* 18:32 15 January 2010 by Kate Ravilious
Earthquake experts are warning that the devastating quake that struck Haiti on Tuesday could be the first of several in the region. They say historical records suggest that not all the energy that has built up in the faults running through the Caribbean region was released in this week's tragedy.
Their fear is that enough energy remains in the fault system to trigger another earthquake of the same scale as Tuesday's.
The last time Haiti was struck by earthquakes of this scale was in 1751 and 1770, when three large earthquakes hit within the space of 20 years. They ruptured the same fault segment as the one that slipped on Tuesday, as well as segments lying further to the east, in Haiti and the neighbouring Dominican Republic.
"Last time round there was a sequence of earthquakes," says Uri ten Brink, an expert on earthquakes in the region from the US Geological Survey in Woods Hole, Massachusetts. "I'm worried, as we might expect the eastern side of the fault to rupture next." Other geologists concur. "Stress transfer along the fault is likely to trigger a chain of quakes," says Bill McGuire from University College London.
Another, larger earthquake could affect surrounding nations as well. The fault that was responsible for Tuesday's quake extends west through Jamaica. Another runs parallel to it in the north, along the southern edge of Cuba and the northern side of Haiti and the Dominican Republic. Historical records suggest that both these faults produce large and destructive earthquakes every few centuries.
"They are dangerous especially when large population centres like Port-au-Prince, Kingston in Jamaica or Santiago in the Dominican Republic are so close to them," says Paul Mann from the University of Texas at Austin, who published a paper in 2008 that forecast a major quake in the region.
The region harbours a third fault to the east, which is a further cause for concern. Unlike the others it lies underwater, where the Atlantic Ocean plate dives underneath the Caribbean plate, creating the Caribbean island chain: it is a submarine thrust fault, like the one that caused the Indian Ocean earthquake and tsunami on 26 December 2004.
Satellite measurements show that the Caribbean plate is moving east over the Atlantic plate at around 2 centimetres per year. Measurements over several decades show that the sum of all earthquakes that strike on "splinter faults" on the Caribbean plate, like Tuesday's, have accounted for around half of the energy associated with this movement, leaving the other half stored up in the system. Some of the remainder may be accommodated by slow creep along the region's faults, but McGuire and his colleagues are concerned that much of the stress may be accumulating on the undersea thrust fault to the east.
If that stress were to be released on the submarine fault, it could trigger a catastrophic tsunami of the scale of the 26 December 2004 Indian Ocean disaster.
McGuire released a report warning of this danger in 2008 (PDF). Along with the entire Caribbean, Central America, the Gulf coast of the US and the north coast of South America would be at risk from such a tsunami.
In particular, geological measurements indicate that stress is building in the section of submarine fault between easternmost Dominican Republic and the island of Guadeloupe. Large earthquakes of magnitude 8.5 to 9.0 could rupture the entire 1000-kilometre length of the fault, McGuire and his colleagues wrote in their report.
From the amount of energy being accumulated by subduction, McGuire and his colleagues estimate that undersea thrust earthquakes could recur every 2000 years or so.
Unfortunately, high rates of natural erosion in the region have long since wiped away the geological signs of the last earthquake along this submarine fault. We know from historical records that there has not been a quake along the fault in 500 years, but the next one could be within the century, or within the next millennium.
"We don't want to scaremonger, but much larger quakes, of magnitude 8 or more, have occurred in this region and will do so again," says McGuire. "Where they are submarine they will present a major tsunami threat, especially as this is such a small area compared with the Indian Ocean."