Chile's 'megathrust': a quite different geological action from Haiti's quake

It would have hurt at any time to see the wreckage Chile suffered on Saturday. Coming on the heels of Haiti's disastrous earthquake, though, Chile's tragedy was especially humbling and hard to fathom.

Two mighty earthquakes hitting the Western Hemisphere in less than two months. Too many. Too much suffering.

Scientifically speaking, though, neither earthquake was a surprise, said Justin Revenaugh, a University of Minnesota professor of geology and geophysics.

And even though Haiti and Chile share hefty measures of shock and loss, the two quakes were quite different.

Before examining the differences, here's an update on conditions in Chile today: At least 708 people have died as a result of Saturday's 8.8-magnitude earthquake and many more are missing. An estimated 500,000 homes were destroyed and some 2 million people were displaced. Amid rampant looting and vandalism, Chilean President Michelle Bachelet dispatched 10,000 troops to restore order and help with rescue efforts.

Now for those geophysical differences in the two quakes that have brought so much destruction to the hemisphere.

Haiti: a strike-slip fault
The disruption beneath the ocean floor in Haiti came along what's known as a strike-slip fault. Two of the gigantic plates that form the Earth's crust ground against each other in a side-to-side motion. Think of setting two bricks on a table and trying to grind them slowly past each other under a lot of pressure. They catch on each other at times and then abruptly break loose, shaking anything you might set on top of them.

There was a risk Haiti would see large waves caused by underwater landslides, but not of killer tsunamis.

The quake off Chile's shore was quite different. It was a "megathrust," similar to the temblor that set off tsunamis in the Indian Ocean in 2004. It occurred on a fault line between two tectonic plates called the Nazca and the South American plate. One plate slid beneath the other and bent, building up tremendous pressure until it broke loose with a mighty snap that lifted the ocean floor.

When that happened under the Indian Ocean in 2004, the 9.1-magnitude quake was so powerful that it set the planet vibrating in oscillations as if it were a ringing bell. Even under Minnesota, the then-frozen ground moved up and down nearly a half-inch as seismic waves spread around the globe. And the resulting tsunamis killed some 230,000 people in 14 countries.

This time there were tsunamis, too — among other places, flooding the Chilean coastal town of Llolleo just west of Santiago. Farther away though, Hawaii and Japan were spared the killer waves that had been feared. Scientists still are studying the tsunami action, but Revenaugh said a key reason it was less severe is that the Chilean quake happened at greater depth than the one in the Indian Ocean.

500 times the energy
The quake in Chile measured 8.8 on the Richter scale, 500 times more powerful than the magnitude 7 temblor that shook Haiti.

Scientists calculate earthquake magnitudes on a logarithmic scale. This means that an earthquake of, say, magnitude 7.2 would produce 10 times more ground motion and release about 32 times more energy than one of magnitude 6.2.

But Chile was relatively lucky. Its quake was centered offshore an estimated 21 miles underground near a relatively unpopulated area, while Haiti's tectonic mayhem struck closer to the surface — about 8 miles underground — and right on the edge of Port-au-Prince, the Associated Press reported.

In all, eight Haitian cities and towns — including this capital of 3 million — suffered "violent" to "extreme" shaking, even though that quake was less powerful. Haiti's government estimates some 220,000 people were killed and 1.2 million were left homeless.

By contrast, no Chilean urban area suffered more than "severe" shaking, the third most serious level on measures set by the U.S. Geological Survey. The quake was centered 200 miles away from the capital and largest city, Santiago.

Another factor was the composition of the coastal ground in the two countries, Revenaugh said. Haiti's hard-hit capital sits on soft soil formed by sediment from the hillsides. Chile rests on more bedrock.

Of course, Haiti's poverty and flimsy buildings were factors too.

"Chile is wealthier and infinitely better prepared, with strict building codes, robust emergency response and a long history of handling seismic catastrophes," the AP said.

Any comparison of the two quakes brings to mind the observation that earthquakes don't necessarily kill people. Collapsing buildings are the culprit.

The sad truth for Haiti and other recently shaken countries is that a quake's magnitude doesn't tell the full story of death and destruction. A magnitude 9.2 quake in Prince William Sound, Alaska, killed 128 people in 1964. Compare that with the magnitude 7.9 quake in Eastern Sichuan China where at least 68,000 people were killed in 2008

No surprise
No one predicted the precise time these quakes would shake Haiti and Chile. But they were expected, Revenaugh said.

A quake as strong as the one that hit Haiti happens somewhere on the planet about 20 times a year, according to Michigan Technological University. Indeed, a magnitude 7 quake rumbled with little notice near Japan's Ryuku islands a few hours before Chile was hit.

Those quakes and the one in Haiti are not related except for the fact that they occurred in parts of the Earth where there is almost constant seismic activity.

Chile's location makes it particularly vulnerable. It suffered the most powerful quake on record, a magnitude 9.5 in 1960 which set tsunamis crashing onto shores in Japan, Hawaii and the Philippines. And 13 quakes of magnitude 7 or larger have hit Chile's coastline since 1973.

Now the surrounding region faces serious worries of aftershocks for days to come.

And even though Chile's death toll is nowhere near that in Haiti, it too is in urgent need of food, water, shelter and recovery aid.

Bachelet, the president, declared on Sunday, "We are confronting an emergency without parallel in Chile's history."

The Chilean government has asked for help in its recovery and the United Nations is ready to rush aid deliveries into the country, the AP reported. Other aid organizations also have sent funds and experts to Chile.

But many relief organizations still are scrambling to address Haiti's needs. And they will be strained to respond to another massive disaster in so short a time.

Sharon Schmickle reports on science, international affairs and other subjects for MinnPost.