Because no seismometers were stationed close to the event, researchers have had trouble pinpointing the quake's exact location. Although the USGS' National Earthquake Information Center in Golden, Colo., placed the epicenter onshore, four other teams concluded that it occurred offshore, within 19 to 32 kilometers of the coast, says Stuart Weinstein, a geophysicist at the Pacific Tsunami Warning Center in Ewa Beach, Hawaii. Harvard researchers calculated that the quake had a magnitude of 7.1 and originated at a depth of 15 km. A quake that starts close to the ocean floor can raise or lower the sea bottom much farther than a deeply buried quake. This displaces more water and spawns larger waves.
Tsunamis bear little resemblance to the breaking waves familiar to beachgoers and surfers.
Moving unobtrusively through the open ocean at speeds of about 800 km per hour, tsunamis hit the coastline like an instantly rising tide, says Synolakis. In 1992, an initially unimpressive earthquake unleashed a series of giant waves that devastated coastal Nicaragua and taught researchers new lessons about tsunamis. Seismologists call this a slow earthquake. The leisurely pace was especially efficient at moving water, and so it pumped up the size of the tsunami. Andrew V. Newman of Northwestern University in Evanston, Ill., has evidence that the July earthquake off Papua New Guinea was also slower than normal.