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Haiti earthquake: Why the Caribbean is a mini ring of fire

The forces that led to the Haiti earthquake are a reminder that the idyllic Caribbean is one of the more geologically active spots on earth, and that a powerful earthquake could strike the region again.

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Floating and diving plates

In plate tectonics, buoyancy counts for a lot. What is now the Caribbean Plate is more buoyant than typical ocean crust. And it's less buoyant than the crust that makes up continents.

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Over time, the North and South American continents migrated west, separated by a small ocean. As they moved, their ocean encountered the more-buoyant Caribbean Plate. It rode over the top of the oceanic crust separating the two continents.

Today, the plate is bounded to the west by the Cocos Plate, which dives under the Caribbean plate just west of Central America. This movement built and sustains the region's volcanoes.

At the Caribbean Plate's southern boundary it slides under the South American Plate. To the east of the Leeward Islands as well as northeast of Hispaniola, the North American Plate is diving beneath the Caribbean.

Where the faults lie

Where plate meets plate head-on, as they do along the subduction zone east of the Leewards, volcanoes form, Lin continues. But where the collision along a subduction zone strikes at an angle, most of the collision energy goes into forming long fault zones, such as the Septentrional-Oriente Fault.

The Septentrional-Oriente Fault zone starts northeast of Puerto Rico, stretches west across the north coast of Hispaniola, slides past the southern end of Cuba, and reaches the Cayman Islands. Then, it takes a quick jog south along the Cayman Trough, the plate's spreading zone.

As for Hispaniola, it sits on a plate fragment known as the Gonave Platelet. The Septentrional-Oriente Fault forms its northern boundary. Its southern boundary is the now-infamous Enriquillo-Plantain Garden Fault, the source of Haiti's current seismic woes.

This fault and an associated undersea trench south of the Dominican Republic form the northern boundary of the Caribbean plate. The motion along the boundary carries the Caribbean Plate eastward along the zone -- the direction of last week's slip.

In 2008, a team led by University of Texas seismologist Paul Mann and Purdue University's Eric Calais calculated that if the accumulated strain along the Enriquillo-Plantain Garden Fault snapped in one event, the resulting earthquake would hit magnitude 7.2. But the projection lacked specifics in time and location. Dr. Mann points out that this section of the fault has never been studied in ways that would yield an average recurrence rate for major earthquakes.

Researchers have raised similar red flags concerning the Septentrional-Oriente Fault.

Carol Prentice, a US Geological Survey scientist who has studied the fault as it runs through the Dominican Republic's Cibao Valley, estimates that the fault is overdue for a major quake. Based on geological studies she and colleagues conducted, they estimate that if the accumulated strain were to be released in a single event, it could trigger a quake with a magnitude of at least 7.5.