A slightly increased risk of a severe earthquake exists in an area south of San Francisco that was jolted by a moderate temblor this week, say state officials who also caution against public alarm.
California's Office of Emergency Services issued an advisory Tuesday saying that for the next five days, the slightly increased likelihood for continued seismic activity was possible for the region rocked by Monday's earthquake, which scientists estimated at between 5.0 and 5.7 on the Richter scale.Scientists from the U.S. Geological Survey, University of California, Berkeley, and the state Division of Mines and Geology "say that this activity could involve an earthquake as large as 6.5 Richter magnitude, although it's not probable that a quake of this size will occur," the advisory said.
It was issued to alert officials of Santa Cruz, San Benito, Monterey and Santa Clara counties "to maintain preparedness and awareness measures."
The chance of a magnitude-6.5 quake on the San Andreas Fault within five days "is a very slight possibility, but we do feel it is our responsibility to make this information public," Office of Emergency Services spokesman Tom Mullins said from Sacramento.
"There is some some concern, although no alarm," said Mike Guerin, a spokesman at the agency's office in Ontario.
Damage from a 6.5-magnitude quake could be comparable to that caused by the 6.2-magnitude Morgan Hill quake on April 24, 1984, acting state geologist Brian Tucker said. That quake injured 27 people and caused $10 million in damage.
Residents of San Jose and the San Francisco Bay area could expect a 6.5-magnitude quake to cause "swaying of high-rise buildings, potentially some fires and some localized areas of more intense damage" to buildings, Tucker said.
Guerin said the advisory, which isn't a prediction, was issued because scientists believe additional stress has been placed on a portion of the San Andreas Fault by Monday's quake, which swayed skyscrapers in San Francisco but resulted in no reports of injuries or serious damage.
Monday's quake was centered on the San Andreas Fault 16 miles south of San Jose, officials said.
Tucker said scientists believe Monday's quake added stress to the San Andreas in part because the section of the fault that snapped had been relatively quiet for years and the temblor occurred near the junction of the San Andreas and Sargent faults. Quakes are more likely to start near fault junctions, he said.
Quake predictions or advisories have been rare in the United States.