A Year After the 2011 Virginia Earthquake: What More Do We Know?

A Year After the 2011 Virginia Earthquake: What More Do We Know? 

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Melanie Gade:  Hello and welcome to USGS CoreCast. I'm your host, Melanie Gade. On August 23rd, 2011, tens of millions of people in the eastern U.S. and southeastern Canada were startled by the sudden onset of moderate to strong ground shaking from a rare magnitude 5.8 earthquake in central Virginia.

This earthquake was among the largest to ever occur on the Eastern Seaboard during the past 400 years. It caused extensive damage in central Virginia, and was likely felt by more people than any other earthquake in U.S. history.

Joining me today is U.S. Geological Survey geologist, Dr. Mike Blanpied. 

So Mike, A year after the event, what more do we know about the cause of the Virginia earthquake?

Dr. Mike Blanpied:  Well, we know that the Virginia earthquake, like all earthquakes, occurred on a fault under the ground. We know from field investigations that that fault was a buried fault that did not expose itself at the surface at the time of the earthquake. So the trick, to understand this earthquake is to understand where was the fault, which fault was it, where was it located, and how big was it? An essential question that we're trying to ask is, is this the biggest earthquake likely to occur, or is there the possibility of something bigger, either here or elsewhere in the region?

If one was to look at the geology in central Virginia, one would see that there are long bands of rocks, and the contacts between rocks, running from Northeast to Southwest, basically the same direction as the Appalachian Mountains.

Between those rock units are surfaces that could act as faults - they could slide - and we know that, throughout the eastern US, there's compression, there's stress, acting on all those surfaces.

The trouble with eastern US earthquakes is they don't happen very often, and sometimes the faults on which they happen haven't had an earthquake for a very long time. Our trick is to figure out which one of those faults - one of those many, buried, not very well-understood faults - had an earthquake on it, back in August.

Melanie:  And Mike, what USGS efforts are currently underway to improve our knowledge about this earthquake?

Mike:  Well, immediately after the earthquake, information was gathered out in the field, identifying damage patterns and so forth. Now that we're past that immediate phase we're undergoing a series of investigations, working in conjunction with the State of Virginia and our university partners, collecting numerous kinds of data, both on the ground and from airplanes flying over the region, bouncing seismic waves off the rocks beneath the surface, and so forth.

This information will all be put together into models that will help reveal the geologic structure, where are the faults underneath the ground. Then, combining that information with the aftershocks that were recorded so well after the earthquake, we should have a much better picture of what happened back in August.

Melanie: So Mike, one final question for you here is:  How has the Virginia earthquake improved scientists' understanding of the seismic hazard in the eastern U.S. in general?

Mike:  We don't have very many earthquakes in the Eastern U.S., and so every single one is an opportunity to learn things that we have not learned before, things we don't understand well enough. A couple of things that we don't understand about eastern US earthquakes as well as we would like to is: how do we identify structures upon which earthquakes occur, and how can we tell how big an earthquake might be?

For example, we might locate a fault in central Virginia that runs for 100 miles. Does that mean that an earthquake of that kind of length, a very whopping big earthquake, is likely to occur? We doubt it, but how can we know for sure? So we're going to study this Virginia earthquake as carefully as we can, to use that information to illuminate our understanding of earthquake hazards throughout the eastern US.

Another issue that we need to know very well is how far shaking travels. An earthquake occurs in a spot, but the shaking affects a region. And those who are in the eastern US recognize that that shaking -- from a moderate-size earthquake by California standards, a 5.8 -- it was felt all the way from Canada to the Gulf of Mexico and all the way out to the Mississippi River by some people.

That's a huge area, and that reflects the fact that seismic waves travel very far and very efficiently in the eastern U.S. A moderate earthquake in the East can cause damage, potentially, over a much larger area than the equivalent earthquake in the western U.S.

In order to predict the likely shaking from earthquakes, in order to build buildings that are sure to withstand that shaking, we need to understand that process very well and how it varies from place to place. The recordings from the Virginia earthquake will improve our estimation of shaking hazard in the East.

Melanie:  Thank you, Mike. This concludes this episode of USGS CoreCast. Visit earthquake.usgs.gov for more information about the USGS Earthquake Hazards Program. And don't forget to follow the USGS on Twitter at twitter.com/USGS, or visit our other social media channels at usgs.gov/socialmedia. CoreCast is a product of the U.S. Geological Survey, Department of the Interior. I'm Melanie Gade. Thanks for listening.