Earthquakes in the Southeastern United States

Fig. 1: History of earthquakes large enough to be felt (greater than magnitude 3) in the eastern United States. These earthquakes are all examples of intraplate earthquakes.

Most students who have ever taken an earth science course learn that earthquakes occur when two tectonic plates interact with each other, which can happen when two plates collide (subduction zone), as two plates slide past each other (transform fault), or as the plates rift apart (rift zone). Japan and Chile each sit near subduction zones, which have created the largest earthquakes in written history. The San Andreas Fault Zone of California is a transform fault zone that can trigger large earthquakes up and down the state. The deadly 2009 earthquake in L'Aquila, Italy was an example of an earthquake in a rift zone.

But what about earthquakes that occur in the center of a continent? In the United States, a large earthquake tore down building in the late 1800 in Charleston, SC; the Eastern Tennessee Seismic Zone sees atleast a thousand small earthquakes each year; and the largest earthquake in the continental U.S. occurred along the New Madrid fault near St. Louis, Missouri in the early 1800s. These earthquakes in the center of tectonic plates are known as intraplate earthquakes, and every earthquake in the map shown here of the eastern United States is an example of an intraplate earthquake.

The New Madrid earthquake was one of the largest in U.S. history, but large, deadly, intraplate earthquakes have also occurred in China, India and Spain, just to name a few. If interactions between different tectonic plates trigger earthquakes at the plate boundaries, then what causes earthquakes within the center of a plate? As tectonic plates have collided and moved around the earth, they've broken up and reformed. What was once a plate boundary or a mountain range may now, on the surface, be nothing more than gentle rolling hills -- like those in Appalachia. However, beneath the surface are remnants of those ancient faults and tectonic plates. When these ancient faults are triggered, an earthquake occurs. Most intraplate earthquakes are small, though occasionally they can get quite large. The two questions nagging seismologists today are: 1) What triggers these ancient faults, and 2) How can we predict where, when and how large one of these earthquakes will be?

These are the questions we're trying to answer at the Virginia Tech Seismological Observatory.