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NR 2001-40
April 18, 2001

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

SAN FRANCISCO BAY AREA'S PERIOD OF LOW SEISMIC ACTIVITY
MAY END SOON IF HISTORIC PATTERN HOLDS
 

SAN FRANCISCO -- If historical trends are an indication, the San Francisco Bay Area -- relatively quiet since the 1989 Loma Prieta earthquake -- is likely to experience potentially damaging earthquakes in the next four to nine years, according to research conducted by the California Department of Conservation's Division of Mines and Geology.

In a paper presented Wednesday at the Seismological Society of America meeting, seismologist Tousson Toppozada, an expert on California earthquake history, noted that there has been a distinctive pattern associated with the four major earthquakes in the Bay Area since 1800. That pattern indicates the possibility of increased activity 15-20 years after the magnitude 7.0 Loma Prieta earthquake.

"We must be clear: This is not a prediction of a specific earthquake," Toppozada said. "At the same time, it is prudent to realize that we will experience activity in the magnitude 5.5 to 6 range again sometime in the near future.

"We really don't know how long the current quiescence in the San Francisco Bay Area can last, but we know it can't last forever. If the past is any indication of the future, we can expect some potentially damaging earthquakes within the decade, possibly by 2004."

Toppozada led a team that researched the pre-instrumental (generally, before 1942) earthquake history of California using newspaper accounts, documents from Spanish missions, letters and diaries. By analyzing and mapping the effects of an earthquake as recorded in those documents, Toppozada and his coworkers were able to compare the historic temblors to earthquakes recorded by modern instruments and determine the approximate epicenters and magnitudes.

Major earthquakes occurred in the Bay Area in 1838 (magnitude 7.4 on the San Andreas fault), in 1868 (magnitude 7.0 on the Hayward fault) and 1906 (the magnitude 7.8 San Francisco earthquake on the San Andreas fault) as well as 1989. Each major earthquake was preceded by a seismically active period at the magnitude 5.5 to 6.5 level and was followed by a period of low seismic activity. The larger the earthquake, the more likely it is to relieve regional stress, leading to longer seismically quiet periods. There were 18 years of low seismicity after the 1838 earthquake and 50 relatively quiet years after the San Francisco earthquake.

Toppozada points out that the 1868 Hayward and 1989 Loma Prieta earthquakes are the most similar of the four major temblors. They were about the same size and the activity preceding them was similar -- 10 magnitude 5.5-6.5 temblors in the 13 years preceding the 1868 event, seven in the 20 years leading up to Loma Prieta.

The 1868 earthquake was followed by about 13 to 15 relatively quiet years. There have been only two magnitude 5.1-5.4 earthquakes in the greater Bay Area since Loma Prieta, south of San Juan Bautista in 1998 and in Napa last year. According to Toppozada, the similarities between the 1868 earthquake and Loma Prieta suggest an increased likelihood for Bay Area seismic activity in the magnitude 5.5-6.5 range within the next few years. Toppozada also pointed out that there are differences between Loma Prieta and the 1868 temblor, most notably that they occurred on different faults.

"The pattern is evident, but certainly not definitive," he said.

"This report is not a signal to panic, but to prepare," DOC Director Darryl Young said. "Earthquakes are simply a fact of life in California, and the increased likelihood of potentially damaging earthquakes in light of this research emphasizes the importance of preparedness and emergency response activities in this region."

Young encouraged local governments to review their earthquake response plans and the strengthening of buildings that are vulnerable to earthquake damage.

In addition to its program to identify and map seismic hazards, the Department of Conservation manages California's earth resources through its programs that safeguard farmland and open space; oversee oil, gas and geothermal wells; ensure mined land reclamation; study earthquakes; and promote beverage container recycling.

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