A scientist is warning that residents of the earthquake-ravaged Caribbean coasts of Costa Rica and Panama should be prepared for an aftershock that could be almost as strong as the original quake.

A jolt measuring 7.4 on the Richter scale shook the area Monday, killing scores of people, injuring hundreds more and leaving thousands homeless.Karen McNally, director of the Charles F. Richter Seismological Laboratory at the University of California, Santa Cruz, said "structures which are weakened but still standing . . . could collapse very easily" if a powerful aftershock occurs.

McNally's laboratory and Costa Rica's National University jointly operate a seismic monitoring system.

In a telephone interview Tuesday, McNally emphasized she wasn't specifically predicting a major aftershock, only stating that it is a possibility based on the area's seismic history.

On April 24, 1916, the same area experienced a magnitude-7.4 quake, followed two days later by an aftershock measuring 7.1 - timing that "leaves us uneasy" about the possibility a large aftershock may be imminent, McNally said. She had no information on casualties from the 1916 quakes.

An earthquake of magnitude 7 on the Richter scale is considered "major" and capable of widespread, heavy damage. The Richter scale is a gauge of the energy released by an earthquake, as measured by the ground motion recorded on a seismograph.

Costa Rican scientists have named Monday's quake the Valle de la Estrella (Valley of the Star) earthquake after a valley near the epicenter, McNally said.

The quake resulted from forces that are squeezing and cracking a plate of the Earth's crust that carries Central America, said McNally and David Harlow, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif.

Under the well-accepted theory known as continental drift or plate tectonics, Earth's crust is broken into giant plates of rock that slowly drift across the face of the planet.

Much of Central America and the Caribbean Sea sit atop the Caribbean plate. The plate is bounded by central Guatemala on the north and the northern edge of South America on the south.