Earthquake leaves Bay area still vulnerable.

Earthquake leaves Bay area still vulnerable

The destructive quake that killed hundreds of people in the San Francisco Bay area this week has not relieved most of the stress stored in the Earth there, leaving the region threatened by an even more destructive shock that may strike anytime in the next few decades, experts warn.

"This is not the big earthquake," says David Russ, assistant chief geologist at the U.S. Geological Survey (USGS) in Reston, Va. In the long run, Russ says, scientists remain concerned over the threat from the San Andreas and other faults in heavily populated parts of the Bay area.

The Oct. 17 jolt, which registered a magnitude of 6.9 on the Richter scale, broke the section of the San Andreas fault that runs through the Santa Cruz mountains, about 80 kilometers southeast of the city of San Francisco and 16 km northeast of Santa Cruz. Seismologists estimate the quake was centered approximately 15 km below the surface.

More than 1,000 km long, the San Andreas fault is divided into sections, and scientists believe that earthquakes break only one or a few connected sections at a time. The Santa Cruz mountain portion, 35 km long, was the southernmost to move in the great 1906 San Francisco earthquake. That shock also broke two sections to the north, including one that runs just west of San Francisco.

Russ says he is not aware of any precursory signs appearing in the days before this week's jolt. However, scientists had been keeping close watch on this part of the San Andreas. Within the last 17 months, the region generated two smaller shocks: a magnitude of 5.2 on Aug. 8, 1989, and a magnitude 5.1 in June 1988.

Coming after many decades of quiescence, those two temblors signaled that strain was again building on this section and that a larger quake might be imminent (SN: 8/19/89, p. 119). In 1988, a group of experts issued a report for the USGS in which they calculated a 30 percent probability that this section of the San Andreas would cause a magnitude 6.5 quake within the next 30 years.

Although this week's quake was the largest in the area since 1906, it did not release the energy stored in sections of the San Andreas closer to San Francisco or in other Bay area faults. These great fractures in the Earth mark an area where the northwest-moving Pacific crustal plate grinds past the North American plate. Instead of sliding smoothly past each other, the plate edges stick together until enough stress builds to rupture the faults, causing an earthquake.

One of the most threatening faults, called the Hayward, runs east of San Jose and northward through the cities of Oakland and Berkeley. "Segments of the Hayward fault remain some of our highest concern. It's been quite a long time since we've had a damaging earthquake there," Russ says.

The USGS report last year estimated a 50 percent chance that the Hayward or the San Francisco section of the San Andreas would generate a magnitude 7 earthquake in the next 30 years. Since these faults run near large cities, they could cause even more damage than this week's shock.

The USGS report says southern California faces an even higher chance of a large earthquake in the next few decades. There is a 60 percent probability that the southern San Andreas will cause a magnitude 7.5 to 8 earthquake in the next 30 years. Because the magnitude scale is logarithmic, a magnitude 8 quake is 10 times stronger than a magnitude 7.

In the next few weeks, as the Earth readjusts itself in the wake of the main shock, thousand of aftershocks, ranging from imperceptible to perhaps as large as magnitude 6 will shake the Santa Cruz area. Some of the larger ones may further damage already-weakened structures. USGS seismologists are setting out portable recording instruments to track the aftershocks, which will help them define the specific region that slipped during the main shock.