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LIANE HANSEN, host:

This is WEEKEND EDITION from NPR News. I'm Liane Hansen.

Rescue operations continue today in Chile after an earthquake struck yesterday. At least 300 people have been killed and homes, bridges and highways in the central part of the country have been devastated.

The magnitude 8.8 quake occurred on one of the most seismically active areas of the planet. In fact, the largest earthquake ever measured stuck not far away, back in 1960. It was a magnitude 9.5. It, too, spawned a tsunami that swept across the Pacific.

NPR's Richard Harris reports.

RICHARD HARRIS: Big earthquakes are usually caused by events that occur on a truly planetary scale. That is, giant chunks of the Earth's crust, tectonic plates, are slowly moving around. And seismologist Kate Hutton at Caltech says the coast of Chile is a perfect example.

Dr. KATE HUTTON (Seismologist, California Institute of Technology): It's very seismically active because it's like California, it's on the boundary between two of the Earth's major tectonic plates. In this case, it's the Nazca plate and the South American plate.

HARRIS: The Nazca plate is moving east at the rate of about three inches a year. And since South America is already sitting to its east, the plate has nowhere to go but down. It plows beneath the South American plate. Over geologic time, the result is obvious.

Dr. HUTTON: The deformation is causing an ocean trench a very deep area offshore and very high mountains, the Andes Mountains, with the volcanoes and everything on shore.

HARRIS: Hutton says that in the short term, though, strain simply builds up in the rock.

Dr. HUTTON: And then when it finally does break, there's a lot of stored energy within the deformed crust that can be released as an earthquake.

HARRIS: And in this case, not just an earthquake, but a tsunami that sharp jolt underwater kicked up a wave.

Dr. HUTTON: It's sort of like sloshing in the bathtub. And if it's a big enough earthquake, then the waves that are caused on the ocean will travel all the way around the Pacific.

HARRIS: This head-on collision of tectonic plates in Chile has been responsible for 13 quakes of magnitude seven or higher since 1973, according to the U.S. Geological Survey. Still, this weekend's quake ranks among the strongest on record.

Dr. BRIAN ATWATER (Researcher, U.S. Geological Survey): There were only three earthquakes in the 20th century that were bigger than this earthquake.

HARRIS: Brian Atwater works for the U.S. Geological Survey out of the University of Washington.

Dr. ATWATER: And those three earthquakes were in Kamchatka in 1952, in Chile in 1960 and in Alaska in 1964.

HARRIS: Of course, there was another giant quake in 2004, the Indonesian quake that triggered the devastating tsunami, so these are very rare events.

By comparison, the quake that devastated Haiti last month was a comparatively modest 7.0. The Chilean 8.8 released 500 times as much energy. So why was the Haiti quake so much more deadly? Well, for one thing, the epicenter was much closer to a densely populated area. Atwater says it's also the case that the quake was surprisingly compact.

Dr. ATWATER: Nearly all the energy from the Haitian earthquake came from a patch just, I don't know, 5 or 10 miles long, and so it's very, very concentrated.

HARRIS: The Chilean quake, by comparison, caused a large area of ground to slip. Atwater says: Don't think of a dot-like epicenter in this case. Think of a failure zone shaped like a giant sausage 70 miles away from the nearest significant city.

Also, in Haiti, the buildings that collapsed were poorly constructed to begin with. Unlike Chile, people there don't have experience building for earthquakes. And besides, Hutton says, Chilean buildings haven't just been built better, but they've been put to the test.

Dr. HUTTON: Probably most of the really vulnerable buildings have already been destroyed in earthquakes.

HARRIS: Of course, a quake of this size still caused lethal damage spread over a large area. But it made a big difference that Chile knew something like this was inevitable and made an effort to be prepared.

Richard Harris, NPR News.

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