Tuesday, May 10, 2016

What Caused the 2011 D.C. Earthquake?

[Smithsonian] On August 23, 2011, a magnitude 5.8 earthquake hit Mineral, Virginia, about 90 miles southwest of Washington D.C. The temblor was the strongest east of the Mississippi since 1944, and was felt by more people than any other quake in U.S. history, reaching 12 states and several Canadian provinces. It shook the nation’s capital so strongly it cracked the Washington Monument and caused 20 million dollars in damage to the National Cathedral.
The earthquake was also  a bit of a mystery. D.C. lies over 1,000 miles from the edge of the North American Plate, away from any fault zones or seismic activity. Earthquakes in the area are rare if not non-existent. But a new study in the Journal of Geophysical Research has a new theory of why the capital shook: Bits of the Earth’s mantle are breaking off underneath the southeast U.S. and sinking. This thins the remaining plate, making it more prone to slip and cause seismic activity.
“This was an interesting finding because everybody thought that this is a stable region, and we would expect regular plate thickness,” Berk Biryol, a seismologist at the University of North Carolina Chapel Hill and lead author of study tells Lauren Lipuma at the American Geophysical Union blog.
Biryol and his colleagues used seismic waves from earthquakes in other parts of the world to study the ground below the eastern United States. Seismic waves act almost like an X-ray, Lipuma explains, with the waves moving quickly through dense, cold rock and speeding up in magma and warmer rocks. Measuring these waves give researchers insight into what is below the Earth's surface. Read More