The strongest earthquake to hit California in two years rolled through the San Francisco Bay area early today, two days after a smaller shock hit the region. Seismologists say the two temblors were unrelated and were not a sign of more serious seismic activity ahead.
Today's quake, measured at 5.3 to 5.6 on the Richter scale at 3:56 a.m. Pacific time, caused only minor damage as objects fell onto beds or people rose barefoot to inspect broken glass. About 10,000 homes in the San Jose area were without power for a few hours.
"I heard of one man hurt diving under a desk," said Nancy Hardaker, a spokeswoman for the state Office of Emergency Services. "He had the right idea -- he just misjudged and bonked himself on the head."
The quake followed a 4.0 jolt that struck the Bay area Saturday morning -- with a quick crack and a sound described by some Oakland and Berkeley residents as resembling a sonic boom -- and a half dozen lesser aftershocks.
State and federal authorities reported more than 160 aftershocks today, including 11 measuring higher than 3.0 on the Richter scale.
The epicenter of Saturday's quake was about 15 miles northeast of San Francisco, while today's shock was centered 15 miles southeast of Fremont.
The two earthquakes originated on small fault lines traversing the major north-south faults in the area, "not on the ones we worry about in the East Bay," said Tom Tobin, executive director of the State Seismic Safety Commission.
Major faults east of San Francisco Bay are the Hayward and Calaveras faults. The San Andreas fault, California's largest, runs through San Francisco west of the bay. "If we had a swarm of earthquakes like this on the Hayward Fault, yes, we would be worried," Tobin said. "I was worried myself when I got the wake-up call this morning."
Last June, a state earthquake prediction council announced that San Diego had an increased probability of a major earthquake after the area received three shocks in the 4.0 range on a single fault in a single day. Residents took simple precautions, Tobin said, but no quake came to pass.
The current seismic activity around San Francisco Bay is not similar. Rick McKenzie, a staff research associate at the Seismographic Stations of the University of California at Berkeley, said Saturday's occurred on minor faults between the Hayward and Calaveras faults and today's was east of Calaveras.
Today's quake swayed high-rise buildings in San Francisco and woke sleepers throughout the area.
"I always think about two things," said Fremont police spokesman Dennis Satariano. "I wonder, Is this all of it, or is this just the beginning? And I wonder where is it centered? I'm thinking, if this is centered in L.A., you're going to have some severe damage and San Francisco has got the Raiders back."
North Oakland resident Karen Chuse said her cat Murphy jumped on her bed just before the quake struck, fur standing on end, and tried to get out of the house. It was unusual behavior for her cat, she thought.
"Just as I was opening the door, the house started to shake," Chuse said.
Though the two recent earthquakes appeared to be unrelated -- and were definitely unrelated to the weekend eruption of Augustine Volcano in Alaska -- they are part of a long-range cycle of larger earthquakes in California, said U.S. Geological Survey spokeswoman Edna King.
Today's quake was the biggest since a 6.2 earthquake in April 1984. On the Richter scale, which measures the energy released, each rise by one number indicates a tenfold increase in intensity. A 5.0 earthquake is considered capable of causing damage, and a 6.2 quake would be nearly 10 times as strong as today's 5.3 quake.