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Haiti’s Geology Points to Big Quakes

The island of Hispaniola, home to both Haiti and the Dominican Republic, sits on the Enriquillo fault, the boundary between the Caribbean and North American tectonic plates. It is a slip-strike fault, where two portions of the Earth's crust are sliding past each other.

Tuesday's devastating earthquake, which centered just 10 miles southeast of the Haitian capital of Port-au-Prince in the southern portion of the island, was not unexpected, says Tim Dixon, a geophysicist with the University of Miami who has studied the geophysics of the area.

Geologists long thought that the plate boundary was located in the northern part of the island, giving it the major earthquake risk. However, Dixon and colleagues began doing careful measurements of the island in the early 1990s and discovered that there was significant fault activity in the south as well .

"The geologists went in there and said 'Hey, there's a huge fault here.'" But while the scientific community was aware there was a potential problem, little was done by Haiti to prepare, says Raymond Russo, a geophysics professor at the University of Florida in Gainesville.

In fact, they found that the Caribbean plate was moving east and that the two parts of the island are moving apart at a fairly quick rate, geologically speaking, the north one centimeter and the south one centimeter per year. For comparison, the plates at the San Andreas fault in California are moving at a rate of about 3.5 centimeters (1.4 inches) a year, Dixon says.

"Given that you've had a one centimeter a year movement for all that time," the 7.0 quake represented a tremendous release of energy, Dixon says of Tuesday's quake.

The historical records back up the findings. "It turns out that there are records of big earthquakes on the southern coast going back to the mid 1600s," says Dixon.

But the south had not had a major quake in 240 years.

"The situation in Haiti is made even worse given the fact that the last major earthquake on this fault occurred in 1770, so the populace has no memory of any similar disasters and preparedness is generally very deficient," says Russo." The majority of buildings were never subjected to any big shaking, so they survived long enough to age and they collapsed immediately.

The last big earthquake in the area, an 8.0 temblor, was in 1946, in the north of the Dominican Republic, says Russo. Haitans hadn't been as aware of the danger. "It's 50 years ago, so a lot of people in a young nation don't remember that. There's a political boundary between them and a language barrier," he says.

The devastation in the area is due in part to the weak infrastructure, says Russo. "The fault is shallow like the San Andreas. It's near the surface, so the shaking is severe and the rolling surface waves really do damage."

While a quake of this magnitude would be considered major in California and would likely cause billions of dollars worth of damage, there would be far fewer deaths and injuries because of better building structures and stronger building codes.

"Haiti is a very poor country with generally poorly built houses and infrastructure" Russo says.

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