Big fault known to span region

By Carolyn Y. Johnson
Globe Staff / January 14, 2010

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The catastrophic 7.0-magnitude earthquake that devastated Haiti on Tuesday afternoon occurred in a region long known to be seismically active, according to geologists, but it’s been more than a century since the earth shook there with such ferocity.

The quake occurred close to the surface along the Enriquillo-Plantain Garden fault, which runs along an east-west line extending from Jamaica to the southern part of Haiti.

“Even though large earthquakes of this size aren’t common there, the fact that there’s a fault system that runs through there makes it not surprising,’’ said John Bellini, a geophysicist with the US Geological Survey’s National Earthquake Information Center in Golden, Colo.

Another large earthquake of about the same magnitude occurred in the area in December 1897, Bellini said.

Like the San Andreas fault in California, the Enriquillo-Plantain Garden fault is a strike-slip fault in which tectonic plates move horizontally in opposite directions, scraping against each other.

“This was a larger earthquake than we had observed on this fault for over 100 years, and the problem we always have as seismologists when we go look at an area and try and convince people you have to take a seismic hazard seriously is they always look at what happened in the past,’’ said John Ebel, director of the Weston Observatory and a professor of geophysics at Boston College.

The earthquake occurred near the surface, making it very destructive. Such shallow strike-slip earthquakes trigger movement known as Love waves, strong horizontal shaking that can bring down buildings, Ebel said.

The region has also been rocked by aftershocks, with at least 35 measured so far, 14 of them over 5.0 magnitude, according to Bellini.

Ebel said large aftershocks will continue in the coming days.

There is no known way to predict when an earthquake will occur, and the tectonic plates in the Caribbean are largely beneath the sea - meaning they are monitored far less than the San Andreas fault. That means geologists know less about the ways in which the tectonic plates normally slip against each other and have less insight into when stresses may be building up that could cause an earthquake.

“Earthquakes aren’t random events; they occur when stresses build up,’’ said Michele Cooke, a geoscientist at the University of Massachusetts Amherst. Information about normal slippage “lets you know a little more, so you can make educated guesses about recurrence intervals for that particular fault’’ - such as whether it’s likely an earthquake will occur in the next decade or half-century.

From a scientific point of view, the quake was not atypical. What made it tragic was that it occurred in a region known to be seismically active, but a country nevertheless ill-prepared.

“It’s in a place that’s probably one of the least likely to be able to handle such a catastrophic event,’’ Bellini said.

Carolyn Y. Johnson can be reached at