Next tsunami may hit any time

2005-06-28 08:50

New York - 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, New York.

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

Roger Bilham of the University of Colorado recently noted that the quake, centered off the coast of Indonesia's Sumatra Island, 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 1.6km wide, 1.6km high and more than 11km long.

The Bay of Bengal and the Andaman Sea lost enough volume capacity from the seafloor uplift to raise the global sea level by about one-hundredth of a centimetre.

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.

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, kilometres and kilometres 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 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. The journal Nature reported that researchers' calculations indicated the March quake brought this area closer to rupture, with the greater threat appearing in the Mentawai segment.

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."

- AP