Nine separate earthquakes have smashed the crockery and rattled the nerves of residents here over the past four days.
The nine quakes, all registering 4.0 or above on the Richter scale, were part of sequence of seismic upheavals on the Greenville fault lying about 35 miles east of San Francisco.
The temblors packed less than a thousandth the strength of the great quake that devastated San Francisco in 1906 but nevertheless cause widespread property damage. There were no serious injuries, however.
The more severe quakes struck on Thursday morning and Saturday evening, registering 5.5 and 5.6 respectively. Their impact was felt over an area stretching nearly 150 miles, running north and south of San Francisco, and about 40 miles across.
The latest in the series of quakes hit at 2:38 a.m. today. In addition, quake-watching scientists say, there have been 100 small shocks measuring between 1.7 and 3.0 since Thursday.
The current wave of quakes follows a pattern of increased seismic activity throughout California in the past year. Dr. Robert Wesson, chief of the office of earthquake studies at the U.S. Geological Survey, says other signs of increased seismicity suggest more and larger earthquakes are on the way.
The levels of radioactive radon gas in artesian well water have risen, suggesting that rock is being squeezed by crustal stresses, far below ground. Precision laser measurements every 12 miles along San Andreas fault tell geologists that the fault has widened perceptibly in the last year.
"This east-west expansion along the fault is disturbing," Wesson said, "because it tends to make it easier for the fault to slip in a north-south direction." Fault slippage could signal a major quake.
The so-called southern California uplift, an elevation of land that sits in the middle of California like a big wrinkle in a rug, has begun to settle from its usual elevation.
Wesson says "disturbing signs" also have been seen in northern California, where the western side of the Hayward fault has been creeping north. In the last year, the northward creep has doubled in one spot and slowed in another.
Wesson says he sees no direct connection between these changes and the quakes along the Greenville fault in Livermore Valley, but the twin quakes above 5 concerned him a little.
"Nobody's surprised when you get one earthquake a year in California that's the equal of 5 in the Richter scale," Wesson said, "but to have two this close in the same week is a little uncommon. It's possible that all the phenomena we're seeing are part of some broad-scale episode that's still to come."
Scientists at the University of California's seismology center in Berkeley, who have estimated the chances at 50-50 that a major quake (7 on the Richter) will hit somewhere in the state in the next decade, also hesitated to connect the Livermore series of temblors to a major earthquake.
"It does not significantly influence the possibility of having a major earthquake in the area," said Dr. Robert Uhrhammer, research seismologist at the center, who added that several additional moderate aftershocks can be expected in the next few days.
Uhrhammer said scientists have been unable to tell whether a quake on one fault can trigger subsequent shocks on that fault -- or ruptures on other nearby faults. Conversely, he said, energy released in quakes does not significantly relieve stress on the fault or reduce the threat of another quake.
Saturday's quake, the most powerful of the recent sequence, caused scattered property damage but no reports of serious injuries. Thursday's quake, however, hit a more populated area and caused millions of dollars of damage in the town of Livermore, some 35 miles southeast of San Francisco. In Livermore, several trailers in a mobile home park were wrecked and dozens of people were injured.
At Lawrence Livermore Laboratory, which conducts nuclear weapons research for the federal government, a tank containing a low-level radioactive isotope ruptured, spilling gallons of the liquid material.
Livermore officials said, however, the leaking material posed no health hazard. They added that there was no damage to the facility's more volatile stores of highly radioactive plutonium and other elements.