Collapsing Mountains Rocked Italy's Earthquake Italy lies between two huge tectonic plates to the north and south, and the Earth's crust on the western side of the country is actually stretching apart. That stretching and gravity pulled down part of the Apennine mountains and caused the 6.3-magnitude earthquake that shook central Italy early Monday.
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Collapsing Mountains Rocked Italy's Earthquake

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Collapsing Mountains Rocked Italy's Earthquake

Collapsing Mountains Rocked Italy's Earthquake

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ARI SHAPIRO, host:

Now, let's take a look underground. As Sylvia just mentioned, central Italy has always been a seismic hotspot. That's because of a complex plate between giant tectonic plates and a spider web of faults running underneath the surface. NPR's Christopher Joyce has more.

CHRISTOPHER JOYCE: The towns that suffered the worst devastation sit in the Apennine Mountains that run like a backbone down the length of Italy. The area is known for its dramatic ridges and cliff-like scarps, the scars left by previous quakes.

Historical records show the town near the quake's epicenter has experienced earthquakes for at least 800 years, and Italy has seen at least a dozen big quakes over the past century.

Geophysicist Paul Earle of the U.S. Geological Survey says Italy is sitting in a geological jumble - giant tectonic plates that form the earth's crust are banging into each other like bumper cars in an amusement park, but in very slow motion.

Dr. PAUL EARLE (Geophysicist, U.S. Geological Survey): Tectonics involve the collision between the Eurasian and African plates. And there are smaller microplates between those plates that cause stresses that generate the earthquakes.

JOYCE: One of those microplates is the Atria plate. It's pushing under Italy from the east. This irresistible force compresses the land and creates high ridges, like the Apennine mountains. These pile up like the ridges in a carpet that's pushed from one side along a wood floor.

Dr. EARLE: But then you also have the opening of a basin on the west end side.

JOYCE: The basin was created because, on the western side of the mountains, the earth's crust is slowly stretching or pulling itself apart. And the mountains above this widening basin are prone to slipping down.

Bob Holdsworth, a geologist at Durham University in the United Kingdom has studied this phenomena. He explains what's happening.

Dr. BOB HOLDSWORTH (Geologist, Durham University): Gravity is actually causing the slow collapse of this mountain chain. And the geological faults — huge fractures that cut through the Earth's crust — are actually accommodating that collapse, so that's why this area is so tectonically active; that's why there are so many earthquakes in this region.

JOYCE: Holdsworth says the faults occur in clusters.

Dr. HOLDSWORTH: And when one of them moves in an event like this, and that produces a significant displacement, which distorts the stress field. And that distortion affects all the faults that surround it. And when you look at the history of movement back through geologic time, you can actually see how those interactions have triggered movements on other faults.

JOYCE: Holdsworth leads a team that will fly to Italy this week to measure movement along the L'Aquila fault, the one that's near the epicenter of the recent quake and a likely suspect in this incident. The hope is to learn which fault will be the next to slip.

Dr. HOLDSWORTH: So, what we're doing is we're using a past record of fault movements to try and predict into the future.

JOYCE: Holdsworth says pinpointing places that could fail some day should be doable.

Dr. HOLDSWORTH: But predicting whether that's going to relate to an event that occurs tomorrow, next week, next month, ten years from now or a thousand years from now, that's much more difficult.

JOYCE: At a magnitude of 6.3, the quake is large but far smaller than the quakes that have hit Southeast Asia in the past few years. But this one hit one right underneath populated areas, and many of the buildings in the region's towns are built of rigid concrete or masonry and are especially vulnerable to collapsing.

Christopher Joyce, NPR News.

SHAPIRO: You can see a map of these tectonic plates at NPR.org.

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