Swarm of East Bay Quakes Spurs Concern About Big One

Feb. 3, 2003
By Yomi S. Wronge, Mercury News

A swarm of earthquakes that rattled homes and nerves in the East Bay beginning Sunday morning has scientists concerned that a bigger quake -- packing more punch than Loma Prieta -- could be on the horizon.

Within hours of the quakes -- centered dangerously close to the northern Calaveras Fault -- experts at the the U.S. Geological Survey in Menlo Park huddled to discuss the possible implications of a series of shakes near a previously calm fault line.

``This part of the Calaveras has been quiet the last 40 years,'' said geologist David Schwartz, noting that a series of small quakes in November believed to be on the Calaveras Fault turned out to be along a new fault in the same area.

Sunday's quakes -- there were more than two dozen -- are significant because of the size and depth of the quakes, and the proximity to the Calaveras Fault, Schwartz said.

``We have some concern about what this represents in terms of a bigger quake,'' he said.

Scientists have estimated the likelihood of a magnitude 6.7 quake or higher occurring along the Calaveras Fault at about 15 percent over the next 30 years. If that happens, Schwartz said, the impact would be felt regionally, from Concord to Pleasanton, to as far east as Tracy.

``A large Calaveras quake would be felt in the entire East Bay and San Francisco,'' Schwartz said. ``The damage would be greater than Loma Prieta, even if the magnitude is similar,'' because the fault is so close to urban areas.

While no one can say with certainty if or when a major quake will hit, experts agree that it's wise to prepare now.

Sunday's quakes ``are Mother Nature's way of reminding us she has more up her sleeve,'' said Bill Ellsworth, chief scientist of the U.S. Geological Survey's earthquake hazards team.

The tremors began at 8:19 a.m. and lasted throughout the day. By midday, the strongest quake had measured magnitude 4.2, with several others measuring between 3.0 and 4.0 in strength. People living as far away as the Santa Cruz Mountains felt the quakes. Those closest to the epicenter reported feeling several quakes.

``We felt anywhere from eight to 10 earthquakes,'' said Christian Bronstein, a manager at the Target store on Bollinger Canyon Road in San Ramon. ``We heard the shelves rattle, but no damage was done, and nothing has fallen.''

It was a little more than two months ago that people in this area experienced five days of temblors -- 135 quakes total -- and Bronstein said the memory is still fresh in people's minds.

In 1990 near Alamo, a swarm of 350 earthquakes during a six-week span with a maximum magnitude of 4.6 shook plenty of nerves. Scientists refer to the wave of small earthquakes as a ``swarm'' because they are relatively the same magnitude and occur in the same area. By comparison, larger earthquakes such as Loma Prieta in 1989, which measured magnitude 6.9, are followed by a series of aftershocks that diminish in magnitude.

http://www.bayarea.com/mld/mercurynews/news/local/5093118.htm