Bay area shock may foreshadow strong quake.

Bay area shock may foreshadow strong quake

While residents of the town of Los Gatos cleared away broken windows and mourned the young man who jumped to his death during last week's earthquake, a few seismologists sought to decipher a message from the temblor that shook the San Andreas fault 13 miles southwest of San Jose. Experts say the magnitude 5.2 quake indicates this region is storing stress that will someday generate a strong earthquake, although that event might lie decades in the future.

In June 1988, a magnitude 5 shock hit the same place along the fault, where it runs a broken complex course through the Santa Cruz mountains. The two tremors followed a 74-year-long quiet period during which this patch caused no jolts of magnitude 5 or greater. The segment is the southernmost part of the San Andreas that moved during the great 1906 San Francisco quake. To the east, the Calveras fault has spawned several magnitude 5 and magnitude 6 quakes in the last decade after a long quiet period.

Seismologist Allan G. Lindh of the U.S. Geological Survey in Menlo Park says the renewed activity fits a pattern researchers have observed in Japan and elsewhere. Called the seismic cycle, this pattern describes how faults enter a period of quiescence after a large earthquake releases most of the stress in the nearby crust. Quiet lasts until the relentless movement of the Earth's plates builds enough stress to generate moderate -- magnitude 5 -- quakes. After a period of such ruptures, which are too small to release much of the stress, another large quake resets the stress levels toward zero and the cycle resumes.

Historical records indicate the southern Bay area was quite active during the century preceding the 1906 rupture. In 1865, a strong shock estimated at about magnitude 6.5 hit the region that broke last week.

In 1981, Lindh, William L. Ellsworth and two colleagues observed that renewed activity in teh Bay area seemed to fit the seismic cycle model, suggesting the area was building toward a large quake. Lindh says the quakes of recent years, including last week's, leave little doubt in his mind.

It's not clear when or where the large Bay area quake will strike. But in an open-file report last year (SN: 7/16/88, p.37), the U.S. Geological Survey offered some assessment of the hazard, saying there was a 50 percent probability that a magnitude 7 shock would hit the area in the next 30 years.