Salvador Quake Viewed as Bad Omen for State

A moderate earthquake that killed 1,500 people in San Salvador in October should stand as a warning to parts of California because it proved that even small temblors are capable of doing great damage under certain conditions, a team of scientists reported Wednesday.

The San Salvador quake measured only 5.4 on the Richter scale but nevertheless caused much damage because its epicenter was only four miles below ground and because the predominant underlying geological structure is pumice from an ancient volcanic eruption.

In California, earthquakes also tend to hit close to the surface, and many of the heavily populated areas are remarkably similar geologically in that they consist of a loosely compacted sedimentary base, the researchers said at an earthquake conference in Santa Barbara.

While that warning may not be new, because scientists have long thought that mild quakes can be killers, what sets the San Salvador quake apart is that it gave scientists their strongest evidence yet to back up their fears.

“What’s exciting about this earthquake is we’re bringing home the numbers,” said David H. Harlow of the U.S. Geological Survey in Menlo Park. Harlow, who presented his findings to the annual meeting of the Seismological Society of America at the University of California, Santa Barbara, led a team that set up an extraordinary array of instruments almost right over the epicenter of the earthquake--two years before it struck.

“Normally, we get there after the quake,” added Randall A. White, also of the survey’s Menlo Park office.

The instruments included 11 seismographs within about three miles of the epicenter, and 10 instruments to measure ground motion within the city of San Salvador. The latter is important because ground motion is one of the most important causes of the failure of structures.

What their instruments revealed was that the quake caused the ground to “accelerate” far more than its size would have suggested, apparently because of the nature of the soil.

In another study presented here, geologist-consultant Rachel Gulliver said that, in the event of a catastrophic earthquake on the southern San Andreas Fault, between 250,000 to 400,000 dwellings would become unsafe, unsanitary or have their water or other utilities knocked out for three weeks or more.

Her estimates are higher than previous ones made by others because new research shows that areas with low water tables are more susceptible to ground failure than previously thought, according to Gulliver, a former Los Angeles building and safety commissioner.