Conditions combined for devastating tsunami in Samoa

Seth Borenstein, Associated Press Yahoo News 1 Oct 09;

WASHINGTON – Because of a lethal combination of geology and geography, the people of American Samoa didn't stand much of a chance. Almost every condition that triggers bad tsunamis was in place this time, generating waves that raced toward the island territory at speeds approaching 530 mph, or as fast as a 747 jumbo jet. And there was almost nothing to slow the water down.

It all started with a type of earthquake that tends to generate strong tsunamis because of the angle at which the ground breaks. Also, the quake was extremely powerful, with a magnitude of 8.0. It struck just below the ocean floor, which means very little lost energy. And it happened in deep water, which means bigger waves.

The deeper water also meant the tsunami sped along the ocean faster. American Samoa happened to be close to the epicenter, about 125 miles, and at just the right angle, with almost no shallow water to slow the speeding waves down.

Put that all together and there was less than 25 minutes, maybe as little as 13 minutes, between the ground shaking and the first tremendous waves swamping Samoa.

And it didn't help that an international computerized system, designed for relief agencies to figure out if they needed to respond, had a computer failure that caused it to pooh-pooh the tsunami's wrath initially.

"This is the kind of earthquake one would expect to be very destructive in the areas close to the epicenter, and unfortunately it was," said Stuart Weinstein, deputy director at the Pacific Tsunami Warning Center in Ewa Beach, Hawaii.

The shaking at the weather service office in Pago Pago, the capital of American Samoa, was so bad that one official immediately called the tsunami warning center in Hawaii, while the island's chief meteorologist phoned homeland security to activate the warning system. Just before 7 a.m. local time, bulletins were issued and alerts aired on TV and radio.

But there wasn't enough time. Four sets of waves 15 to 20 feet high hit. As of Wednesday afternoon, the death toll had climbed well over 100.

"It's one of those heart-wrenching situations where you have some time, but what can you do? It's not much time," said Eric Geist, a tsunami specialist and geophysicist at the U.S. Geological Survey in Menlo Park, Calif.

Tsunamis are towering waves triggered by earthquakes. They can top 100 feet, and can stick around for as much as an hour, recede violently, then come back hours later.

In some ways, the geological conditions were even worse for Tuesday's tsunami than they were during the devastating 9.0-magnitude quake and tsunami that killed more than 150,000 people in Asia in 2004. But this time, there were fewer people in harm's way in the middle of the Pacific Ocean.

The key factor this time was the type of earthquake. It was an "outer rise" quake — one that breaks the sea floor in a way that concentrates the energy and pushes up at the water to create a wave, said Bruce Jaffe, an oceanographer and tsunami specialist at USGS in Santa Cruz, Calif. Strong quakes are usually a different type, called a thrust event.

The area where it hit is no stranger to quakes, getting a few magnitude-6-to-7 ones per year, said Peggy Hellweg, a geophysicist at the Berkeley Seismological Laboratory. Because quakes are measured on a logarithmic scale, a magnitude-8 is 1,000 times stronger than a magnitude-6 in terms of energy released, Hellweg said.

Tuesday's quake was the fourth-strongest outer rise on record, Geist said.

This quake was also relatively shallow in the ground, only 11.2 miles under the sea floor. That's important because the closer the quake is to the surface of the ocean, the less energy dissipates as it travels through the ground.

It was also in deep water. Initial estimates are that there was well over 3 miles, maybe even 4 miles, of water above the shaking ground, Geist said. That means more water displaced, and thus bigger waves. If there had been only 1 mile of water above the quake, the waves would have been about 11 feet smaller, Geist estimated.

That deep water also was responsible for the blinding speed of the tsunami. The deeper the water, the faster a tsunami travels.

This water was so deep that the tsunami could have been zipping along at 530 mph, Geist said. Usually, a tsunami slows down when it hits shallow water. Around the United States, for example, the shallow continental shelf slows downs waves dramatically.

Samoa didn't have that protection until just before the tsunami reached the shore. And by the time it hit, it was still coming at 30 mph.

And by the time you see a tsunami, "it's usually too late to outrun it," Geist said.

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Associated Press writer Mark Niesse contributed to this story from Honolulu.

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NOAA animation of the Sept. 29 tsunami: http://nctr.pmel.noaa.gov/samoa20090929.html

NOAA tsunami safety information: http://www.noaa.gov/features/protecting(underscore)0409/tsunami.html

U.S. Geological Survey Earthquake Information Center: http://earthquake.usgs.gov/eqcenter/

Samoa 'too close to epicentre' for long warning
Hannah Devlin, Times Online 30 Sep 09;

The tsunami that that swept the South Pacific islands was caused by a powerful subsea earthquake.

A sophisticated network of seismometers around the world pinpointed the location and strength of the quake within minutes of it occurring and sent out a tsunami alert.

But the proximity of Samoa and American Samoa to the epicentre meant that it was only a matter of minutes before their coastlines were hit by 3m-high waves, giving villagers little time to escape to higher ground.

