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http://www.nytimes.com/2010/02/28/world/americas/28quake.html?pagewanted=print
February 27, 2010
Underwater Plate Cuts 400-Mile Gash
By HENRY FOUNTAIN
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.
Paul Caruso, a geophysicist with the United States Geological Survey in Golden, Colo., said the quake on Saturday generated about 500 times more energy than the Haitian quake, which was measured at magnitude 7.0. “But even though this quake is larger, it’s probably not going to reap the devastation that the Haitian quake did,” at least on land, Mr. Caruso said.
For one thing, he noted, the quality of building construction is 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 200 miles southwest of the Chilean capital, Santiago, and 60 miles from the nearest town, Chillan.
In some respects, the Chilean quake is similar to the 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.
When they occur underwater, thrust-fault earthquakes are far more likely to create tsunamis than tremors 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.”
That is what occurred in 1960. A tsunami generated by that quake devastated Hilo, Hawaii, with waves as high as 35 feet, killing 61 people. The tsunami reached as far as Japan, reaching northern parts of the main island, Honshu, about a day after the quake and killing 185 people and destroying more than 1,600 homes.
February 27, 2010
Underwater Plate Cuts 400-Mile Gash
By HENRY FOUNTAIN
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.
Paul Caruso, a geophysicist with the United States Geological Survey in Golden, Colo., said the quake on Saturday generated about 500 times more energy than the Haitian quake, which was measured at magnitude 7.0. “But even though this quake is larger, it’s probably not going to reap the devastation that the Haitian quake did,” at least on land, Mr. Caruso said.
For one thing, he noted, the quality of building construction is 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 200 miles southwest of the Chilean capital, Santiago, and 60 miles from the nearest town, Chillan.
In some respects, the Chilean quake is similar to the 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.
When they occur underwater, thrust-fault earthquakes are far more likely to create tsunamis than tremors 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.”
That is what occurred in 1960. A tsunami generated by that quake devastated Hilo, Hawaii, with waves as high as 35 feet, killing 61 people. The tsunami reached as far as Japan, reaching northern parts of the main island, Honshu, about a day after the quake and killing 185 people and destroying more than 1,600 homes.