Risk of fresh Haiti earthquake could be greater than previously thought

Scientists had thought the Haiti earthquake a year ago released stress on a well-known fault. It didn't. Instead, it revealed faults that scientists didn't know existed.

By , Staff writer

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    Haitian police lower the national flag in front of the damaged presidential palace on the first anniversary of the magnitude-7.0 earthquake, in Port-au-Prince, Haiti, Wednesday.
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A year after a devastating earthquake struck Haiti, killing an estimated 316,000 people and leaving a million homeless, scientists say the region around the capital faces additional earthquake risks that weren't apparent prior to Jan. 12, 2010.

In the tragedy's aftermath, researchers have conducted dozens of studies and have found that:

• The fault responsible was not the fault many initially identified as the culprit, but appears to have been a previously unknown hidden fault. That means centuries of accumulated strain remains on scientists' initial suspect, the Enriquillo-Plantain Garden fault.

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• Strain has increased along this fault to the east and west of the region containing the fault that ruptured. The segments of the Enriquillo-Plantain Garden fault under greater strain include one that runs much closer to Port-au-Prince than the fault linked to last January's quake.

• Aftershocks have revealed what appears to be an undersea fault offshore from the epicenter of last year's quake – a fault whose historic rupture rate, along with estimates of the magnitude of the quakes it could generate, are unknown.

What emerges is a picture of faulting in the area far more complex than the picture researchers had prior to last January's temblor – a picture they are trying to fill in.

"We need to better characterize the hazard in the region so that we know all the different faults that are players" contributing to the region's seismic hazards, says Gavin Hayes, a geophysicist at the National Earthquake Center in Golden, Colo., a member of one of two groups that independently exonerated the Enriquillo-Plantain Garden fault.

Charting the quake

The quake's epicenter was roughly seven miles southeast of the coastal town of Leogane, some 20 miles west of Port-au-Prince on the country's southern peninsula.

The Enriquillo-Plantain Garden fault, in effect part of a boundary between the North American and Caribbean plates, runs through the area.

But when members of Dr. Hayes's team arrived and hunted for fresh surface evidence of a rupture along the fault in that area, they found none. Typically, a strike-slip fault such as this, in which one crustal plate slides along the edge of another, would show fresh jogs in once-straight stream beds or other signs of lateral shifts. But the team found none.

Instead, after reviewing seismic information, along with information from satellites and boots-on-the-ground visits to various locations in the area, the team found that the landscape between the fault and the coast has been raised by as much as two feet. Once-submerged corals along the coast were now high and dry.

That suggested a hidden thrust fault, a conclusion borne out by modeling experiments built on the data the team collected.

"Most or all of the slip was on the Leongane fault," says Carol Prentice, an earthquake scientist at the US Geological Survey's office in Menlo Park, Calif., and another member of the team. But, she cautions, it's a modeled fault and will require more study to determine if it truly has a real-world counterpart.

A different group, same result

Another group, led by Purdue University geophysicist Eric Calais, came to a similar conclusion. Both groups published their results last October in the journal Nature Geoscience.

In follow-up studies using the presence of the newly proposed fault as a starting point, a group led by Jian Lin of the Woods Hole Oceanographic Institution calculated the effect the quake had on strain along the Enriquillo-Plantain Garden fault. They found that the quake had increased strain along the fault to the east and west of the Leongane fault's rupture zone.

The eastern segment runs past Port-au-Prince a scant three miles from the city center.

"That is a major concern," Dr. Lin says.

But a few months ago, he continues, an analysis of aftershock data revealed something new: another possible fault on the sea floor not far offshore from Haiti's capital.

The fault's capacity for large earthquakes and how frequently the occur are unknown, he says.

Researchers say they hope data from a seismic monitoring network they have established there will help resolve many of the remaining questions scientist have about the area's earthquake risk.

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