San Francisco Bay area quake odds go up, scientists say

Thursday, October 14th 1999, 12:00 am
By: News On 6

OAKLAND, Calif. (AP) -- There is a 70 percent chance that a large
earthquake will shake the San Francisco Bay area in the next 30
years, according to a new federal study released today.

The new conclusion: There are a number of faults slicing through
heavily developed areas around San Francisco, more chances they
will rupture and more people who will be affected when they do.

The estimate from the U.S. Geological Survey is slightly greater
than that found by a 1990 study because it spreads the earthquake
hazard over a much broader region. It also lowers the bar for a
large earthquake.

The study lowered the definition of a large earthquake to
magnitude 6.7 to match the strength of the 1994 Northridge quake,
which killed 57 people and caused $20 billion in damage in Southern
California. The 1990 study had estimated a 67 percent chance of a
magnitude 7.0 earthquake by 2020.

"The whole purpose of our research is to use science to
contribute to safer communities," USGS geologist David Schwartz
said. "Although we can't predict or prevent earthquakes, we can
all prepare for them."

His team of 70 scientists developed a new set of computer models
that considered the interaction of faults.

The study looked at the earthquake hazard from the Pacific Ocean
to the Sacramento Delta about 40 miles inland. The area has seen
rapid development since 1989, when the Loma Prieta earthquake shook
the area with a 6.9 magnitude quake just as a World Series game in
San Francisco was about to start. That quake killed 67 people and
caused $6 billion in damage.

The highest odds for a specific fault line -- 32 percent -- was on
the Hayward-Rodgers Creek fault, which stretches from Fremont to
Santa Rosa, cutting through the heavily populated East Bay area.

The southern section of the fault last ruptured in 1868. The
northern section has not ruptured since between 1640 and 1776,
making it overdue for a major shakeup.

The San Andreas fault, which runs from San Jose to north of San
Francisco, had a 21 percent probability of a large quake.

The rate of large quakes was high in the late 1800s but dropped
after the catastrophic 1906 San Francisco earthquake, probably
because the San Andreas fault slipped so much that strain was
reduced over most faults in the region, the study found.

Strain on those faults has been slowly building up and strong
quakes began to occur in the 1980s, although not yet at the level
of a century ago.