Chile Earthquake, Tsunami Warnings: The Science of Disaster

Large aftershocks continue after 6th strongest quake on modern record.

February 27, 2010, 11:09 PM

Feb. 28, 2010 — -- A day after Chile's great earthquake, the ground is still shaking, remnants of the resulting tsunami still slosh across the Pacific -- and there is not a geologist alive who is truly surprised that the catastrophe happened where it did.

Chile is right on the "ring of fire" -- the fault lines, all around the perimeter of the Pacific Ocean, that make for some of the most violent and frequent earthquakes on the planet.

The one that struck on Saturday morning will probably rank sixth on the list kept by the U.S. Geological Survey of the worst earthquakes since 1900.

Just off the Chilean coast, one giant plate of the earth's crust is pushing against the one that holds up South America. It is what geologists call a subduction zone, where one tectonic plate is trying to slide under another, and tremendous tension has built up in the ground.

On Saturday the ground gave way -- violently.

"These regions of the earth are capable of producing the largest-magnitude earthquakes in the world, as you've seen in this magnitude 8.8," said Paul Earle, a seismologist with the U.S. Geological Survey, in an interview with ABC News.

"This particular fault, this is a huge earthquake -- this ruptured an area of about 400-500 km [300-400 miles]," said Earle.

The epicenter of Saturday's earthquake was only about 140 miles north of the single strongest earthquake recorded in modern times. A quake of magnitude 9.5 struck on May 22, 1960, also off the coast of Chile, killing 1,655 people, injuring 3,000 and leaving another 2 million homeless. It set of a tsunami far more destructive than Saturday's; 61 people died in Hawaii and at least 138 in Japan.

If there was anything to soften the blow on Saturday, it was that the center of the earthquake was 60 miles out at sea. Also important is that Chile, which has been hit so hard before, mandates the construction of buildings that are more likely to survive an earthquake.

That is in sharp contrast to last month's disaster in Haiti, where the government has put the death toll at 220,000 and the precise number will never be known.

Earthquake Aftermath: Chile and Haiti

The victims in Haiti, scientists say, were doomed by poverty and proximity.

The epicenter of the Jan. 12 earthquake was just 10 miles from Port-au-Prince. And as one seismologist put it, the cinderblock buildings there, in the poorest country in the western hemisphere, were barely able to withstand gravity -- let alone a magnitude 7.0 earthquake.

Beneath it was beneath the Pacific floor -- and because one tectonic plate pushed up over the other -- a bulge of water was forced toward the ocean's surface. That is why tsunami warnings were issued across the Pacific basin, as far away as Antarctica and Alaska.

On the open ocean, a tsunami can spread at hundreds of miles per hour, mostly under the ocean's surface. If you were in a ship in a tsunami's path, you might not even feel it as it raced by beneath you.

But as it moves into shallow water, it is slowed by the sea floor, and the force has nowhere to go but up, often onshore. That is what happened in the Indian Ocean tsunami of 2004, when more than a quarter of a million people are believed to have died.

"They come ashore, typically, at 35 miles an hour," said Jenifer Rhoades, who heads the tsunami program at the National Weather Service, part of the National Oceanic and Atmospheric Administration. "You cannot outrun a tsunami."

The danger is not over. Those giant plates, deep in the earth, are still grinding against each other. The aftershocks Saturday were strong enough that they would have made headlines on their own. And there will be more.

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