Exact Intensity, Epicenter Unclear : Data Problems Frustrate Experts Studying Quake

The earthquake that struck San Salvador Friday was probably a very shallow temblor centered somewhere in the intricate maze of faults that underlie the city, U.S. seismology experts said.

It may be days or longer before scientists locate the epicenter precisely, experts cautioned, or have an exact measure of the quake’s intensity, which was initially measured at 5.4 on the Richter scale.

Furthermore, scientists fear that they may never have very good information from the site itself because the Salvadoran earthquake-monitoring network has been strapped for funds. A shortage of chart paper for their seismographs may have prevented scientists from recording data when the quake struck.

U.S. scientists have not yet been able to contact their Salvadoran counterparts for further information, and first reports from the region have varied widely.

Seismologists from the U.S. Geological Survey in Menlo Park left Saturday for El Salvador to install a portable monitoring network in hopes of measuring aftershocks that should reveal the precise location of the quake’s epicenter.

Virtually all of Central America is seismically active, according to geologist Lloyd Cluff of Pacific Gas & Electric, who studied the region extensively before joining the utility. One major feature of El Salvador is a so-called subduction zone along its western coast--the region where the Cocos tectonic plate is sliding under the Caribbean plate.

Massive earthquakes, typically with a magnitude of 7.0 or higher on the Richter scale occur when the plates, which are normally locked together by friction, shift abruptly. The 8.1 quake that ravaged Mexico City in 1985, for example, was caused by slippage of the Cocos plate under the North American plate.

Subduction events normally occur deep within the Earth and cause damage over a widespread area. Because the damage Friday seems to have been limited primarily to the area around San Salvador, however, experts think the quake belonged to a second category that is caused by stress and deformation within the Cocos plate itself.

Just east of the Central American subduction zone lies a string of volcanoes and associated faults that stretches from Guatemala through Nicaragua to Costa Rica and that are caused by such deformation.

Quakes associated with this fault system are generally located very close to the surface of the Earth, usually have a magnitude below 7.0 and cause damage only in a limited area. The Dec. 23, 1972, temblor that killed 6,000 people in Managua, Nicaragua, was such a quake.

If an imaginary line were drawn through the center of the Central American volcanoes, it would pass directly through San Salvador.

Scientists thus believe, Cluff said, that Friday’s temblor occurred on the local fault system and was very similar to the Managua temblor.

First reports from the region have not been consistent. Costa Rican scientists said the quake was centered about 55 miles northwest of San Salvador and had a magnitude of about 7.0. Guatemalan scientists said it was centered about 6 miles northeast of San Salvador and had a magnitude of about 6.5.

Those disparities arise because most Central American earthquake networks were created to provide information primarily about nearby earthquakes, according to Ed Malaressi, a former director of the Costa Rica seismology network who is now a graduate student at UC Santa Cruz.

“The farther these networks are away from the earthquake center,” he said, “the less accurate their results are.”

Monitoring systems such as those at Caltech, in contrast, are designed to study quakes that are distant from the system. Normally, the U.S. National Earthquake Information Center in Colorado would combine data from such locations around the world and calculate the precise location and magnitude of the temblor.

But the data about Friday’s quake is taking a long time to come in, Malaressi said, which suggests that the San Salvador event was “not an ideal earthquake to read.”

Malaressi also noted that the Salvadoran earthquake network “has very serious budgetary problems.” U.S. scientists speculate that Salvadoran scientists may not have been able to record Friday’s quake because they could not afford chart paper for their seismographs.

If that is the case, much valuable data will have been lost.