Haiti quake reveals previously unknown fault

IGUAÇÚ FALLS, Brazil — The devastating quake that slammed Haiti on January 12 occurred on a previously unrecognized fault zone, report scientists who are still trying to determine the implications for the region’s long-term seismic risk.

The newly discovered fault hasn’t been officially named yet but is informally known as the Léogane fault, after one of the Haitian cities that sits directly atop it, study leader Eric Calais told Science News.

Just after the magnitude-7 temblor struck, scientists presumed that the epicenter of the quake was located on the well-known Enriquillo fault, says Calais, a geophysicist at Purdue University in West Lafayette, Ind. In fact, Calais notes, he and his colleagues published a paper in 2008 suggesting that the Enriquillo fault, which runs east-west through a long valley south of Port-au-Prince, Haiti, was ripe for a magnitude-7.2 quake.

But data collected after the quake didn’t jibe with the notion of an Enriquillo-spawned quake, Calais reported August 10 at a geophysics conference called the Meeting of the Americas. For one thing, the edges of that fault are vertical and the two sides slide past each other horizontally, but comparisons of space-based images taken before and after the quake revealed that the area north of the fault had been forced substantially upward, as well as southward, during the event. During a post-quake field survey along the coastline west of Port-au-Prince, scientists also found that formerly submerged corals died when the quake lifted them above the tides by as much as 60 centimeters.

Altogether, these data point to a different culprit and suggest that this previously unknown fault is about 30 kilometers long, says Calais. Unlike the Enriquillo fault, which extends vertically into the ground, the newly discovered fault dips steeply northward into the earth at an angle of about 60 degrees.

“This is part of a whole system of faults that we hadn’t recognized before,” Calais notes. The fault had escaped detection largely because Haiti has no network of seismometers, and the neighboring Dominican Republic has only a few such instruments.

During the January quake, deep parts of the Léogane fault slipped past each other as much as 5 meters. Despite the significant slippage that occurred at depths between 5 and 20 kilometers, there was — unusually, Calais says — no rupture of the ground at Earth’s surface.