Danger of S.F. ‘Bay Mud’ in Quake Told

Data from the 1985 Mexico City earthquake indicates tougher building codes are required to deal with an unusually viscous clay ringing San Francisco Bay and supporting the city’s dense, high-rise financial district, a UC Berkeley professor told the state Seismic Safety Commission on Thursday.

Civil engineering professor H. Bolton Seed said the clay, known misleadingly as “Bay mud,” is similar to soil beneath the section of Mexico City most damaged by the Magnitude 8.1 temblor that killed an estimated 10,000 people.

San Francisco, he said, is the only earthquake-prone urban area in the United States where this type of clay supports high-occupancy buildings. He said he knows of no similar clay near metropolitan Southern California, which scientists say is statistically more likely to have a major quake before San Francisco.

Seed stressed that even on the suspect clay, existing building codes are probably adequate to minimize structural damage in a 7.2 temblor, which is roughly equivalent to the 1971 Sylmar quake.

He added, however, that a Magnitude 8.2 earthquake, equivalent to the great 1906 San Francisco temblor, would probably produce damage levels similar to those seen in Mexico City. Most at risk in such an unlikely event, he said, would be mid-rise buildings between 10 and 20 stories. Because of their middling height, he said, they would be most susceptible to quake-generated vibrations.

Such a powerful earthquake is unlikely in the near future, Seed added. More likely, he told the commission, is a Magnitude 7.2 temblor. The Bay Area has a 50% chance of experiencing a quake of that strength over the next 30 years, he said.

Commission Chairman Lloyd Cluff, a geoscientist at Pacific Gas & Electric Co., said the effect of a major earthquake on the clay described by Seed “could be catastrophic” but added he thinks that building codes can be rewritten to avoid a disaster.

“It (Seed’s study) is not telling us everything is a mess,” he said. “It is telling us we need to look at what we’re doing.”

In particular, Cluff said the commission will reexamine certain structures, such as concrete-frame buildings, that if built on this type of clay “may not do as well (in an earthquake) as we had hoped.”

Before the devastating Mexican temblor, Seed had postulated that waterlogged clay would amplify the ground shaking precipitated by earthquakes and transmit much more stress to buildings than anticipated. He, Cluff and other earthquake experts meeting in San Francisco, said an analysis of Mexico City seismographs and damage inventories confirmed his thesis.

“Their clay is different from our clay,” he said, “but the difference is minor in terms of the things we are talking about.”

He added that the effects are likely to be exaggerated because San Francisco is between two major faults, while Mexico City was over 200 miles from the epicenter of its quake.

According to Seed, certain water-saturated clays--such as those along San Francisco Bay and on the bottom of the dry lake bed on which Mexico City is built--are especially susceptible to earthquake vibrations. The extent of this susceptibility was in doubt until the Mexico city temblor.

Seeds applied his clay theory to a U.S. Geologic Survey soil map of the Bay area and noted many tracts built atop Bay mud. However, the map did not show the clay layer’s thickness--a key variable in estimating quake danger--so a more complete soils survey is needed to fully assess the threat, he said.

One good thing was indicated by the map, he noted. Most of the suspect clay lies beneath undeveloped land.