Tuesday, March 29, 2005

Why didn't this quake cause a killer tsunami?

By Michael Coren
CNN.com
Monday, March 28, 2005 Posted: 9:03 PM EST (0203 GMT)

(CNN) -- Why do some earthquakes spawn tsunamis that kill thousands while others hardly stir a ripple on the ocean surface?

"That's one of the mysteries of seismic science," said Caroline Bell, spokeswoman for the U.S. Geological Survey. "We are hoping that with an increase in the seismic [monitoring] system ... in the Atlantic, Caribbean and around the world, [we] will be able to determine why some offshore earthquakes cause tsunamis and some don't."

Monday night's quake in the Indian Ocean was centered 60 miles north on the same fault line of the earthquake on December 26, 2004, that launched tsunamis that left more than 300,000 people dead or missing.

"This looks like a fraternal twin of the December 26 earthquake," said Kerry Sieh, a professor of geology at the California Institute of Technology. "It's not a duplicate. It occurred a little bit further south, a couple kilometers further south. But it's the same type of earthquake."

The December quake was registered 9.0, the biggest in 40 years. The magnitude of the quake Monday night was recorded between 8.5 and 8.7, according to various monitoring agencies, but caused only a slight wave.

The USGS reported that a tide gauge near the Cocos Islands in the eastern Indian Ocean recorded a "small" tsunami, but its size was unclear. The agency said no major tsunami was observed near the epicenter of the earthquake.

Robert Cessaro of the Pacific Tsunami Warning Center said Monday's quake could have sent its energy farther to the south than last year's quake.

"All that pressure to the north would have been relieved" by that quake, Cessaro said. "We think this event probably ruptured to the south, with the beam of energy probably propagated to the south toward Mauritius and ... Rodrigues [islands]."

Tsunamis occur due to a number of factors from undersea landslides to massive displacement of the Earth's crust. Volcanoes also may launch the killer waves that can tower 32 feet high (10 meters) and sweep miles inland.

After examining the fault line of an earthquake, geologists can usually determine what occurred.

But "it does not help us predict if an earthquake will produce a tsunami," Bell said.

The USGS, working with a number of agencies of other governments, hopes a global network of seismometers, buoys and relay devices will allow tsunami predictions in time to save lives.

One of the leading theories is that the wavelength of earthquake readings -- the frequency at which the ground shakes -- may indicate a coming tsunami, but that is still far from certain.

Finding a seismic smoking gun for tsunamis will finally allow researchers to predict the killer waves.

"That's one of the goals of the USGS, to keep these events from becoming the disaster they are [today]," Bell said.

The warning came swiftly Monday after the earthquake rumbled off the coast of northern Sumatra, sparking panic among people living along the Indian Ocean.

Within hours, tens of thousands of people who survived a killer tsunami just three months earlier heeded evacuation orders to abandon their homes for higher ground. The temblor shook the ground from Indonesia to Thailand.

Although the size of the earthquake can play a large part in gauging its damage, it is not always an accurate indicator of its potential to produce a tsunami.

Jan Egeland, the U.N. disaster relief coordinator, said tsunamis usually occur when an earthquake reaches a magnitude of 8.5.

Yet earthquake fault lines, the point at which plates of the Earth's crust rub together, can stretch for hundreds of miles and quakes may erupt in different ways anywhere along that fault. That makes precise predictions hard.

"It's so common for people to interpret the epicenter as a point from which everything radiates," said Brian Atwater, a USGS geologist at the University of Washington.

"[In the December earthquake], the energy was radiating from fault rupture that was 1,300 kilometers long. ... The fault just unzipped," Atwater said.

"It took seven minutes from start to finish, and all along the way it's radiating energy and displacing sea floor and generating a tsunami."

That makes rapid and accurate predictions virtually impossible for developing countries with poor infrastructure.

False alarms, which have been common in the past, dilute the effectiveness of subsequent warnings.

Since the massive waves may travel at speeds exceeding 500 mph over deep water, scientists have scant time to interpret data.

"You're damned if you do and damned if you don't in this business," Atwater said. "It's a tough choice."

CNN

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