Saturday, January 16, 2010

Haiti's fault rupture boosts long-term risk of Jamaica quake

By Tom Randall and Meg Tirrell:

NEW YORK, USA (Bloomberg) -- The magnitude 7 earthquake that killed as many as 100,000 people in Haiti this week may increase the likelihood of a future quake in Jamaica, according to seismologists who study geologic risk.

When aftershocks subside in the coming weeks, Haiti’s prospects of another earthquake will plummet, while areas west along the same fault line will see increased seismic pressure, said Stuart Sipkin, a seismologist at the US Geological Survey in Golden, Colorado. It could take decades or a century for the pressure to rupture on the western edge of the fault in Jamaica.

A similar quake flattened the Haitian capital of Port-au- Prince 240 years ago, so long ago that most residents were unaware they were at risk, said Roger Musson, who advises engineers on regional dangers for the British Geological Survey. The 1770 upheaval was part of a string of westward-moving temblors that culminated in Jamaica in 1907, he said.

“In Haiti, there’s not been earthquakes in living memory; now it’s likely that the stress will be increased on the next segment along,” Musson, the agency’s head of seismic hazard, said in a telephone interview. However, he added, “You are constantly surprised by earthquakes doing things that they’re not supposed to do.”

Haiti lies near the eastern end of a fault line between the North American and Caribbean tectonic plates -- massive subterranean sections of the earth’s crust that move at about the speed that human fingernails grow, Sipkin said.

When the two passing tectonic plates get stuck together, pressure builds until it is relieved through a violent movement of earth, Sipkin said.

It probably took about 20 to 30 seconds for the fault to break, said Kate Hutton, a seismologist at the Seismological Laboratory at the California Institute of Technology in Pasadena.

“People probably felt it for longer,” Hutton said today in a telephone interview. “People’s perception of time slows down when they get really stressed.”

The Haiti earthquake was a “worst-case scenario,” a shallow rupture in the earth that ripped through a densely populated and poorly constructed city, said Pedro de Alba, professor of civil engineering at the University of New Hampshire in Durham. The depth of the rupture is important, because if it occurs deep in the earth, much of the energy is absorbed by rock, he said.

“A shallow earthquake is the worst possible kind,” de Alba said in a telephone interview today. “Pressure was building up for quite a long time.”

De Alba said the probability of a future quake west along the fault line has increased, “but to what extent we simply can’t predict.”

January 16, 2010