Another earthquake in Haiti?

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The World's science reporter Rhitu Chatterjee has been talking with geologists. They say the Haitian earthquake didn't originate in the place they had anticipated. They had been looking at a different fault line. One that's yet to rupture.

Long before last year's earthquake, geologists had warned the Haitian government that a big quake was around the corner. Studies had showed that a fault that runs along the southern part of Hispaniola was due to rupture. So when the earthquake struck last January, geologists thought that's the fault that had ruptured.

?Most of us assumed that the earthquake was on the Enriquillo Plantain Garden fault,? said Carol Prentice, a research scientist with the US Geological Survey.
The Enriquillo Plantain Garden fault or the Enriquillo fault runs east-west through Haiti and the Dominican Republic. It marks the boundary between two tectonic plates. The North American plate, which lies north of the fault, is moving west. The Caribbean Plate on the south is moving east.

The Enriquillo fault is what geologists call a Strike-Slip fault, where the two plates move past each other horizontally. The last big earthquake on the fault was back in the 1700s. Since then, the fault has been building up a lot of pressure.

?Typically, a strike-slip earthquake of that magnitude will cause surface rupture along the fault zone,? said Prentice.

Soon after last year's quake, Prentice and her colleagues began looking at satellite images for surface ruptures. But they found no breaks in the earth's surface along the fault line.

?What we did see instead on this very high resolution imagery is evidence that the coastline had actually been raised in areas up to the west of Port Au Prince,? said Prentice. ?And you can see in the imagery that there were coral reefs sticking out of the water.?

Strike-slip faults like the Enriquillo do not raise or lower land when they rupture. So the absence of surface ruptures and the raised coastline were pointing to a different mechanism, and a different fault.

Geologists now blame last year's earthquake on a previously unknown fault, named the Leogane fault.

?This is a fault that dips into the earth pretty much under the city of Leogane,? said Eric Calais, a geophysicist at Purdue University, and a science advisor to the United Nations Development Program in Haiti.

That is one of the reasons that the town of Leogane was hard hit by the quake, said Calais. ?It was essentially located right on top of the piece of the crust that ruptured during the earthquake.?

The findings bring some good news. The Leogane fault is unlikely to rupture in the near future, because most of the pressure that had built up was released during last January's earthquake.

But, there is some bad news too. The quake that scientists had long predicted has yet to happen.

That's because the 90 percent of the Enriquillo fault ?has not yet ruptured and has been building up seismic energy over 250 years,? said Calais.

Geologists can't predict exactly when the fault will rupture. But Calais said it could happen at any time. And when it does, it could produce an earthquake even more powerful than the one last year.