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Friday, June 17, 2005

Research on fossils may offer clues on when tsunami will hit

Geologists studying fossils in Alaska and Oregon have discovered what they believe is a signal that occurred a few years before major coastal earthquakes in the past.

Seismologists have known for some time that really big quakes with the potential to create a killer tsunami hit the Pacific Northwest coast every 500 years on average. But the interval in between can vary from just a few centuries to 1,000 years. The last one struck the area in 1700, so it is not out of the question that another could hit in the near future.

On the other hand, there could be hundreds of tsunami-free years before the next one rolls in.

So what's a city planner to do? Ask the bugs, says a team led by geologists Jere Lipps of the University of California-Berkeley and Andrea Hawkes of Dalhousie University in Nova Scotia.

The team studied microscopic fossils known as foraminifera that lived in coastal marshes in Alaska and Oregon. They found that a few years before several large earthquakes in the past, freshwater foraminifera died out and saltwater species suddenly appeared. This happened because the coast dropped slightly in elevation, allowing salt water to infiltrate the marshes at high tide, Lipps said.

Two to five years later, a major earthquake struck. Four of the five quakes the team studied from the past 3,000 years, including the 1964 Alaska earthquake, were followed by a tsunami, the group reports in the current issue of GSA Bulletin produced by the Geological Society of America.

The earthquakes were located in subduction zones where an oceanic tectonic plate is being forced beneath another plate. Sometimes the two plates stick together and quakes occur when they suddenly become unstuck and slip. The elevation drop the team discovered may have happened because plates stuck and the upper plate was bent down as the ocean plate tried to push under it, Lipps said.

"It's a very interesting idea that there would be these land level changes that would precede the earthquake," said Brian Atwater, a U.S. Geological Survey geologist based at Seattle's University of Washington. But Atwater would like to see evidence in more locations to corroborate the finding.

Lipps' team is working to find that evidence in mangrove swamps in Mexico and marshes in New Zealand, and Lipps also hopes to go to southeast Asia where the devastating tsunami struck in December. The hope is that by placing instruments that measure small changes in the dip of the crust known as tiltmeters in subduction zones, scientists might be able to issue a warning a few years in advance that a major earthquake and tsunami is likely.

And there may be reason to believe the Pacific Northwest is due for another wallop like the magnitude 9 that struck the region in 1700 and sent a tsunami racing across the Pacific all the way to Japan where it damaged coastal villages.

A group led by Harvey Kelsey at Humboldt State University studied the record of past tsunamis in lake sediments in Oregon. They found that over the last 4,600 years, tsunamis have tended to come in clusters of three or four every 1,000 years with quiet stretches of around 1,000 years in between. The work also appears in the current GSA Bulletin.

Prior to the 1700 tsunami, there was a period of about 700 years with no giant waves. So that event could have been the start of a new cluster of tsunamis, raising the likelihood of another big wave in the near future.

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1 Comments:

Anonymous Anonymous said...

There are people who will be making bets on this.

11:59 PM  

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