Tuesday, February 9, 2010

Haiti's earthquake: The dangers aren't over as tectonic plates readjust."Not only Haiti"

FAYE FLAM, The Philadelphia Inquirer

PHILADELPHIA -- The deadly earthquake that felled much of Haiti's capital last week broke 250 years of strain -- a tension that had built slowly across the nearby fault as it resisted the inexorable tug of drifting tectonic plates.Quake_intensity.jpg
Geologists who have studied the complicated fault system in the area say more quakes could follow last week's disaster, which killed an estimated 50,000 to 100,000, according to the Pan American Health Organization. Often quakes such as this trigger others nearby in a domino effect, the experts say. In coming years, other sections of the same fault are likely to rupture, threatening not only Haiti but also the Dominican Republic and Jamaica.
While scientists cannot predict earthquakes, they can make rough forecasts and are in the process of creating a global database that flags areas facing the gravest danger.
Though geologists have found Haiti a difficult country in which to work, a handful of them had identified the danger there by carefully mapping out the region's faults, studying historical records of earthquakes, and using a high-precision Global Positioning System to measure the opposing motion of the two tectonic plates that meet there.
A prominent study published in 2008 showed that Haiti was sitting on several active faults whose silence in recent decades had grown ominous. CAYMAN_IS_QUAKE.jpg
Haiti lies on the margin between the North American and Caribbean plates, which are grinding past each other in an east-west direction at a speed of about an inch a year, said Ross Stein, a geophysicist with the U.S. Geological Survey in Menlo Park, Calif
In the big picture, these plates drift smoothly around the planet. But zoom in to fault lines around their borders and whole sections of earth jerk, stick and slip. On Jan. 12, the fault probably displaced the ground by 2 to 3 feet, Stein said. That's an insignificant distance on the scale of the continents, but it's enough to topple a capital.

The section of fault that ruptured near Port-au-Prince had been "locked" since 1751, Stein said. That was when the last big earthquake was recorded there -- a lesser disaster because of a much smaller population. Caribbean_faults.jpg
"If you'd gone by recent occurrences of small to moderate earthquakes, you'd have gotten this misimpression that this is a pretty quiet area," he said. But it was the proverbial calm before the storm.
When geologists describe an area as locked, what they mean is stuck, Stein explained. "It's really a product of friction," he said. "If faults were made of Teflon, we wouldn't have any earthquakes."
When enough tension builds up along the stuck zones, they eventually jerk free, violently catching up with the drift of the plates.
The magnitude of the quake was estimated at 7.0, which was small compared with the 9.3 quake that hit Sumatra in 2004, leading to the infamous tsunami.
In magnitude, the event in Haiti was comparable to California's Loma Prieta earthquake, which shook the Bay Area during the World Series in 1989, killing 63 people.

The Haiti quake packed so much more deadly force for several reasons. Its epicenter was on the outskirts of a heavily populated area and the structures in Port-au-Prince are more vulnerable to collapse than were those near San Francisco.
Another factor adding to the Haiti quake's intensity was the shallow depth of its epicenter. Deeper quakes tend to shake more gently because the seismic waves have more space to dissipate before they rumble to the surface.
Stein compares it to the ripples moving outward from a skipped rock -- losing force the farther they move out. "If you're close, you get clobbered numerous times," he said.
Though Haiti is cut by several active faults, relatively few geologists have worked there. "It is a difficult country to work in," said geologist Paul Mann of the University of Texas, who mapped much of the faulting in the region and collaborated on the 2008 paper pointing out the risk of a major earthquake in Haiti."It's a little unsafe in areas," he said, though it was less so when he started working there in the mid-1980s. Back then, the country was run by the dictator Jean-Claude "Baby Doc" Duvalier and was tightly controlled. But in recent years the situation has become more dangerous.
Still, he and his colleagues were able to map the 100-mile east-west fault, which stretches all the way from the Dominican Republic, through Haiti's western peninsula, along the sea floor, and into Jamaica. Mann named it the Enriquillo-Plantain Garden fault for the two points at its ends.
It is considered a strike-slip fault -- similar to the San Andreas -- because each side moves horizontally with respect to the other, said Mann.
Another fault runs north of Port-au-Prince, he said. That fault has been accumulating tension for 800 years and is considered overdue.

While Mann has used historical records to retrace the past, geologists in other regions use what's called trenching. They dig down and study breaks in the strata to mark past quakes.
In Jamaica and the Dominican Republic, they've been able to use trenching to read the history of previous earthquakes on neighboring regions of the fault. It's been too difficult to do that in Haiti, Mann said.
And earth scientists are divided on how best to predict the future from the past, said USGS's Stein. Some say quakes are going to occur most often where they have occurred in the past. But others focus on areas of active faulting that haven't moved in decades -- areas where strain is accumulating.
"Both these sides have their proponents, and they duke it out at every meeting," Stein said.
At the same time, scientists can now use GPS to precisely measure the motion of the plates in the region -- another piece of information about potential risk.
"This is not like the GPS in your car," said Purdue University geophysicist Andrew Freed. "We use $10,000 instruments," he said, which can measure the motion of one point relative to another within a millimeter per year.
"You can actually see the motions of the plates themselves," he said, and you can measure the way the plates are bent, stretched or squeezed by one another.
Taken together, the signs all led Mann and colleagues to forecast in 2008 that some time in the next century, the area near Port-au-Prince could suffer an earthquake of magnitude 7.2.
That's in the right ballpark, said Freed, though the way magnitude is measured on a logarithmic scale, a 7.2 would unleash more than twice the energy of a 7.0.
The bad news -- as if it could get worse -- is that the slipping of the Enriquillo-Plantain Garden fault near Port-au-Prince actually increased tension on neighboring sections of the same fault, Stein said.
"Sometimes you have a falling-domino series of earthquakes triggering other earthquakes," he said. That's why they tend to come in clusters.
You don't simply relieve stress with an earthquake like this, said Purdue's Freed. "You reorganize the stress field."
If any of the neighboring sections were already near the breaking point, he said, the quake in Haiti could kick them over the top.
View full sizeThat's often the pattern, he said, as fault ruptures "zip down the line."
The scientists can't predict exactly when this will happen. It could be later this year or decades from now.
Mann said his calculations show there is even more tension on the other fault on the north side of the island on which Haiti is located. "They could have a 7.5 there," he said.
Many other places are vulnerable to even more deadly earthquakes, said Stein: Bangladesh; Java; and the cities of Istanbul, Turkey; New Delhi; Quito, Ecuador; Lima, Peru; and Bogota, Colombia, are all disasters waiting to happen. "Haiti is not alone," he said.
With cities growing ever more crowded, construction quality is going to go down, not up, he said. Some argue that the world is likely to see a million-death earthquake in the 21st century.
As a first step in dealing with this, Stein said he and others in his field were putting together a publicly accessible database -- the Global Earthquake Model -- that would provide risk estimates for the world.
Stein considers knowing seismic risk to be a human right. He said the massive 2004 Sumatra quake and associated tsunami taught the geology community that they needed to do more.
Preparing is another matter. Even if you warn people that it's dangerous to live on steep hillsides, Mann said, poor people will go there because they have no other choice.
In Japan, an early-warning system picks up the first sign of a quake and automatically stops high-speed trains and other dangerous operations a few seconds before the worst shaking begins.
While such a system is probably beyond the means of Haiti and other nations in the world's danger zones, Stein said that with enough help, a good building code could at least be enforced for schools and hospitals.
"People need to know they are at high risk," he said. "That is the prerequisite to doing anything about it."
(c) 2010, The Philadelphia Inquirer.










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