occurred since sometime between 1640 and 1776, said Schwartz, who has done an extensive study of the Hayward fault.

Thursday’s quake occurred at 6:40 p.m. at a depth of eight miles, three miles southeast of Berkeley, near the Caldecott Tunnel in the suburb of Piedmont.

It was felt throughout the Bay Area. It did light damage, which mainly involved items falling off shelves, in Oakland and Berkeley. There was a 3.0 aftershock an hour later.

About 5,000 people described what they felt on the Geological Survey’s Web site.

This indicated the quake was felt most strongly in Oakland, Berkeley and San Francisco at a modified Mercalli intensity level as high as V, which means that some liquids were spilled, small unstable objects were displaced or upset, doors swung open or closed and pictures moved.

Since 1868, there has been the huge 1906 San Francisco quake along 200 miles of the San Andreas fault, plus strong quakes in Long Beach in 1933 and the San Fernando Valley in 1971 and 1994.

The San Francisco quake killed more than 700 people, the Long Beach quake 120 and the two San Fernando Valley quakes 115 people between them.

The Hayward fault is considered by many scientists to be more hazardous because the next quake along it may occur sooner.

They also note that the San Andreas fault passes offshore from San Francisco, and thus the city would escape the strongest shaking.

Schwartz said Friday that the recurrent interval of a 6.5 quake along the northern segment of the Hayward fault may be about 200 years. That would make it overdue.

He said a 6.9 quake also might occur on the southern segment, the one that ruptured in 1868.

Lucy Jones, scientist in charge of the Geological Survey office in Pasadena, said Friday that the chances of a comparably sized quake along the Newport-Inglewood fault in Orange and Los Angeles counties is “very low, probably just several times in the millennium.”

All this is not simple guesswork. It is based on research into slip rates along the faults. But it is uncertain.

Several large earthquakes, such as the 1994 Northridge quake, have recently occurred on unknown faults.

Still, because of its proximity to heavy population and the relative frequency of quakes, Schwartz and Jones believe the Hayward fault should be considered most dangerous at this time.

Schwartz said, however, that the 3.9 quake Thursday night does not increase the likelihood of a big one immediately.

“A 3.9 can pop off essentially anywhere in the region at any time,” he said, without necessarily presaging a larger one.