Deep Rumblings, Part 2

The magnitude 8 earthquake that occurred this past February along the New Guinea Trench, about 45 miles north of Biak Island in the western Pacific, came as a complete surprise to Biak inhabitants and geologists alike. In principle the trench is far from inert: it lies at the boundary where the Australian and Pacific plates collide. The wrinkle of mountains that runs the length of New Guinea, with peaks as high as 15,000 feet, testifies to the power of the collision. Yet the 440-mile-long trench itself had never produced any major tremors. More perplexing still, seismic studies have shown no evidence that ocean crust is being subducted--thrust down into the hot mantle underlying the trench--which is the process that results in quakes at other deep-sea trenches. We see traces of subduction further to the east and west, but not in this particular area, says Northwestern University geophysicist Emile Okal. It may be a very young system, where subduction is just beginning to take place. Biak residents had reason to regret the geologic novelty: the February quake and the resulting 20-foot tsunami killed 100 and washed away more than 600 homes.