Strain of plate sliding off Chile caused long rupture

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 1/2 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 yesterday along what is estimated to be about 400 miles of the zone. The quake generated a tsunami, with small waves forecast to hit the West Coast of the United States, and larger ones for Hawaii and elsewhere in the Pacific.

Jian Lin, a geophysicist with the Woods Hole Oceanographic Institution, said that the quake occurred in an area just north of the site of the 1960 earthquake, with very little overlap. “Most of the rupture today 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 the area where the quake occurred yesterday. Although there had been smaller quakes in the area in the ensuing 50 years, Lin said, none of them 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 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. The Chilean earthquake occurred along a thrust fault, in which most of the motion is vertical.

Lin said his calculations showed that the quake yesterday was 250 to 350 times more powerful than the Haitian quake.

Paul Caruso, a geophysicist with the US Geological Survey in Golden, Colo., noted that at least on land, the effects of the Chilean tremor might not be as bad. “Even though this quake is larger, it’s probably not going to reap the devastation that the Haitian quake did,’’ he said.

For one thing, he said, the quality of building construction is generally better in Chile than in Haiti. And the fact that the quake occurred offshore should also help limit the destruction. In Haiti, the rupture occurred only a few miles from the capital, Port-au-Prince. The rupture yesterday was centered about 200 miles southwest of the Chilean capital, Santiago, and 60 miles from the nearest town, Chillan.

In many respects, Lin said, the Chilean quake is similar to the 9.0-magnitude Indonesian earthquake of Dec. 26, 2004. 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 part of the fault zone.

When they occur underwater, thrust-fault earthquakes are far more likely to create tsunamis than quakes on strike-slip faults, said David Schwartz, of the geological survey in California.

That is what occurred in 1960. A tsunami generated by that quake devastated Hilo, Hawaii, killing 61 people. Hilo is vulnerable to tsunamis because its narrow bay funnels the water, increasing wave heights, which in 1960 reached 35 feet. But the tsunami also reached as far as Japan, hitting northern parts of the main island, Honshu, and killing 185 people and destroying more than 1,600 homes.