Chile quake similar to 2004 Indian Ocean tremor

The magnitude-8.8 earthquake that struck off the coast of Chile early Saturday was a megathrust type and occurred along the same fault responsible for the biggest quake ever measured, a 1960 tremor that killed nearly 2,000 people in Chile and hundreds more across the Pacific.

Both earthquakes took place along a fault zone 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 3 ½ inches a year.

Earthquake experts said the strains built up by that movement, plus the stresses added along the fault zone by the 1960 quake, led to the rupture Saturday along what is estimated to be about 400 miles of the zone. The quake generated a tsunami.

Jian Lin, a geophysicist with the Woods Hole Oceanographic Institution in Massachusetts, said the quake occurred just north of the site of the 1960 earthquake.

"Most of the rupture [Saturday] picked up where the 1960 rupture stopped," said Lin, who has studied the 1960 event, which occurred along about 600 miles of the fault zone and was measured at magnitude 9.5.

Like many other large earthquakes, the 1960 quake increased stresses on adjacent parts of the fault zone, including where the quake occurred Saturday. Although there had been smaller quakes in the area in the ensuing 50 years, Lin said, none had been large enough to relieve the strain, which kept building up as the two plates converged. "This one should have released most of the stresses," he said.

Experts said the earthquake appeared to have no connection to a magnitude-6.9 quake that struck off the southern coast of Japan late Friday. The Chilean event also appeared to have no connection to the magnitude-7.0 quake that occurred in Haiti on Jan. 12.

The Haiti quake occurred along a strike-slip fault, in which most of the ground motion is lateral. The Chilean earthquake occurred along a thrust fault, in which most of the motion is vertical.

In many respects, Lin said, the Chilean quake is similar to the Indian Ocean earthquake of Dec. 26, 2004, which is variously rated as having a magnitude of 9.1 to 9.3.

That quake, which also occurred along a thrust fault, generated a tsunami that killed more than 200,000 people around the Indian Ocean.

And like the 1960 Chilean quake, the Indonesian quake increased stresses nearby: It was followed, three months later, by an 8.7-magnitude quake on an adjacent portion of the fault zone.

When they occur underwater, thrust-fault earthquakes like the one in Chile are far more likely to create tsunamis than quakes on strike-slip faults, said David Schwartz, an earthquake geologist with the geological survey in Menlo Park, Calif.

In a thrust-fault earthquake, "The fault that causes the earthquake breaks the surface, and pushes the water up," Schwartz said.

"It pushes an awful lot of water. And that water has to go somewhere."

Material from The Associated Press is included in this report.