Science / Medicine : Shells Reveal Earthquakes

Studies of hearths and discarded seashells at old Indian sites suggest that huge earthquakes struck the Northwest in the last 300 to 400 years, far more recently than previously believed, according to geologists Rick Minor and Wendy Grant of the U.S. Geological Survey. They told a Geological Society of America convention that they had correlated sudden, dramatic changes in land levels to disruptions in Indian communities.

Previous evidence suggested that the last major quake in the region was 3,000 or more years ago. The quakes are believed to have been caused by one of the Earth’s plates diving under another at a fault line beneath the Pacific Ocean.

Chuck Steele of the Federal Emergency Management Administration said the discovery of such a recent quake could affect a Geological Survey study of the likelihood of a quake in the region and how to deal with it. The new information is significant because Oregon has been regarded as seismically “benign.”

Grant said carbon dating of hearth sites and layers of shellfish eaten by people living near the ocean shows periods where habitation abruptly ended. These periods can be correlated to evidence of dramatic drops in land levels followed by a tsunami, a giant wave associated with large earthquakes, she added.