Looking for Clues to Predict Future Tsunamis

Scientists think they may have found a precursor to the kind of earthquake that sent a devastating tsunami crashing into wide areas along the northern Indian Ocean, killing at least 200,000 people. If they are right, it may be possible to warn coastal residents a few years before such an earthquake strikes, thus allowing time for protective measures to be taken.

But if they are wrong, they will join a large collection of scientists who have tried and failed to figure out when an earthquake is about to hit.

The technique applies to subduction zones, like the Pacific Northwest and parts of Alaska, where the oceanic plate is being pushed under the continental plate. Those regions are capable of generating what scientists call "megathrust" earthquakes, frequently resulting in tsunamis.

The technique won't work in seismically active regions like Southern California, where subduction is not the reason faults move. But it may prove to be a lifesaver in many areas of the globe where subduction rules the day.

Nowhere is that need any greater than along the northwest coastline of North America, which many experts believe is past due for a catastrophic quake.

"We think we're on to something," says Jere Lipps, professor of integrative biology at the University of California, Berkeley.

But, he adds quickly, it needs to be confirmed by other researchers.

That Sinking Feeling

Lipps and a team of researchers have found evidence that in the years preceding a megathrust earthquake, the coastal area subsides, or sinks, just about a foot. It's subtle, but it shows up clearly in the life and death of tiny microorganisms that dwell right along the high tide line.

Their most compelling evidence was found in Alaska, where a 9.2 magnitude quake struck on Good Friday, 1964, devastating much of Anchorage and a number of other communities and sending a tsunami that killed dozens along the west coast.

The researchers found that between five and 15 years before that deadly quake struck, the coastline near Anchorage sank about a foot.

The subsidence was so gradual that "even people who lived there didn't notice," says David B. Scott, director of the Center for Environmental and Marine Geology at Dalhousie University in Halifax, Nova Scotia.

A report on the research will be published in the May/June issue of the Geological Society of America Bulletin.

Scott has spent 30 years studying tiny, shell-like creatures called foraminifera and thecamoebians, that are "about the size of a pinhead, or smaller," says Lipps, who like Scott is a micropaleontologist. These organisms live in the freshwater mud along the coast, and are very vulnerable to salt water.

Scott and Lipps, along with Andrea Hawkes of Dalhousie and Rod Combellick of the Alaska Division of Geological & Geophysical Surveys, collected 12-foot deep cores from Alaska and Oregon to see if the biological record could tell them anything about events leading up to major quakes.

The Alaska quake was particularly important because scientists know exactly when it occurred. The other quakes are historical, including one in Alaska 1,800 years ago and four in Oregon 3,000; 1,840; 1,670 and 300 years ago. But those dates are not as precise as the Alaska quake, "which we know down to the minute it happened," Scott says.

Thus it is possible to reconstruct the timetable for changes leading up to the 1964 quake with considerable confidence.

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