Haitian quake no shock to geologists

by Mary Caperton Morton
Thursday, January 5, 2012

Although the magnitude-7.0 earthquake that shook Haiti on Jan. 12 was a horrible shock for millions of people in and around the Haitian capital of Port-au-Prince, the event came as no surprise to geologists. For years, scientists familiar with the geology of the region had been warning that the island of Hispaniola, which Haiti shares with the Dominican Republic, was overdue for a major quake.

In March 2008, several research teams from the United States presented data at the 18th Caribbean Geological Conference in Santo Domingo in the Dominican Republic, warning that Hispaniola’s two major east-west trending strike-slip faults, the Enriquillo-Plantain Garden Fault in the south and the Septentrional Fault in the north, were both due for a major event. Using GPS data, the teams calculated that either fault could produce a magnitude-7.2 quake at any time.

“As with all earthquake forecasting, the big problem was that we couldn’t provide any information about timing,” says Paul Mann, a geologist at the University of Texas at Austin who presented at the conference. “We couldn’t say if the earthquake would happen in two days or 10 years.” Eric Calais, a geophysicist at Purdue University in West Lafayette, Ind., who also presented at the meeting says, “We had talked to a number of government officials about the risk and they were very receptive. They just didn’t have enough time to do much to prepare for such an event, especially with Haiti’s other pressing problems.”

Then on Jan. 12, a magnitude-7.0 earthquake struck along the Enriquillo-Plantain Garden Fault, about 25 kilometers southwest of Port-au-Prince, leveling the capital city and killing untold thousands. The proximity of the epicenter to the major city and Port-au-Prince’s lax building codes combined to directly affect more than 3 million people, setting off a massive international aid effort. Aftershocks as great as magnitude 5.9 continue to shake the region a week after the initial event. It appears that the fault slipped up to 4.5 meters in places.

Unfortunately, Mann says, even though this most recent event released stress, it doesn’t mean Hispaniola is safe from more seismic activity. If anything, the island is now at even greater risk, he says. Along strike-slip faults, like the Enriquillo, a rupture on one segment of the fault can often increase stress on the adjacent segments. “This event ruptured about 80 kilometers, or about one-tenth of the fault’s total length,” he says. Segments to the west and east of the rupture could have been brought closer to failure by the fault movement, he says, which could result in another large earthquake.

Furthermore, it is possible that the rupture on the Enriquillo-Plantain Garden Fault could affect the stress on the Septentrional Fault in northern Hispaniola, says Jian Lin, a geophysicist at Woods Hole Oceanographic Institute in Massachusetts. Overall, the Caribbean is very seismically active, Lin says. Earthquakes tend to be large, but few and far between. Currently, the North American Plate is diving beneath the Caribbean Plate just north of Hispaniola. For a long time, the Septentrional Fault was thought to accommodate the bulk of the stress building from the plate movement. But scientists don’t have a clear picture of how the faults affect stress on one another. “These two faults are very similar,” Lin says. “They run the same direction and have the same motion. It’s quite unusual to see parallel faults like that and we don’t quite know how they interact.”

Part of the problem is that geologists have only a sparse record of the quake history of Haiti and the rest of the Caribbean, says Carol Prentice, a paleoseismologist with the U.S. Geologic Survey in Menlo Park, Calif. Large earthquakes last struck Haiti in 1751 and 1770, but exactly how strong those events were and on which faults they occurred is unknown, she says. Even today, Haiti lacks its own seismic network. The Dominican Republic has a small one, but most of the data about this most recent event came from seismic networks in the United States and elsewhere.

The lack of instrumentation in Haiti is somewhat hampering geologists' efforts to assess the island’s renewed seismic hazard potential, Prentice says. Calais and colleagues hope to travel to Haiti in the coming weeks to reassess some of the GPS benchmarks they installed on the island as well as survey the trace of the fault at the surface. “Search and rescue is obviously priority number one,” says Calais, who adds that many of his Haitian colleagues have not been heard from since the quake. “But as soon as we can, we’d like to go in and try to figure out what the future holds. It’s likely that Haiti is not done quaking.”

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