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Why Was the Virginia Earthquake Felt So Widely?

Earthquakes are not rare in the eastern half of North America, and when big ones happen they are felt far and wide
virginia-shake-map



Courtesy of USGS

On August 23, just before 2 P.M. Eastern time, the entire eastern half of the North American continent rocked back and forth for a few moments. From northern Ontario down south to Georgia and inland as far as western Ohio and Tennessee, the 5.8-magnitude quake centered in Mineral, Va., had some broad reach.

But much more powerful earthquakes in the U.S. West rarely make their presence felt beyond the immediate vicinity. A temblor in Los Angeles is unlikely to affect San Francisco, while an earthquake that rocks Washington, D.C., rings bells in Boston. At the same time, eastern North America hasn't been at the seismically active edge of a tectonic plate since the Appalachians began to rise, more than 250 million years ago, and it now sits hundreds of kilometers from any such edge. So why does Virginia have a seismically active zone, anyway?

To find out why East Coast quakes travel well—and why the region has an undeserved reputation as immune to quakes—Scientific American spoke with geophysicist Julie Dutton of the U.S. Geological Survey.

[An edited transcript of the interview follows.]

Why do earthquakes in the eastern U.S. travel so far?
It has to do with the basic Earth structure. On the West Coast you have a lot of deformity—cracks, breaks, faults—and when an earthquake occurs a lot of that energy is dissipated. It's not felt as far because it has to travel through a lot of different rock formations and structures that are looser.

On the East Coast, you have condensed, old rock that, when an earthquake occurs, the energy is transferred very smoothly and easily over a long distance because there are not a lot of things to disrupt that energy path.

What is this East Coast rock made out of?

It's a harder bedrock, so that when an earthquake occurs the energy is transferred much easier from one place to another.

But the West Coast seems crumbly and fractured, and therefore easy to shake.
With that kind of crumbly, broken-up crust, the earthquake energy is lost in the movement of the crust from one area to another. The energy goes and hits a fault and some of that energy is lost in the fault. Energy is lost much quicker than if you have this solid mass where the Earth is just one big rock.

But there are still faults on the East Coast. Why don't they dissipate the energy?

You are dealing with much older rock. It's a part of the craton [old, solid, relatively stable areas of rock at the center of tectonic plates] that hasn't been broken up as much. There are lots of little faults.

What are those little faults, like the one that caused this earthquake, remnants of? And why are they still slipping after all this time?
The rocks are still dealing with stresses and strains. The whole continent is still moving, it's not just at the plate boundary. It just builds up stress over thousands of years and doesn't allow that break—then you can have a larger earthquake.

In this area, the Virginia seismic zone, which is 120 kilometers across in both directions, it's just a part of the continental crust that has some more stress and strain than some of the other regions. What exactly is the reason for that is not quite known.

So what can this earthquake be attributed to?
We have earthquakes on the East Coast quite often, more frequently than people realize. There are faults all throughout the eastern region that do cause earthquakes. It just takes a larger quake before people become aware of this.

In this case, it's not related to the Tohoku quake [in Japan] or other earthquakes. Mountains were made in the region a long, long time ago, and there are still stresses and strains. We don't know exactly which fault is to blame [though the leading candidates are the nearby Lakeside and Spotsylvania faults]. That is something that will be extensively investigated. It is an area that is prone to earthquakes, and we could see more in the future.

But earthquakes as big as this one are relatively low-probability events?
The Virginia seismic zone is known for earthquakes, but to have one of this magnitude was quite unusual. It's not something that people are prepared for because, in recent history, it hasn't occurred. It doesn't mean it hasn't happened in the past.

Will this shake up other faults in the region, causing more earthquakes?
That's a possibility for some earthquakes. Some earthquakes will completely release the stress of a region, and then it's in equilibrium. But sometimes you have an earthquake and it builds up the stress in a different region because that area is not allowing the release of energy. It can go either way.

It's a very difficult thing to figure out how exactly that stress is transferred. Is it locked up somewhere else, or is all the stress and strain in equilibrium?

So what can we expect in future?
We experience unusual earthquakes like this one all over. It is strange for Virginia, but it is not strange for us to see something that we weren't anticipating. We've had a large earthquake, but the chances of having another one are pretty small. It is not going to increase the seismicity of the region. Chances are this is an isolated event and you probably won't see it again in future. But you never know.

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