By Richard A. Lovett
In addition to smashing buildings and killing more than 200,000 people, Haiti's devastating January 12 earthquake produced two 3-meter tsunamis, scientists announced on February 24 at a meeting in Portland, Ore.
The discovery was made by Hermann Fritz, a coastal engineer at the Georgia Institute of Technology in Savannah, who spent six days in Haiti in early February documenting stories from fishermen and aid workers.
When the 7.0-magnitude earthquake struck, computer models predicted that a tsunami would hit the nation's south shore--but with a height of just 20 centimeters, said Fritz. And, indeed, the waves that crossed the Caribbean were tiny, lapping harmlessly on distant shores. But in Haiti itself, his on-site survey revealed that the waves may have reached as high as three meters--and that several meter-high waves struck along a 100-kilometer stretch of shoreline, all the way into the neighboring Dominican Republic.
One site Fritz visited was the town of Jacmel, on the southern coast of Haiti, where he found boats and boulders washed ashore and walls knocked down by the water. In one place, water had surged a meter deep into the barracks of a group of United Nations soldiers, but the soldiers, from Sri Lanka, had already left. "They knew all about tsunamis," Fritz said, noting that their country had been hard hit by the 2004 Indian Ocean tsunami.
Elsewhere, he found signs of three-meter tsunamis. In one case, along the shores of the Bay of Port-au-Prince on the western coast of Haiti, waves had rushed 70 meters inland, killing a grandfather and two young grandchildren who had stopped to watch.
The discovery that there had been such a large tsunami is surprising, says Seth Stein, a geophysicist at Northwestern University in Evanston, Ill. That's because the earthquake was of a type known as strike-slip, in which the plates shift sideways. "When an earthquake generates a tsunami, it's because the sea floor went up and down," he says. "An earthquake that is pure strike-slip doesn't do that."
But though earthquakes can cause tsunamis, a combination of other factors can contribute to triggering them too. In the Bay of Port-au-Prince, the waves seemed to have been caused by underwater landslides. Satellite images and on-the-ground photos show that the coastline has changed. In one case, Fritz says, a palm tree slid from the shore into 7-meter-deep water. And the waves came in almost immediately, on about a 15-second cycle. Such short-period waves, Fritz says, are a signature of landslide-induced tsunamis.
The south shore is a different matter. "We still have mysteries to solve there," Fritz says. To begin with, the wave cycle was longer, in the order of five minutes, and sizeable waves hit a much longer section of coastline. This indicates that the tsunami came from a larger source.
Eddie Bernard, director of the US National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, Wash., says one possibility is that the earthquake might not have been confined to the main fault, with additional seabed shaking coming from a "splay" fault. "Think of it like hitting a brick with a hammer," explains Bernard. "You have the main fault and all these faults going sideways." But if that had been the cause of the tsunami, aftershocks should have been felt along a splay fault near the tsunami zone, and that was not the case.
The south coast tsunami was more likely to have been caused by multiple underwater landslides, says Roger Bilham, a seismologist at the University of Colorado in Boulder, who was one of the first to visit the stricken country.
"In the past century," says Bilham, who published an analysis of building practices in Haiti recently in Nature, "Haiti has decimated its forest cover, resulting in enhanced erosion and piles of near-shore sedimentation. The surface waves from the earthquake triggered a bunch of submarine slides along the coast."
Yet another hypothesis comes from Costas Synolakis, director of the Tsunami Research Center at the University of Southern California in Los Angeles, who thinks that the shaking land may have caused water to vibrate, "like a plunging wavemaker."
"This is something that we had been suspecting might be possible, and Haiti may well be the smoking gun," he told Nature.
Whatever the cause, one of the things that Fritz observed was that many residents failed to react appropriately, gathering on the shore to watch when the ocean receded before the tsunami on the south coast struck, rather than running to high ground. That's a strong indicator that tsunami education is needed in Haiti and probably other Caribbean nations.
"If the tsunami had been bigger, it would have killed all these onlookers," Fritz says.