Underwater Plate Cuts 400-Mile Gash

The magnitude 8.8 earthquake that struck off the coast of Chile early Saturday morning occurred along the same fault responsible for the biggest quake ever measured, a 1960 tremor that killed thousands in Chile and hundreds more across the Pacific. Both earthquakes took place along a fault line where the Nazca tectonic plate, the section of the earth’s crust that lies under much of the Eastern Pacific Ocean south of the Equator, is sliding beneath another section, the South American plate. The two are converging at a rate of about three and a half inches per year.  Earthquake experts said the strains built up by that movement, plus the stresses added along the fault line by the 1960 quake and smaller ones in the intervening years, led to the rupture on Saturday along what is estimated to be about 400 miles of the fault. The quake generated a tsunami, with wave heights of about five feet recorded along the Chilean coast and larger waves forecast for Hawaii and elsewhere in the Pacific. The quake was centered about 140 miles north of the center of the 1960 earthquake, which ruptured more than 650 miles of the fault and was measured at magnitude 9.5. The fault is a thrust fault, in which most of the ground motion during a quake is vertical. Experts said the earthquake appeared to have no connection to a magnitude 6.9 quake that struck off the southern coast of Japan on Saturday. The Chilean event also had no connection to the magnitude 7.0 quake that occurred in Haiti on Jan. 12. That quake, which is believed to have killed more than 200,000 people, occurred along a strike-slip fault, in which most of the ground motion is lateral.