The Geological Column: The summer quake of 2011

Annie Craddock

As you know, the east coast experienced an unexpected earthquake this summer. Were you able to feel if from where you were? I was in Charleston, SC at the time and surprisingly didn’t feel a thing. The episode was a 5.8 magnitude earthquake that caused a lot of damage primarily in Virginia and the Washington, D.C. area.

This area had been identified as an active seismic zone since the mid 19th century, mostly creating very small earthquakes that did not cause any damage.

The last large earthquake felt in the seismic zone was estimated to be a 4.8 in 1875 — seismographs were not invented then — and a 4.5 in 2003. Both of these caused minor damages, but none were nearly as severe as the quake this August.

Because the quake happened at the shallow depth of 6 km, a significant amount of damage occurred. When quakes occur at deep levels, energy is able to dissipate before felt on the surface. But when a quake is shallow, the energy is near its original velocity and highly destructive.

The earthquake was caused by slip along a fault as a reverse fault. In other words, the rocks on one side of the fault moved downwards. Usually this occurs when the crust is being stretched.

In this particular case, however, the plate on which the earthquake occurred was not regionally stretched, but pushed west by a rift occurring in the middle of the Atlantic Ocean.

Even so, the regional geology shows that the area should be an inactive zone. Geologists know that the quake happened on a smaller fault in the field. This also makes the reason for the movement harder to understand.

Because predicting earthquakes is not certain, it is especially important to know the geological history of an area before you buy a house, or at least to get the right insurance.