Chile, Haiti Quakes Explained

MICHELE NORRIS, host:

Significant aftershocks continue to rock Chile two days after a magnitude 8.8 earthquake brought down buildings and bridges and triggered a tsunami. The death toll stands at more than 700. Some areas have yet to be reached. And yet, it's already clear that the devastation in Chile will not reach the levels that we saw in Haiti after the 7.0 earthquake there in January.

For an understanding of the differences between the two quakes, we've turned to Walter Mooney. He's a seismologist with the U.S. Geological Survey in Menlo Park, California. Thanks so much for being with us.

Dr. WALTER MOONEY (Seismologist, U.S. Geological Survey): It's my pleasure, Michele.

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NORRIS: Many people right now point to the fact that the Haiti quake was smaller in magnitude. But when it comes to earthquakes, you're not just looking at magnitude, it seems like you're also looking at the frequency of shaking. Could you explain that concept to us, what shaking means?

Dr. MOONEY: Sure, Michele. Every earthquake of course causes some ground shaking. But, you know, the number of shakes per second is different for a very big earthquake. There you have relatively fewer shakes per second because it's a big earthquake like a cello. And you have an earthquake like Haiti, which I would compare to a violin with a higher pitch. And so the number of shakes per second is going to make a difference in how buildings and bridges and hospitals and schools respond.

Schools and hospitals are usually one story and two story, and they don't do very well with these high frequency shakes. In contrast, a tall building, like the 12 and 14-story apartment buildings in Chile, they don't do very well with these long period, few shakes-per-second kind of waves. So, with this frequent shaking near to the source, near to the epicenter, that kind of circumstance like in Haiti, people don't have a chance. They can't evacuate and the buildings come down very, very quickly. In contrast, in Chile, the earthquake would've started a little bit more with a rumble and people would have some chance to evacuate.

NORRIS: Does this have something to do with depth also?

Dr. MOONEY: Oh, boy. Depth off an earthquake is a really important parameter. And we can recall the earthquake that occurred only about a year ago in L'Aquila, Italy. Here, the earthquake was only a 6.3, but it ruptured right up to the surface with devastating results, the reason being that the city was sitting basically right on the fault. So, any structure that had not been designed with earthquake engineering in mind, it never had a chance and it came down right away.

NORRIS: It's very unsettling to see these back-to-back earthquakes. Where else in the world are seismologists predicting major quakes right now?

Dr. MOONEY: Boy, we have a lot of places that we're very, very concerned about. Tehran is one city in the Middle East where we have more than 10 million people living right on a fault. You know, the population pressure is so great for people to move to the capital cities and to occupy that area that's - it's an economic pressure. Other cities that we worry about a lot is Caracas, Venezuela, Quito, Ecuador, Lima, Peru and, well, you know, the list goes on.

Northern India is a highly populated area that hasn't seen a really big earthquake for about 100 years. And we worry about that region as well. So, I would have to say that, you know, we don't have a list of the top five. Unfortunately, our list of the top concerns would probably be 25 mega cities, all of which are in peril from the great earthquake.

NORRIS: Walter Mooney, thank you very much.

Dr. MOONEY: You're welcome.

NORRIS: Walter Mooney is a seismologist with the U.S. Geological Survey in Menlo Park, California. Transcript provided by NPR, Copyright NPR.