San Salvador: small quake, big problems.

San Salvador: Small quake, big problems

Last year's San Salvador earthquakeprovided a graphic demonstration of how local geology can turn a relatively small seismic event into a killer. Researchers who studied the quake also said at last week's Seismological Society of America meeting in Santa Barbara, Calif., that the lesson may go beyond the city of San Salvador: Sections of San Francisco have similar geology, suggesting that a small quake in the Bay Area could do more damage than is currently suspected.

The meeting provided a forum for thefirst full scientific postmortem of the Oct. 10, 1986, earthquake, which had a magnitude of 5.4. The quake occurred when two of the tectonic plates that compose the earth's outer shell suddenly slipped past each other 8 kilometers underground, just beneath the edge of town. Though its strength was not sufficient to rupture the ground, the event nevertheless released enough energy to cause landslides and topple buildings, killing 1,500 people. Another 10,000 suffered injuries, and a quarter of a million people were left homeless.

While 5.4 magnitude quakes are likelyto be felt by anyone in the vicinity, they generally don't damage buildings. There were several such quakes in California in 1986, and they caused few problems.

The root of the San Salvador quake'sdestructiveness traces back to several local volcanic eruptions, ending in A.D. 260, which spewed a layer of ash that collected in deposits up to 25 meters thick, says Michael J. Rymer of the U.S. Geological Survey (USGS) in Menlo Park, Calif. When the 3-second-long quake hit last year, the ash amplified the waves to four or five times the strength they had when traveling through solid rock, according to Rymer.

Certain parts of San Francisco, especiallylandfill areas, are composed of loose, rocky soil that could similarly amplify the power of a small quake, Rymer says.

Amplification was also critical in amagnitude 8.1 quake in 1985, which traveled several hundred kilometers from its epicenter to inflict severe damage on Mexico City (SN: 9/28/85, p.196; 10/5/85, p.214). While that quake showed that a seismic wave from a strong quake can travel vast distances and become destructive due to local amplification, the San Salvador quake showed that amplification from a local, weak quake can also be damaging, says the USGS's Randall A. White.

A second factor in the damage wasthe frequency of the earthquake wave in San Salvador. Whereas the Mexico City quake affected buildings five to 20 stories high, the higher frequency of the San Salvador seismic wave destroyed one- to five-story buildings, the predominant type in that city.

The USGS researchers were helped intheir San Salvador analyses by a fortuitous placement of equipment. Scientists had put in measuring devices two years before the quake, and there were six monitors operating within 6 kilometers of the quake's center. "I don't think we've ever had one that close to the instruments,' says White. "The formulas predicted many things, but it was unusual to finally get the data.'