Lake Bed ‘Vibrates Like Jello,’ Worsens Damage

Mexico’s devastating earthquake was caused by friction between two overlapping pieces of the Earth’s crust on the country’s Pacific Coast, the same forces that caused three major quakes there in the 1970s and have long made the region tremor-prone, seismic researchers said Thursday.

Although the epicenter of the earthquake appeared to be roughly equidistant from Mexico City and Acapulco, Mexico City suffered far worse damage because the inland capital sits “on an old lake bed of soft clay that vibrates like a bowl of Jello, accenting the vibrations,” said Paul Jennings, chairman of the Caltech’s division of engineering and applied science.

Mexico City “usually suffers when a quake occurs along the coast,” said Karen McNally, director of the Charles Richter Seismological Laboratory at UC Santa Cruz and a specialist on Mexican earthquake activity.

Kate Hutton, staff seismologist at Caltech, said the Cocos Plate, a piece of earth on Mexico’s west coast, is “being shoved down underneath the coastal part of the North American Plate. It’s that strain involved in the relative motion of those two plates that’s causing these quakes.”

The two plates form what is called a subduction zone, a configuration that differs from California’s San Andreas Fault. On the San Andreas, two continental plates “are moving horizontally with respect to one another,” Jennings said. The Mexican and California formations are “different seismologically, but in terms of the consequences to buildings and people, they’re very similar.”

Seismologists at the National Earthquake Center in Golden, Colo., were still trying to pinpoint the precise epicenter late Thursday. Early in the day, they reported that the epicenter was offshore, but later they said it appeared to be on the coast, at 18.1 degrees north latitude and 102.3 degrees west longitude. Other American scientists planned to travel to Mexico to locate and study the epicenter.

Mexico’s two most recent major quakes occurred in a four-month period. In November, 1978, five coast-centered quakes--the largest registering 7.9 on the Richter scale--rocked Mexico City in a 15-hour period, killing at least nine and injuring about 500. Two older buildings collapsed and at least 750 others sustained damage. In March, 1979, another 7.9 earthquake killed one person and injured 21.

The nation’s most deadly quake occurred in 1973, when 527 people died in regions south and east of Mexico City.

Roger Scholl, technical director of the Berkeley-based Earthquake Engineering Research Institute, which sent investigators to Mexico to study the impact of the 1978 and 1979 quakes, said Thursday’s damage appeared considerably worse, based on early news reports.

Santa Cruz’s McNally, who had gone to Mexico in 1978 because she expected a major quake and had monitoring equipment in place when it occurred, said Mexico’s earthquake-preparation consciousness “fluctuates with other priorities of their government.”

Scholl said Mexico “is a relatively earthquake-sophisticated country. I don’t think their construction requirements are quite as good as ours, but as far as their general knowledge, they have a lot of capable people down there.”

Caltech’s Jennings, who has also studied the damage done to Mexico’s buildings by past earthquakes, said a number of newer Mexico City structures have been equipped with “strong-motion instruments,” designed to measure the stress a quake puts on a building.

“The population flood has meant a lot of buildings have gone up that are not as good as they might have hoped, but there are some very good buildings,” Jennings said.