More than 100 small earthquakes shook this town 26 miles east of San Francisco last week, reminding residents of the Bay area that terra firma isn't quite as firm as they'd like to believe.
The series of temblors, which reached a maximum of 3.9 on the Richter scale, did not cause any damage, but such activity is a chilling reminder of what may be in store for the Bay area within the next 30 years, said Dr. Robert Uhrhammer of the U.C. Berkeley Seismological Laboratory.
The "swarm," which started Nov. 24 at 7 a.m., affected the area around San Ramon. A swarm is not a typical earthquake, Uhrhammer said, but it is not by any means unusual.
Swarms account for two to five percent of all seismic activity, he said, and differ from "classic earthquakes" in that they lack a central, defining quake. Swarms are like a series of aftershocks that have "no temporal pattern," he said. They seem to appear out of nowhere and slowly disappear in the same inscrutable manner in which they began.
Slight shocks continued Tuesday, but the main activity was over, Uhrhammer said.
"So it looks like the swarm has subsided," he said.
Swarms have struck near San Ramon in the last few decades; in 1990, a swarm hit near Danville, and in 1970 a swarm shook the town of Alamo.
While it's too early to tell what kind of effect the latest swarm may have on the region, Uhrhammer said the quakes are a reminder that future activity is likely to be anything but benign.
The swarms near San Ramon, he said, shook the earth around the Calaveras fault, one of seven major fault lines in the Bay area.
There is an 18 percent probability that a major earthquake-of magnitude of 6.7 or higher-will take place along this fault in the next 30 years.
And that's just one of seven fault lines. The "aggregate probability" for a major quake, over 30 years, on any one of these lines, Uhrhammer said, is 70 percent.
"The thing that people have to remember," he said, "is the potential of serious earthquakes is very real here in the Bay area."
All Bay residents can do, in order to brace themselves against "The Big One," is act like good Boy Scouts and be prepared, according to Bob Anderson, senior geologist for the Seismic Safety Commission.
That means, he said, keeping a cache of food and potable water (enough for three days per family member), necessary medicine and a portable radio as well as a flashlight loaded with fresh batteries.
Other than that, families should have a simple, clearly understood plan of action, he said. Every family member should know how to turn off the house gas line, and every family member should know the location of a predetermined "rally point" where the family would meet during an emergency.
And the hot water heater should be firmly strapped down.
"That's a bare bones minimum," he said.
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