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Timing of Next Indonesia Tsunami a Mystery

June 27, 2005

When will the next Indonesian tsunami strike? With last December’s tragedy and a second large earthquake in the area just three months later, that’s more than just an abstract scientific question.

But there is no scientific answer.

“If anyone tells you they know, they’re pulling your leg,” says geophysics professor Rob McCaffrey of Rensselaer Polytechnic Institute in Troy, N.Y.

The Dec. 26 tsunami, the most devastating in recorded history, was triggered by the biggest earthquake in 40 years and the second largest ever recorded.

Roger Bilham of the University of Colorado recently noted that the quake, centered off the coast of Sumatra, was so big:

_It released about enough energy to power the United States for six months.

_It set off small local earthquakes in south-central Alaska.

_At least to a minor extent, “no point on Earth remained undisturbed.”

_The associated shifts in the ocean floor displaced enough water to fill a tank one mile wide, one mile high and more than seven miles long.

_The Bay of Bengal and the Andaman Sea lost enough volume capacity from the seafloor uplift to raise global sea level by about four-thousandths of an inch.

_The tsunami struck not only nearby lands, but eventually in weakened form reached the Antarctic, the Arctic Ocean and the east and west coasts of the Americas.

Nobody could have seen a quake this big coming, he said. And nobody can accurately predict the next one in that area with accuracy either, he said in an interview.

Scientists do know of nearby fault areas that have caused earthquakes in the past and are accumulating pressures that will probably lead to more quakes someday. But scientists are “very bad” at predicting when, Bilham said. Researchers can make mathematical calculations, but the real data they need lie out of reach, miles and miles deep in the earth.

It’s hard enough to predict when a ripe apple is going to fall from a tree, he said. “Earthquake prediction is much more difficult than that, because you don’t even see the apple. It’s underground.”

Scientists are concerned chiefly about a fault area that lies southeast of December’s rupture and the March earthquake, which didn’t cause a tsunami.

If you follow the fault line from the site of the December quake to the area of the March quake and keep going, you come to segments under the Batu and Mentawai islands.

Earthquakes don’t always follow each other along a fault line like dominoes falling, but it’s hard to ignore that possibility here. In the May issue of the journal Nature, researchers reported that their calculations indicated the March quake brought this area closer to rupture, with the greater threat appearing in the Mentawai segment.

The Batu-Mentawai area produced a large earthquake in 1833, triggering a tsunami that devastated the adjacent coast of Sumatra. But because of the quake’s location, most of the tsunami’s energy flowed out into the open Indian Ocean. It passed south of Sri Lanka and India, which were hit hard by the Dec. 26 tsunami.

So if that section of the fault ruptured again and a similar tsunami appeared, its threat to distant lands would be somewhat lower than that of December’s, said Thorne Lay of the University of California at Santa Cruz.

And when might that happen?

Since the two recent quakes to the north probably increased the fault stress in the Batu-Mentawai area, “it’s likely to come sooner than it would have otherwise,” he said. “But we just don’t know whether that’s days, months, years, decades, even centuries from now.”

On the Net:

U.S. Geologic Service site on tsunami: http://walrus.wr.usgs.gov/tsunami

Nature site on Dec. 26 tsunami: http://npg.nature.com/news/specials/tsunami/flash/map.html




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