Spring earthquakes rattle the globe.

Spring earthquakes rattle the globe

Planet Earth seems restless. In the last five weeks, nine strong earthquakes have wracked various spots on the globe.

In the deadliest event, a magnitude 6.4 shock killed more than 100 people in northern Peru. While initial reports of the May 29 temblor placed its epicenter offshore, newer information indicates it actually centered near the town of Moyobamba on the eastern flank of the Andes mountains, geophysicist Darrell Herd told SCIENCE NEWS. In terms of plate tectonics, the Peru earthquake results from the subduction of the Pacific plate as it dives underneath the South American plate and pushes the Andes up the Amazon basin.

Herd, chief of the Latin American branch at the U.S. Geological Survey in Reston, Va., attributes the large number of deaths to the local architecture and possibly to quake-generated landslides. Many people in the area live in adobe houses that readily collapse under shaking, he says. Geologists and relief workers have had trouble reaching the remote region, which is beset by drug traders and members of the Shining Path revolutionary group.

In central Africa, Sudan suffered its largest known earthquake, a magnitude 7.2 shock that struck the southern section of the nation on May 20 and was followed four days later by aftershocks with magnitudes of 7.0 and 6.6. Despite their strength, the quakes apparently caused minimal damage in this sparsely populated region, says Badaoui Rouhban of the United Nations Educational, Scientific and Cultural Organization in Paris, France. "The U.N. mission that went there was unable to detect serious damage," he told SCIENCE NEWS.

The quakes in the Sudan probably relate to the East African rift zone, a tectonic structure that is slowly pulling apart the eastern margin of the continent. However, geoscientists remain unsure about the shock's tectonic significance because they occurred in a seismically quiet region on an ill-defined section of the rift zone, says Bruce Presgrave of the National Earthquake Information Center (NEIC) in Golden, Colo. Researchers hope to learn more about the behavior of the rift zone in this region as information on the Sudan earthquakes trickles in from seismic stations around the world.

According to seismologist Adam M. Dziewonski of Harvard University, May's most puzzling quake originated deep within the planet, about 610 kilometers below the island of the Sakhalin in the western Pacific. Earthquakes this deep normally occur in a subducting piece of the ocean floor. But the May 12 shock was an isolated event some 200 km west of the seismically active zone that defines the downward-thrusting Pacific plate in that area.

Geophysicists debate what ultimately happens to subducting slabs. Some argue that the diving plates reach a boundary at about a depth of 700 km and flatten out, while others suggest they plunge even deeper. The recent deep quake, however, does not quite fit either model, raising new questions of what is going on deep beneath subduction zones, Dziewonski says.

Recent temblors have also shaken Romania (magnitude 6.5 and 6.0 on May 30 and 31), the Philippines (6.9 on June 14) and Kazakhstan in the Sovient Union (6.8 on June 14). The apparent clustering has led some to wonder whether the number of quakes worldwide is increasing, says Waverly Person of NEIC. But Person maintains these quakes only seem more numerous because many have rocked populated areas where they attract widespread attention. In fact, the last few years have proved relatively quiet, seismically speaking. Since 1900, quake experts have recorded about 18 events each year measuring magnitude 7.0 or larger, but the past decade brought only 11 such shocks a year on average, notes Person. As for great quakes - those above magnitude 8.0 -- the century's statistics show about 10 per decade, but the 1980s had only four.