Jessica Robertson: Hello and welcome to USGS CoreCast. I'm your host, Jessica Robertson.
In light of the recent earthquake in Virginia, today we are talking about East Coast Earthquakes; the history, the geology of this area versus the West Coast and more. We are joined with David Russ, who is the USGS Regional Executive for the Northeast area. Thank you for joining us today, Dave.
David Russ: It's my pleasure to be here.
Jessica Robertson: The size of the earthquake we felt yesterday was a magnitude 5.8. Is that common or unusual for this area?
David Russ: That would definitely be unusual. We have earthquakes in the east, as most people are aware, but not very large and generally not as large as 5.8 where we start to get damage and strong motion that could affect structures.
Jessica Robertson: I know that earthquake in Virginia was felt widely in the surrounding states. So far, the USGS "Did you feel it" website received a 122,000 reports from citizens in 8,000 zip codes; why was this earthquake felt so widely and is that unusual?
David Russ: This earthquake that occurred yesterday was felt around the Atlanta area in Georgia, up through all the northern states and into Canada, so very large area indeed.
It's unusual from the point of view that we don't usually have earthquakes large enough in the eastern part of the country to cause energy from an earthquake to be propagated such large distances.
It wasn't unexpected from the point of view that when you do have an earthquake this large in the eastern part of the country, that it would be felt over such a wide area. The reason for that is that the earth's crust in the east is different than in the west, where we have most of our experience from earthquakes.
In the east, the earth's crust is a bit older and more rigid. It tends to propagate seismic energy more efficiently, causing it to ring like a bell, perhaps, when it does take place. Whereas in the west, the warmer crust tend to absorb seismic energy; it doesn't or propagate quite so far.
Jessica Robertson: So Dave, what are the largest earthquakes that have happened in the east?
David Russ: There haven't been a lot, but those that have occurred are in Charleston, South Carolina in 1886, about a magnitude 7.3. There was an event that occurred offshore of Cape Ann, north of Boston, Massachusetts in 1755. That event, it's hard to calculate exactly what the magnitude was, given its antiquity. So, the event that occurred yesterday, although small by damage in earthquake or disaster standards, was large for the east.
Events in the east that are truly catastrophic are considered to occur maybe only about one in a 1,000 years, or even less frequent, and it's hard to calculate that with any great precision in part because we haven't had enough earthquakes to base it upon.
Jessica Robertson: Are there any special dangers for earthquakes in the east?
David Russ: I would say one of the dangers, first of all, is a lack of knowledge of where there're geological faults that might be capable of generating a damaging earthquake. Therefore, it's hard for us to know how to plan properly and to be prepared.
Obviously, the eastern part of the country is one of the older parts of the country. The built infrastructure is aged and much of it not designed to resist strong shaking from an earthquake, therefore, more likely to potentially to fail.
In addition, because many people in the eastern part of the country have not during their lifetimes or the lifetimes of their families, experienced an earthquake. There's a general tendency not to think that it's a problem in the east or that it's a hazard, because of that lack of understanding, not knowing how to properly respond in the event of an earthquake.
First, most people are startled, so they try to understand, "What is this I'm feeling?" They don't know, necessarily, if it's an earthquake that they should duck and cover, that they should try to get under a hard object, hard thing like a desk or a doorway or a table, they might run out of the doors too soon and get caught in falling debris. One of the dangers is simply, knowledge and preparedness, and lack of that.
Jessica Robertson: Dave, can you tell us why are earthquakes in the east different than in the west?
David Russ: Most earthquakes of a damaging size occur along the margins of these great crustal plates that the earth is broken into. In the west, we have the boundary of one of these plates; that's what the San Andreas Fault is. The motions between plates are more dynamic and they produce larger earthquakes. In the east, we happen to be in the middle of one of these crustal plates and the stresses that accumulate there, and therefore generate earthquakes, are not the same as those along the plate boundary.
We don't have a good understanding yet about the actual stresses that do accumulate to cause earthquakes in the center of a plate, which is where we have the eastern United States, so it's hard for us to understand exactly when the next bigger earthquake is going to occur and where that might be.
Jessica Robertson: Thank you for joining us today, Dave. Is there anything else you want to share with our listeners?
David Russ: I would like the listeners to be aware of the fact that there're always lessons to be learned following any earthquake. Even in events such as that occurred in Virginia yesterday, a magnitude 5.8, which by most accounts isn't deemed to be a large devastating earthquake but what it does demonstrate is that the earth is a dynamic active body. It can generate earthquakes almost anywhere, including relatively low seismic risk areas such as the eastern U.S.
It's important for us to study events such as this because they are the key to mitigating hazards, reducing damage and to improve our understanding of earthquakes, and therefore be better prepared to respond and have less hazards and loss from them.
A magnitude 5.8 earthquake occurred in Virginia on August 23, 2011. Join us as we talk to David Russ, who is the USGS Regional Executive for the Northeast Area, about that event as well as earthquake risk, history and geology along the East coast.