Yes, Virginia, it was an earthquake.

It was immediately apparent from the news coverage on Tuesday, August 23, 2011 that people on the East Coast of the United States are not at all accustomed to having the ground move beneath them. Reactions ranged from “I thought it was a terrorist attack” to “Scary!”, and the story displaced other national and even international news stories for days.

People generally did the wrong thing during the quake – many ran outside, even though FEMA recommends that people who are inside a building stay where they are , drop, take cover, and stay away from windows and other glass (http://www.fema.gov/hazard/earthquake/eq_during.shtm). Even the US Geological Survey office in Reston, Virginia was evacuated!

On the West Coast, where I happened to be during the quake (in Seattle), people in general (and news anchors in particular) took great pleasure in comparing the Virginia quake to others of similar magnitude that happen much more frequently in the seismically active Northwest. Lots of ribbing and mocking – mostly good natured – took place in the days that followed the quake.

East Coast earthquakes are much less common than West Coast earthquakes, because the eastern edge of the United States is what is called a passive continental margin. It is the edge of a continent (North America), but it is not the edge of a plate, and the edges of plates are where most seismic activity tends to occur. The edge of the plate on which the North American continent is riding is in the middle of the Atlantic Ocean, at the Mid-Atlantic Rift. That is where the North American Plate is moving apart from the Eurasian Plate (and, further to the south, the African Plate); earthquakes (and volcanic activity, as well) do indeed occur along that rift boundary.

Actually, East Coast earthquakes happen all the time; they’re just usually not this powerful. The August 23 quake was the strongest ever measured in Virginia. The causes of many of these intraplate earthquakes are not thoroughly understood. Some involve the activation of old, deep-seated faults. The area in which this particular quake occurred, the Central Virginia Seismic Zone, had experienced seismic activity before, but not along known, measured faults. The largest known historic earthquake in this area was a bit father to the west, in 1875, and was likely a bit less powerful than the 2011 quake. 

One geophysicist explained the cause of intraplate earthquakes like this: Imagine a plate the size of North America that is being jostled around on all sides by interactions with other plates. Obviously most of the earthquake activity will take place around the edges, but you’re also bound to be building up some stresses in the middle of the plate, too, which will eventually cause earthquakes to occur in the middle of the plate.

The details of the 2011 and historic quakes in the Central Virginia Seismic Zone can be found at the USGS Earthquake Hazards website (http://earthquake.usgs.gov/) . This is a truly rich resource that you should check out, if you haven’t already.

Of real concern is the lack of earthquake preparedness in the East. The general panic and uncertainty about what to do highlights this lack of preparedness. Another aspect is the general state of earthquake resistance of buildings in the East. Many older buildings are actually better suited to survive earthquake shaking than modern high-rises, which – unless specifically engineered to be earthquake-resistant, as they are typically now on the West Coast – are more likely to be rigid and subject to failure during seismic shaking. An interesting site with much information about cutting-edge research on earthquake engineering, damage assessment, and architecture is the Consortium of Universities for Research in Earthquake Engineering (http://www.curee.org/).

An additional educational resource that is worth checking out is Teachable Moments, provided by the University of Portland and IRIS Education and Outreach (IRIS is Incorporated Research Institutions for Seismology). You can find a Teachable Moment about the August 23 Virginia earthquake at http://www.iris.edu/hq/retm, and you can sign up to be on the Teachable Moments list-serv at the IRIS website (http://www.iris.edu/hq/programs/education_and_outreach/).

 Barbara Murck