Reading Between the Fault Lines

Repeated patterns of seismic activity in the Bay Area over the past two centuries point to the likelihood of a resumption of damaging earthquakes there in the near future, according to a new study.

The author of the study, Tousson R. Toppozada, a leading seismologist with the state Division of Mines and Geology, believes that there is a strong possibility of quakes up to 6.5 magnitude sometime in the next few years.

"It is prudent to realize that we will experience activity in the magnitude 5.5 to 6.5 range again sometime in the near future," Toppozada said at a recent meeting of the Seismological Society of America in San Francisco "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's theory is based on a review of seismic activity that shows that the four largest earthquakes in the Bay Area since 1838 were preceded by moderately strong quakes and followed by relative calm.

Scientists believe that the largest quakes--magnitude 7.0 and above--relieve stress and bring stability for a period of years. The experts say they do not fully understand the mechanics of seismic stress. But the record in the Bay Area indicates that eventually the pressure builds to the point where a series of moderately strong quakes culminate in a big one.

Toppozada said that the precursor shocks, usually about 6.0, signify that an area is vulnerable to a much larger seismic event.

The San Francisco quake of 1906, with an estimated magnitude of 7.8, was the largest studied by Toppozada. The 1906 quake was the last powerful jolt in the Bay Area for more than 60 years.

A 5.8 temblor in Santa Rosa in 1969 ended the period of quiescence, but it took the much larger 1989 Loma Prieta quake, south of San Jose, to relieve the stress and usher in another period of calm.

There were similar, but shorter, periods of seismic quiet after the two other major events studied by Toppozada: the 7.4 earthquake centered south of San Jose in 1838 and the 7.0 quake at Hayward in the East Bay in 1868.

There has been a 12-year period of quiet since the Loma Prieta quake, but it is not expected to last as long as the calm that followed the 1906 quake.

Toppozada said that, in general, the larger the quake, the longer the subsequent period of seismic quiet. It stands to reason, he said, that the 7.8 San Francisco quake relieved more pressure than the 7.0 Loma Prieta quake.

The belief that patterns of seismic activity will recur has not always been borne out.

Scientists with the U.S. Geological Survey said in 1985 that, based on 19th and 20th century quake records, they could predict, with 95% certainty, that a quake of about magnitude 6 would take place by early 1993 near Parkfield, on the San Andreas fault in Monterey County. No such quake has yet occurred.

The historic record, however, does show that several major quakes in the Bay Area were preceded by a number of strong temblors.

For instance, there were 10 quakes with magnitude 5.5 to 6.5 in the 13 years preceding the 1868 Hayward quake, 13 quakes in the 26 years before the 1906 San Francisco earthquake and seven in the 20 years leading up to the Loma Prieta quake.

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

No one, he said, can give a precise date for an end to the present seismic quiet and a resumption of strong activity, but the broad parameters are clear.

Darryl Young, director of the state Department of Conservation, the parent agency of the Division of Mines and Geology, said Toppozada's concerns should be taken seriously.

"This report is not a signal to panic, but to prepare," 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."

In another report at the Seismological Society meeting, David Schwartz of the Geological Survey reiterated his estimates that there is about a 70% chance of a major quake in the Bay Area in the next 30 years.

According to the Working Group on California Earthquake Probabilities, which Schwartz led in 1999, the chances of a major earthquake occurring on the Hayward fault are greater than on the San Andreas.

The entire Hayward fault system extends north from San Jose through Oakland and Berkeley and north to the Rodgers Creek fault in Sonoma County.

Schwartz put the chance of a rupture somewhere on the Hayward fault system at 39% in the next 30 years.

Toppozada said cycles of seismic activity have not been discerned in the Los Angeles area.

Los Angeles has experienced little seismic activity since the 6.7 Northridge quake seven years ago. But Toppozada said no conclusions can be drawn from such a comparatively short period of calm.