Oddball quake in Costa Rica.

At first glance, the seismic shock that ravaged parts of Costa Rica and Panama last month appears misplaced. Major earthquakes in Central America usually occur along the Pacific coast or in the volcanic highlands, says James Dewey of the National Earthquake Information Center (NEIC) in Golden, Colo. But the magnitude 7.6 quake on April 22 hit neither of these regions, striking instead on the eastern side of Costa Rica.

Geophysicists believe the Pacific coast most often spawns intense shocks because it is here that a piece of the ocean floor, the Cocos plate, crashes into and dives underneath Central America. This disappearing act -- called subduction -- also feeds the chain of volcanoes running like a twisted spine between North America and its southern sister. The puzzling April earthquake, however, occurred far from the subduction zone, apparently ruling out a direct connection, says Bruce Presgrave of NEIC.

An explanation for this seismic conundrum may lie under the waves of the Pacific ocean along a plateau called the Cocos Ridge. Sitting on the Cocos plate, the raised region is being jammed down into the subduction zone. Yet the ridge appears unwilling to plunge underneath central America, Dewey says. That reluctant subduction, he suggests, may generate stress over an unusually broad region extending to Costa Rica's Caribbean coast, where the recent quake occurred. Records indicate that a similarly sized quake may have struck the same location in 1913. The recent temblor killed 80 people and left almost 70,000 persons homeless, according to NEIC.

Dewey cautions that scientists lack important information about the quake at this time, making it difficult to pin down an explanation. A clearer picture should emerge in the next few months as data arrive from seismic stations around the world.