The death toll from yesterday's magnitude-7.0 earthquake in Haiti has already reached the thousands and may climb higher. But seismologist William McCann says things could have been a lot worse. The quake ruptured only a part of the fault segment that caused a magnitude-7.5 quake over 200 years ago, he says, and that quake was about five times more powerful than the one that struck near Port-au-Prince yesterday.
McCann, who works at Earth Scientific Consultants in Westminster, Colorado, has spent decades studying the earthquake risks in the Caribbean. Nothing like yesterday's quake had struck Haiti since 1770--and that's what worried him. "We were concerned because it's been 240 years since the last major earthquake on this fault," he says. "Centuries have passed, and this area has been extremely quiet." That meant stress was building on the San Andreas-like fault just 16 kilometers south of Port-au-Prince's population of 1 million--without any quakes to relieve the strain.
When the east-west fault finally ruptured at 4:53 p.m. local time, residents of Port-au-Prince felt "very strong" shaking, while "violent" shaking struck the coast to the west of the city, according to estimates by the U.S. Geological Survey. That meant "moderate potential damage" in Port-au-Prince, but such projections can't take account of local conditions, notes McCann. The capital city is built on sediments, not bedrock, he says, so the whole valley would have trembled like a bowl of jelly. Haiti, the poorest country in the Western Hemisphere, "lacks a reasonable building code" that could have required reinforcement of structures to resist shaking. And the country "has almost no enforcement" of what code it has, he says. "This could be thousands and thousands of people dead."
Still, says McCann, many more people could have been killed. If the entire fault segment that failed in 1770 had let loose again, it would have produced another magnitude-7.5 quake, and Port-au-Prince would have been utterly devastated, he says. What's more, the potential for great destruction exists elsewhere on the island of Hispaniola, which Haiti shares with the Dominican Republic. In 1751, a magnitude 8.0--32 times more powerful than yesterday's temblor--struck farther along the same fault system off the southern shore of the Dominican Republic, where the fault dives under the island. A couple of months later, a magnitude 7.5 occurred between that great quake and the future 1770 event. And a separate active fault system slices across the north coast of the island.
In the grand scheme of things, says McCann, yesterday's "quake is not really big," so plenty of accumulating strain remains to be released. And that means the region needs to be prepared for stronger quakes to come. "We may be coming out of the quiet time we've had" for the past 60 years, says McCann, with a return to the turmoil of the 18th century a real possibility. "People should be better prepared than they are."
The magnitude-7.5 quake that ruptured part of the fault segment near Port-au-Prince over 200 years ago was about five times more powerful than the one that struck near Port-au-Prince on 12 January, not 20 times as originally reported.