mardi 2 février 2010

A Deadly Quake in a Seismic Hot Zone

Published: January 25, 2010; revised: January 27, 2010

Source: The New York Times

To scientists who study seismic hazards in the Caribbean, there was no surprise in the magnitude 7 earthquake that devastated the Haitian capital, Port-au-Prince, two weeks ago.

Except, perhaps, in where on the island of Hispaniola it occurred.

“If I had had to make a bet, I would have bet that the first earthquake would have taken place in the northern Dominican Republic, not Haiti,” said Eric Calais, a geophysicist at Purdue University who has conducted research in the area for years.

The fault that ruptured violently on Jan. 12 had been building up strain since the last major earthquake in Port-au-Prince, 240 years ago. Dr. Calais and others had warned in 2008 that a quake could occur along that segment, part of what is called the Enriquillo-Plantain Garden fault zone, although they could not predict when.

But about 100 miles to the northeast is a long segment of a similar fault, the Septentrional, that has not had a quake in 800 years. Researchers have estimated that a rupture along that segment — and again, they have no idea when one might occur — could result in a magnitude 7.5 quake that could cause severe damage in the Dominican Republic’s second-largest city, Santiago, and the surrounding Cibao Valley, together home to several million people.

“You can imagine the strain that has accumulated there,” said Paul Mann, a senior research scientist at the University of Texas, referring to the Septentrional fault. “It’s been going on for longer and accumulating faster. Therefore it’s going to produce a stronger earthquake.”

The recent quake on the Enriquillo fault and the forecast for the Septentrional are bleak reminders that the Caribbean is an active seismic zone, one with many hazards. Major earthquakes have regularly devastated the region’s cities, including the Jamaican capital, Kingston, which was destroyed twice in three centuries. An eruption of Mount Pelée killed 30,000 people in Martinique in the Lesser Antilles in 1902, and it and other volcanoes are currently active along that island arc on the Caribbean’s north and eastern reaches. Earthquakes and landslides along the Puerto Rico Trench, an undersea fault zone, have the potential to cause tsunamis.

The Haitian quake itself might have added to the risks, researchers say. Dr. Calais and colleagues and a team including Ross Stein of the United States Geological Survey in Menlo Park, Calif., have each calculated the stress changes on the Enriquillo fault that occurred when a 30-mile segment, centered in Léogâne about 18 miles west of Port-au-Prince, gave way this month. Although the results are preliminary, the work shows that stresses have increased just west of the segment and just east, within three miles of Port-au-Prince.

“This earthquake has increased the risk on other segments of that fault and perhaps on other faults as well,” Dr. Calais said. “The numbers are well within the range of stress changes that have triggered earthquakes on other faults.” But he said the quake probably did not increase the likelihood of a major tremor on the Septentrional fault.

The Haitian quake has produced a large number of aftershocks, about three times as many as quakes of similar magnitude in California and elsewhere, Dr. Stein said. But the intensity and frequency of those aftershocks have followed the patterns of other earthquakes, he said. Last Thursday, the geological survey issued a statement estimating that there was a 3 percent likelihood of a 7 magnitude aftershock in the next 30 days, and a 25 percent chance of one of magnitude 6. (On Wednesday, the area experienced a strong aftershock that was initially rated at 6.1 but was revised to 5.8.)

Of some concern, researchers said, was that none of the aftershocks have occurred in the area of increased stress nearer to Port-au-Prince, where ordinarily some might have been expected.

“One possibility is that these are simply calculations, and they may be wrong,” Dr. Stein said. “The other possibility is, O.K., this fault is fundamentally locked in some fashion, on pretty much all scales, and might be capable of popping off something large.”

In its statement, the geological survey cautioned that near the capital, “the fault still stores sufficient strain to be released as a large, damaging earthquake during the lifetime of structures built during the reconstruction effort.”

The region’s seismic activity is due to the movement of the Caribbean tectonic plate, which can be likened to a finger pushing its way against two larger plates, the North American and South American. Along the boundaries, the relative eastward movement of the Caribbean plate, at the rate of less than an inch a year, creates strike-slip faults, shallow fissures whose sides slide in relation to one another in an earthquake.

On the island of Hispaniola, which comprises the Dominican Republic and Haiti, the Caribbean-North American boundary stresses are expressed in numerous strike-slip faults, including the Enriquillo and Septentrional, which are relatively long and roughly parallel.

“It’s a bit unusual to have two parallel faults like that,” said Uri S. ten Brink, a geophysicist with the geological survey in Woods Hole, Mass. “It may simply be that for some reason there was already a weakened area further south.”

Dr. ten Brink’s main area of research is the Puerto Rico Trench, north of Puerto Rico and the United States Virgin Islands. This is a subduction zone — where the North American plate is sliding under the Caribbean, creating the potential for earthquakes and undersea landslides that can set off tsunamis.

“We’re trying to see if it’s a similar situation to the Sumatra fault,” Dr. ten Brink said, referring to the Indonesian subduction zone where a large earthquake in December 2004 created a tsunami that killed a quarter of a million people. Scientists have not yet found evidence of large subduction earthquakes on the Puerto Rican Trench, he said, “but that’s the $64,000 question.”

Because of the proximity of the trench to American territory, Dr. ten Brink and others have been able to obtain financing for their studies. But in other places around the Caribbean, research money has been hard to come by.

Haiti, for example, has no seismometers, meaning there has been no way to measure all the small tremors that might help characterize the Enriquillo fault. Researchers have relied on a network of 35 benchmarks to measure fault movement. Last week Dr. Calais, Dr. Mann and others were planning a trip to Haiti to make more accurate measurements for their stress calculations, and to install devices to monitor the fault zone continuously for a year or more.

Much of what is known about the seismic activity around Port-au-Prince has been gleaned from historical accounts of previous quakes. While far from precise, these accounts suggest a century-long, westward-marching sequence of quakes along the fault, beginning with one in 1751 in the Dominican Republic at the fault’s eastern end and including the 1770 earthquake that destroyed Port-au-Prince.

That raises the possibility that the Jan. 12 earthquake could be the beginning of a new sequence occurring over decades, with each successive quake redistributing stresses along the fault. “It’s certainly possible and it’s really something we’re very concerned about,” said Carol S. Prentice, a geologist with the geological survey in Menlo Park. Such sequences have been observed on other faults, including the North Anatolian in Turkey.

The Septentrional fault’s history is better known, largely because Dr. Prentice and others have done basic research on a segment in the Dominican Republic. The study involved digging trenches across the fault and looking for rupture lines in the sediments. By finding higher sediments that are unruptured, the dates of quakes can be determined.

Researchers said that more study was needed on the Septentrional and Enriquillo faults and elsewhere in the Caribbean, and that governments needed to prepare better for the inevitable.

There are already signs that the Haitian quake has prompted concern elsewhere in the region, at least among the general population. Dr. Mann said he was on Jamaican radio programs in the past two weeks to discuss the hazards.

“They know they’ve been destroyed twice,” he said. “They know their construction is not the best. All those things have put the whole country on edge.”

“The whole region is fearful.”


Note from the author.- This article has been revised to reflect the following correction:
Correction: January 27, 2010 An article on Tuesday about seismic hazards in the Caribbean referred incorrectly to the Septentrional fault, where some experts had thought an earthquake might first occur on the island of Hispaniola. A long segment in the Dominican Republic, not the entire fault, has gone without a quake for 800 years.

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