More earthquakes to come

The Cause of Haiti's Latest Earthquake: Is the Worst Yet to Come? A Look at the Seismic Science in the Caribbean.

Jeneen Interlandi

In the seven days since a 7.0 earthquake struck the island of Haiti and decimated its capital city, at least 14 aftershocks measuring 5 or above have been recorded by the United States Geological Survey—including a 6.0 quake in Haiti just this morning. The event confirms a new report by Woods Hole seismologists which found that not only would such shocks be likely to continue, but the already devastated island nation would face great risk of significant future calamity.

Haiti and its neighbors sit above two tectonic plates (the North American and the Caribbean) that slide awkwardly past one another in an east-west direction at about an inch a year. The 100-mile border between these two plates, known as the Enriquillo-Plaintain fault line, extends from the Dominican Republic through Haiti all the way to Jamaica. Last Tuesday’s rupture occurred when a segment of the plates that had been stuck together since 1751 (when the last earthquake occurred) jerked themselves free, releasing 250 years of built-up friction from the earthquake’s epicenter and displacing just enough ground to topple Haiti’s fragile and ill-prepared capital.

Not all of that tension was released by the quake. As this map—created by UNAVCO, a nonprofit geology consortium—shows, a good deal of it has merely been shifted to other segments of the same fault line, collecting in two spots in particular: 20 miles west of the original epicenter, near the port village of Mirago├óne (already the site of several vigorous aftershocks) and 10 miles east, near the Dominican border (where aftershocks have yet to be reported). Eventually, say seismologists, the tension in both places will be released by seismic events equal to or greater than that of Tuesday's quake.

Of course, when that might happen is anybody’s guess. "We know that both spots will rupture at some point," says Jian Lin, a Woods Hole geophysicist and the report's lead author. "But it could be in 10 months, or 10 years—or even 100 years."

While earthquakes are generally difficult to predict, data on the tectonic plates underlying the Caribbean have been especially paltry. Part of the problem is that in much of the region, the plate boundaries themselves are below sea level and thus inaccessible to scientists. The other problem is that political instability and dire poverty have made the region a challenging one for seismologists; until very recently, no consistent earthquake-monitoring efforts were even attempted. (Compare that with the San Andreas fault in California, where both sides of the plate boundary are above sea level and have been heavily monitored for decades.) While scientists have been studying the region more closely since the 2004 Indian Ocean tsunami spurred a public outcry for better disaster preparedness, they say not enough data has accumulated to make reliable forecasts.

What seismologists do know is that between 1751 and 1770, Haiti and its environs were struck by three major earthquakes, each about 10years apart. They also know that the Enriquillo fault is not the only one to transect Haiti: a second fault line, north of the Enriquillo, has been accumulating tension for roughly 800 years and is considered overdue for a major quake. When one connects these dots, it becomes clear that the Caribbean is an active seismic zone and that more earthquakes, like the aftershock that hit today and the original quake that devastated Haiti, should not come as a surprise. Now the question remains: if we know they're coming, how can we protect cities in the Caribbean so that the devestation in Haiti never happens again?