Earthquakes usually strike remote areas, not such places as Haiti, Chile, Mexico

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Wednesday, April 14, 2010

First Haiti, then Chile, and now Mexico. Why all of a sudden do there seem to be so many earthquakes?

Actually, there are no more earthquakes happening than usual; it's just that these three quakes happened to strike areas where a lot of people live, so we heard about them.

According to Walter Mooney, a seismologist with the U.S. Geological Survey (he's a physicist who studies earthquakes), most earthquakes occur in remote parts of the world. "There are 14 to 17 magnitude-7 earthquakes on Earth every year," Mooney said. "Only a few are near population centers."

A magnitude-7 quake is big enough to do a lot of damage. But how much damage an earthquake does depends on the buildings in the area. In many earthquake-prone regions, governments require especially strong construction standards so that buildings can withstand a lot of shaking. The earthquake in Mexico earlier this month, which was also felt in Southern California, was measured at 7.2, but it did relatively little damage because of strict building standards. In Haiti, though, where few structures were well built, the magnitude-7.3 quake in January destroyed huge parts of the capital city and killed more than 200,000 people.

What's frustrating to scientists is that they have no way to tell where an earthquake will strike next; they can only make educated guesses. "As we go for longer and longer periods of time, then the probability slowly increases" that an earthquake will happen soon in a given region, Mooney said. "But the Earth is very complicated, and we are unable to do better than giving a probability."

Many scientists were not surprised by the big quake in Chile because the last time an earthquake struck the same region was in 1835. (It was recorded by Charles Darwin when he was doing research on the animals and plants in South America). Haiti hadn't had an earthquake since 1770. Both areas were due for a strong earthquake.

In North America, the most active area for earthquakes is the West Coast, from the Mexican border up to Alaska. But there are other areas in the United States that seismologists say also have the potential for a big earthquake sometime soon, including Memphis, Tennessee; St. Louis, Missouri; and Charleston, South Carolina. All three last experienced earthquakes in the 1800s.

-- Margaret Webb Pressler


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