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NR 2002-48
November 17, 2002

Contact: Ed Wilson
Mark Oldfield
Don Drysdale
(916) 323-1886

EARTHQUAKE SWARMS ARE UNCOMMON IN MOST PLACES,
BUT NOT IN THE SAN RAMON AREA

SACRAMENTO – The San Ramon area is experiencing a swarm of minor earthquakes that while uncommon in most of California is not unprecedented in that area.

Between 6:54 Sunday morning and noon Wednesday, 106 earthquakes had taken place in the San Ramon area. Seven of those were greater than magnitude 3 – not likely to cause damage, but strong enough to be felt. The largest was magnitude 3.9.

Previous swarms near the Calaveras fault in Contra Costa County have not heralded a larger, damaging earthquake, and scientists at the Department of Conservation’s California Geological Survey are hopeful that the current phenomenon will follow that historic trend.

“We can’t be 100 percent certain that there will not be a larger event, but that would not be consistent with what has been recorded in the past,” State Geologist Jim Davis said. “Most likely, the swarm will persist for a few weeks, as has been the case in the past. It wouldn’t be surprising to see an event as large as magnitude 4.5. We are keeping an eye on what’s going on and reporting the scientific data to the Office of Emergency Services and officials in the San Ramon area.”

Earthquakes generally occur in a series: foreshocks, main shocks and aftershocks. In a swarm, there are a series of perhaps hundreds of earthquakes, typically minor and generally of a similar size, without an outstanding main shock or largest event.

“Swarms don’t follow the regular pattern of a main shock followed by a series of aftershocks that decrease in magnitude and frequency,” said Chris Wills, a Department of Conservation supervising geologist. “Typically, if there’s a magnitude 3.5 earthquake, it’s followed by something in the 2.5 range, and then aftershocks of decreasing magnitude. In the swarm we’re seeing now, there have been several magnitude 3.5 to 3.9 events with a number of magnitude 2.5 to 3.5 quakes. Although swarms do decrease in activity over time, it is common to have several earthquakes of similar size over several days to weeks”

The swarm currently shaking the San Ramon area was at first thought to be associated with the Calaveras fault. However, analysis shows that it actually is occurring on a two-mile long fault that is perpendicular to the Calaveras. This fault has not been mapped at the Earth’s surface, and it is possible it is entirely a subsurface fault.

Earthquake swarms occurred in the nearby Danville area in 1970 and 1976. The 1970 swarm included 353 earthquakes from May 22-June 19, the largest being magnitude 4.3 and 4.2 temblors. In 1990, the Alamo area, 10 miles north of San Ramon, experienced minor earthquakes over a six-week period, including a magnitude 4.6 temblor and 13 quakes greater than magnitude 3.0.

“There have been a number of swarms in recent years in this area,” Wills said. “There really isn’t a good model for what’s happening here, but it seems as though there’s a series of small faults near the end of a major one (the Calaveras) at work. These faults seem to abut the Calaveras fault in an area where the activity on the Calaveras is decreasing from south to north. It’s a fascinating geological area.”

However, the stress released by these minor earthquakes is insignificant compared to larger events that can occur on faults in the bay area. According to a 1999 study of earthquake probabilities in the San Francisco Bay Area, there is a 70 percent likelihood of a magnitude 6.7 or greater earthquake between 2000 and 2030. The probability for the Calaveras fault producing such a temblor was placed at 18 percent.

“Historic records tell us that we should expect earthquakes in the magnitude 5.5 range every decade or so in the Bay Area,” Department of Conservation seismologist Tousson Toppozada said. “There hasn’t been anything of that size in the Bay Area since the magnitude 6.9 Loma Prieta earthquake in 1989.”

The northern section of the Calaveras fault ruptured in a magnitude 5.5-6 earthquake in 1861. Surface cracking of 1.5 miles occurred, and there were reports of minor damage in what was then a sparsely populated area. That earthquake reportedly was felt as far away as Sacramento, Petaluma and Santa Cruz. That 1861 earthquake was not part of an earthquake swarm. Slightly larger earthquakes have occurred on the Calaveras fault further to the southeast. The most recent was a magnitude 6.2 earthquake in 1983 that caused $8 million in damage in a sparsely populated area of Morgan Hill.

The Department of Conservation’s California Geological Survey studies earthquakes to help Californians plan and build earthquake-resistant communities. CGS maintains a network of strong ground motion seismographs that combined with instruments maintained by Caltech, UC Berkeley and the USGS forms the California Integrated Seismic Network (CISN). This newly expanded, combined network allows scientists to study the ground motions from earthquakes, leading to advances that result in more earthquake-resistant structure.

CGS also studies the distribution of historic earthquakes and evaluates faults, combining that information to prepare maps showing the potential for ground shaking, fault rupture, liquefaction and seismically induced landsliding.

On December 20, CGS will release the newest Seismic Hazard Zone maps for Alameda County. These maps impact the planning community, developers, property sellers and real estate agents. If property is located in a zone of required investigation, where liquefaction or landslides could occur during a large earthquake, the local building department must require geologic studies before projects are permitted. Also, property sellers and real estate agents must inform potential buyers if property they're selling is in a zone, as is the case when property is in a designated flood or wildfire zone.

In Contra Costa County, Alquist-Priolo Earthquake Fault Zones were established by CGS along the Hayward, Calaveras and Concord faults in the 1970’s, and revised in the last two decades. These maps define zones to ensure that structures for human habitation are set back from known faults. Because of this zoning, much of the development in the San Ramon area has included detailed studies so that buildings are not built directly over the Calaveras fault. This will greatly reduce the damage from a major ground-rupturing earthquake.

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