Underwater Plate Cuts 400-Mile Gash
Published: February 27, 2010
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 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 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 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 on Saturday along what is estimated to be about 400 miles of the zone, at a depth of about 22 miles under the sea floor. The quake generated a tsunami, with small surges hitting the West Coast of the United States and slightly larger ones in Hawaii and other parts of the Pacific. A 7.7-foot surge was recorded in Talcahuano, Chile.
Jian Lin, a geophysicist with the Woods Hole Oceanographic Institution, said the quake occurred 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 Mr. 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 Saturday. Although there had been smaller quakes in the area in the ensuing 50 years, Mr. 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 evening. Nor was the Chilean event linked 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.
Mr. Lin said his calculations showed that the quake on Saturday was 250 to 350 times more powerful than the Haitian quake.
But Paul Caruso, a geophysicist with the United States 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 on Saturday was centered about 60 miles from the nearest town, Chillan, and 70 miles from the country’s second-largest city, Concepción.
In many respects, Mr. 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, just 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. “When they slip, the fault that causes the earthquake breaks the surface, and pushes the water up,” he said. “It pushes an awful lot of water. And that water has to go somewhere.”
The waves the quake produces travel across the ocean at high speed. Along the way, their height can be measured by buoys linked by satellite. But the height of the waves when they make landfall, and their potential for destruction, often depends on local topography and the profile of the nearby sea floor. A shallow shelf, for example, can amplify the waves.
The tsunami that was generated by the 1960 quake devastated Hilo, Hawaii, killing 61 people. Hilo is particularly vulnerable to tsunamis because its bay and narrow harbor funnel the water, increasing wave heights, which in 1960 reached 35 feet. But the tsunami also struck as far as Japan, hitting northern parts of the main island, Honshu, about a day after the quake and killing 185 people and destroying more than 1,600 homes.