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Sandy signs of prehistoric shaking.

Sandy signs of phrehistoric shaking

If seismic history repeats itself, the Wabash Valley along the Indiana-Illinois border will someday host a strong earthquake. Geologists last week reported finding evidence that an intense temblor -- of estimated magnitude 6.2 to 6.7 on the Richter scale -- shook this region sometime before the arrival of European settlers.

Stephen Obermeier of the U.S. Geological Survey in Reston, Va., and colleagues made their discovery by searching the Wabash Valley area for features indicating liquefaction -- a process that occcurs when strong shaking turns buried sediments into a pressuarized liquid. The slurry can escape upward through fractures and erupt at the surface.

To date the liquefaction deposits, the researchers relied on geological and archeological information from nearby layers of soil. Most of the shaking-produced features formed during a single earthquake sometime between 7,500 and 1,500 years ago, the scientists conclude in the March 1 SCIENCE. The distribution of the deposits helped them determine the quake's size.

During the last 200 years, the Wabash Valley region has generated five slightly damaging eartquakes, ranging from magnitudes 5 to 5.8. The new evidence demonstrates that a far stronger earthquake rattled the area in the recent prehistoric period. Because the valley continues to experience sporadic seismic activity, including many small shocks eacy year, Obermeier says: "I have to believe that this area is a good candidate for a future strong earthquake."

Earthquake fatalities rose in 1990

Last year proved particularly lethal in terms of earthquakes. Seismic catastrophes caused almost as many deaths worldwide in 1990 as during the entire decade of the 1980s, the U.S. Geological Survey reported last month. Most of the fatalities in 1990 resulted from a major earthquake in Iran on June 20. The magnitude 7.7 shock killed an estimated 50,000 people and left 60,000 injured.
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Title Annotation:evidence of an earthquake in the Wabash Valley of Illinois and Indiana, includes worldwide earthquake fatalities
Publication:Science News
Date:Mar 9, 1991
Words:307
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