“These islands are right on the subduction zone, which means the official warnings don’t always come soon enough,” said Professor John McCloskey, a geophysicist at the University of Ulster.

Instead, he said, there has been an increased focus on educating people to move immediately to higher ground and stay there for several hours if they feel a tremor that lasts for more than around 30 seconds.

Unconfirmed reports from Samoa and American Samoa suggest that in some places people did heed the indicators of an imminent tsunami and that many evacuated their homes. At other locations farther from the quake, the warnings were broadcast on TV and radio.

Seismometers recorded the initial earthquake event at 6.48am Samoa time (5.48pm GMT) this morning. The US Geological Survey’s rapid reporting system registered the quake at magnitude 8.1. Others put it slightly higher at 8.3 or slightly lower at 7.8. The complexity of the calculation involved in determining the strength of an earthquake means that some uncertainty is to be expected.

Three minutes later, the Pacific Tsunami Warning Centre in Hawaii sent out a “tsunami watch” warning to islands in the vicinity, including Samoa, American Samoa, the Cook Islands and Fiji, predicting that an earthquake of this size was likely to result in a tsunami.

Computer modelling allowed the centre to predict arrival times and the likely height of incoming waves. It warned that American Samoa, which is around 200km (124 miles) from the epicentre, was likely to be hit by 3m waves within ten minutes and Samoa within 20 minutes of the quake.

Several minutes later, the centre updated the warning saying that local sea-level readings confirmed that a tsunami was taking place.

The quake occurred at the north end of a large north-south tectonic plate boundary, close to the 10,000m-deep Tonga-Kermadec Trench. Where the two plates meet, the Pacific Plate is being driven under the Australian Plate, a phenomenon known as subduction happens.

Early calculations suggest that the rupture was around 200-300km long and that the plates moved around 4-7m. Scientists expect an event of this magnitude to occur around once or twice a year.

The earthquake was considerably smaller than the Indian Ocean quake on Boxing Day in 2004, which had a Magnitude of 9.3.

“The earthquake magnitude and deformation were much less than in the rare huge earthquakes of magnitude 9.0 and higher, such as that off Chile in 1960, and the Boxing Day 2004 earthquake off Sumatra,” said Gary Gibson, the chairman of the executive committee of the International Seismological Centre.

“The fault length for those was greater than 1,000km, and the plates moved 10-20m or more relative to each other.”

Whether a tsunami is triggered by an earthquake depends on a number of factors, but primarily the amount of movement of the plates. The depth of the sea determines the speed with which the tsunami moves towards the shore. In this case, the tsunami is believed to have reached up to 430mph (692km/h).

Earthquakes frequently result in secondary quakes and scientists are currently calculating the likelihood of a second event.

New Sumatra quake takes seismologists by surprise
Yahoo News 1 Oct 09;

PARIS (AFP) – The huge earthquake that hit Sumatra occurred at a deep, unexpected location, illustrating the dangerously complex geological mosaic in this area, a seismologist told AFP on Thursday.

The 7.6 magnitude quake struck on Wednesday 80 kilometers (50 miles) beneath the sea, 45 kilometers northwest of the city of Padang, according to US Geological Survey (USGS) data.

The fault line where this happened runs parallel to Sumatra and is called the Sunda Trench.

It marks a "subduction" zone, where one plate of Earth's crust rides on top of the other.

To the west is the Australia plate, which is moving northeast at about five centimetres (two inches) a year).

The Australia plate is being forced under, or subducted, by the Sunda plate, which lies to the east.

Scientists had long feared a major earthquake would occur on the part of the trench near Padang.

They considered it vulnerable to a so-called quake "cascade" that began with the notorious 9.1 quake of December 26, 2004 that unleashed the Indian Ocean tsunami.

"Cascade" events can occur in long, badly-stressed faults. The stress of a large earthquake causes the next section of a fault to weaken and then rupture, in a domino-like effect.

But Sandy Steacy, a professor at the Environmental Sciences Research Institute at the University of Ulster in Northern Ireland, said Wednesday's quake was not part of this chain reaction.

"The event yesterday was kind of strange, it wasn't what we would have expected," she said.

"It appears if the most recent earthquake didn't occur on the interface of the subduction zone. It occurred within the plate that is being subducted, the part that's going down beneath the interface."

Steacy said that seismologists were working hard to calculate what had happened, but her bet was that the quake had occurred deep below the seabed on the tongue of the Australian plate, contorting as it was forced under the Sunda plate.

"Clearly you are going to get faults in that kind of situation, because you're taking a slab, you're bending it, you're pushing it down, so you're going to get material breakage there," she said.

"I think once calculations are done, they will show that the stress had increased along that structure," she said.

"I suspect that structure, nobody even knew it was there. We don't have any way of mapping the faults in the subducting slab because it's so deep. It's only by having earthquakes on it that gives you an indication."

French expert Robin Lacassin, of the Institute of the Physics of the Globe in Paris, agreed that an "as-yet unknown mechanism" had unleashed Wednesday's quake.

"It happened at some depth, around 80 kilometres. It appears to have occurred in the subducted plate, beneath the face where the two plates meet," he said.