Tuesday’s magnitude 7.0 earthquake was the largest recorded in Haiti since 1770, but more than a dozen quakes of that size have shaken the country’s Caribbean neighbors repeatedly over the past centuries.
That’s because Hispaniola, the Caribbean Island Haiti shares with the Dominican Republic, sits along the border between two large tectonic plates — the North American plate to the north, and the Caribbean plate to the south.
Those plates are slowly sliding past each other, as the Caribbean plate moves west-to-east. Between the two, “there’s a series of interconnected fault lines,” says geophysicist Jian Lin of the Woods Hole Oceanographic Institute, who studies the area.
Two major fault lines run through Haiti; Tuesday’s earthquake near Port-au-Prince occurred along the Enriquilla-Plantain Garden Fault that runs through the south of the country. Another fault, the Septentrional Fault, runs through the north.
The fault that caused Tuesday’s quake is a “strike-slip” fault — the same type as the San Andreas Fault in California — which means that the plates are sliding horizontally past one another, rather than one slipping under the other.
“It might be slightly less active [than the San Andreas Fault], but also the reason [that you hear less about it] is the same reason that you hear about murders in Washington, D.C. but not somewhere in the Amazon — if it’s close to us, close to our heart, we talk about it,” says Uri Ten Brink, a geophysicist at the U.S. Geological Survey.
Ten Brink also says that less is known about the geology of Haiti than that of nearby Puerto Rico or the Virgin Islands, because Haiti is such a difficult and dangerous place to do research.
“I was looking today very quickly at some papers that have been written on the area, and there have been so few,” he says. “That’s unfortunate.”
U.S.G.S. researcher Carol Prentice told the Christian Science Monitor that a team of researchers is already planning to visit Haiti to gather data about the quake.
“A lot of the data we need to collect are pretty ephemeral,” she said.
Meanwhile, Ten Brink says, the devastated country is still necessarily not out of the danger zone. Aftershocks from a large quake can come days or even a month later. “The larger the magnitude, the longer it takes,” he says.
Boston — The magnitude 7 earthquake that leveled much of Haiti's capital Tuesday – the strongest temblor to hit the country in some 200 years -- may have increased strain on a segment of the same fault that lies across the border in the Dominican Republic.
That concern, based on calculations made during the first 24 hours after the quake hit, may ease with additional on-the-ground data, cautions Purdue University geophysicist Eric Calais, who has spent years studying faults on Hispaniola, the island both countries share.
But since the mid-1980s and the advent of precision satellite measurements of ground movement, plus other high-tech advances, earth scientists have developed an increasing respect for the ability of the slip of one fault to increase the strain on other, nearby faults, or on a different segment of the original fault.
In any effort to track changes in strain, "I would focus on the eastern termination of the fault towards the Dominican Republic, pending more information," he says. The reason: The rupture slid to the east. While strain would build at both ends of the ruptured segment, another rupture farther to the west would occur in a sparsely populated, hard to reach portion of Haiti. To the east, however, the fault traces a path through the mountains separating Haiti from the Dominican Republic and into the more-heavily populated southwestern portion of Haiti's neighbor.
Already, teams of researchers are planning to visit the stricken area to take the full measure of the quake before erosion or rebuilding efforts erase key pieces of evidence.
"A lot of the data we need to collect are pretty ephemeral," says Carol Prentice, a US Geological Survey researcher who is part of a relatively small group of scientists who have studied the area.
Accurate measurements of ground movement -- seen in offset stream beds or displaced roads, for instance -- help establish the amount of strain released. Careful monitoring of the strongest aftershocks, which can die off fairly quickly after a quake, help establish how deep in the crust the quake's break-point was. Tracking the quake's aftermath over the long term will help researchers better understand the slow rebound that occurs along the fault after the initial snap.
Why Haiti's fault got less attention
The tragic event has struck a broad patch of the globe -- the Caribbean -- well known for earthquakes and volcanic activity. But while islands such as Puerto Rico, Jamaica, and the Antilles chains are fairly well studied, Haiti hasn't received the same attention, since working in a country that is no stranger to poverty and hardship can be downright unsafe.
Still, scientists have pulled together an increasingly clear picture of the broad geophysical context in which Monday's event took place. Hispaniola sits squarely atop a boundary between two plates in Earth's crust: the North American Plate and the Caribbean Plate. The Caribbean Plate is trying to slide past a particularly tough block of crust underlying the Bahamas. The two are grinding past each other, much like the motion along another plate boundary -- California's San Andreas Fault.
Tuesday's quake occurred along the Enriquilla-Plantain Garden Fault, an east-west crack in the Earth that also runs west through Jamaica and out to the Cayman Islands. But Hispaniola also is home to another major fault, which cuts across its northern interior and heads toward Cuba.
These two fault systems trace their origins to a broader interaction between the North American plate and the Caribbean Plate. The North American plate is plunging under the Caribbean plate north of Hispanola and Puerto Rico. But one portion of the North America plate dubbed the Bahamas Platform is too bouyant to make the plunge easily, reseachers say. The result: The giant collision is deforming Hispanola.
Over millions of years, Hispaniola's northern fault, the Septentrional, has been the major pathway for building and releasing strain from the sideways motion of the two plates. But some researchers now argue that activity may be shifting; the Enriquilla-Plantain Garden Fault may slowly be taking over as the main strain-relief valve for the plate boundary, explains Uri ten Brink, a geophysicist with the US Geological Survey's office in Woods Hole, Mass.
In effect, the region has two parallel plate boundaries. "That's quite unusual," he says. Indeed, given the number of people who live on the island, as well as the number who visit, "this was really a wake-up call to me to try and understand this system."