Three seconds after 1:51 p.m., August 23, 2011, a 5.8 magnitude earthquake rattled Virginia.1 With an epicenter 3.7 miles under the ground, located some 41 miles northwest of Richmond, the ground shaking was felt as far away as New York City and even in Toronto, Canada.
In Washington, DC, only 83 miles from the epicenter, visitors inside the observation deck atop the Washington Monument were thrown about by the force of the shaking; falling mortar and pieces of stone caused minor injuries, though all the people inside exited safely.2 Damage occurred throughout the metropolitan Washington area, but the Washington Monument was among the significantly damaged structures. Cracks, spalls, and displacements of stones and joints throughout the building required a major repair process.
Why an Earthquake in Virginia?
Why would the Southeastern USA experience such a devastating earthquake, a region that sits snugly in the middle of a tectonic plate? The ground shaking usually happens in areas that rest on the edges of tectonic plates, such as in California.
Berk Biryol, a seismologist at the University of North Carolina Chapel Hill, and his colleagues set out to investigate a potential cause.3 They put together 3D images of the uppermost part of the earth’s mantle, which is just below the continental crust and comprises the bottom of the North American tectonic plate. The tectonic plates today move at the pace of growing fingernails, moving atop a layer of warm, pliable rock called the Andrew Snellingsthenosphere (from the Greek ἀσθενής asthenḗs meaning “weak” + sphere meaning “globe”).
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