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How could an earthquake happen in Virginia?

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According to the U.S. Geological Survey, the small town of Mineral, Virginia was was the epicenter of a 5.9 magnitude earthquake earlier this afternoon, making today's quake at least equal in intensity to Virginia's largest earthquake on record, which the USGS says occurred in Giles County, Virginia in 1897.

Reports of the quake — which occurred at roughly 1:51 PM EST and is said to have lasted between 20 and 30 seconds — and its effects are popping up all along the East Coast, with people in regions as far and wide as Martha's Vineyard, Pittsburgh, and Milwaukee taking to twitter en masse to report their feeling the quake.


The USGS is asking people to inform them if they felt the quake. And the USGS have good reason to take interest in the extent of the tremors — this degree of geological activity, while not unheard of along the East Coast, is exceedingly rare.


What is the likelihood of earthquakes in Virginia?
While the USGS's seismic hazard map does indicate that the risk for earthquakes in Virginia is slightly higher than that of other areas along America's Eastern Seaboard, when you get right down to it the chances of an earthquake of this magnitude along the majority of the U.S.'s Atlantic coast is relatively rare.

Geologically speaking, Virginia — along with much of the rest of the eastern coast of the North American continent — is actually right around the middle of a tectonic plate (compare this with cities like San Francisco, which rest pretty much smack dab on the westernmost edge of the North American tectonic plate — see the red along the western coast of the USGS seismic hazard map? That's what we're talking about here).

Tectonic plates, you'll recall, are the dozen or so enormous rigid slabs of the Earth's crust that, when they slip and shift along one another in a violent and unexpected fashion, are known to cause major earthquakes. (Image via)


The thing is, the easternmost edge of the plate on which America's East Coast rests is in the middle of the Atlantic Ocean, and historically speaking tends to shift at the leisurely pace of 2-3 centimeters per year. What's more, since Virginia and the rest of the East Coast reside on top of the middle of the plate, conditions tend to be relatively stable, with large-magnitude earthquakes instead tending to affect places like San Francisco, L.A., and Japan — places that are right on the edge of a tectonic plate.

Due to its location, Virginia is instead prone to experiencing "intra-plate" seismicity (i.e. tectonic movement within a plate as opposed to at its boundaries) like it did today. But intra-plate activity is typically much less noticeable.


So why was today's quake so uncharacteristically strong?

In an interview with NPR, seismologist John Vidale said that the Earth's crust actually has ancient faults in many places, but that "most of them don't move very much...the mystery is really what's pushing the faults to make it move now — and there are a lot of theories."


Vidale said one of these theories is that the plate is in the process recovering from the end of the last glacial period, relieving itself of pressure incurred while the region was still covered in ice by rising and putting stress on the Earth's crust.

Chrisopher Bailey, chairman of the geology department at William and Mary, told NPR that another explanation could be that the current movements of the North Atlantic tectonic plate are putting pressure on the same ancient faults mentioned by Vidale.


According to David Spears, Virginia's state geologist, there are three such faults in Virginia alone.

"An area of central Virginia, along a line that runs from Fredericksburg to Gereensboro, N.C., used to be a plate boundary," Spears said. "Perhaps there's some leftover stress in the crust."


When was the last time a quake like this hit the Virginia area?
All this is to say that a 5.9 earthquake in Virginia is big news. For a sense of perspective, consider that just last year the USGS reported a 3.6 magnitude Earthquake in Montgomery County, Maryland — which, like the town of Mineral, Virginia, is very close to the D.C. area. The USGS classified last year's Montgomery County earthquake as a "significant earthquake." At the time, Mike Blanpied provided this summary of the historical seismicity of the Maryland/Virginia region, prepared by the USGS National Earthquake Information Center (NEIC):

Earthquakes in Maryland and Northern Virginia are uncommon but not unprecedented. In the past four decades, there have been about a dozen magnitude 2.0 and higher earthquakes within 100 km of this recent earthquake. The largest earthquake on record in Virginia is a magnitude 5.6 in Giles County on May 31, 1897. More recently in the broader area, a magnitude 4.1 earthquake struck Lancaster County, Pennsylvania on April 22, 1984. This Pennsylvania earthquake caused slight damage at Conestoga, Lampeter, Mt Nebo and New Providence and was felt from West Virginia to Connecticut. A series of small, felt earthquakes spanning March to July 1993 occurred near Columbia, Maryland, within 33 km (20 miles) of the July 16, 2010 earthquake. The largest in this series was a magnitude 2.7 in March, 1993. On May 5, 2003 a magnitude 3.9 event struck near Cartersville, Virginia about midway between Charlottesville and Richmond. That earthquake was felt widely in central Virginia and in parts of Maryland. The most recent earthquake in this region was a magnitude 2.0 event on May 6, 2008 near Annandale, Virginia.


Note: the "largest earthquake on record" reported in this official USGS statement from 2010 is not misreported; the summary states the largest earthquake on record in Virginia as being 5.6 in Giles County, while the USGS website ranks the Giles County earthquake at 5.9.


Also listed here is a catalog (too large to include here) of Virginia's 15 largest earthquakes between the years 1774 and present day. Again — today's quake ranks at least as high as the highest magnitude quake on this list.

Why is a 5.9 magnitude quake in VA worse than a 5.9 magnitude quake in CA, and how will infrastructure be affected?
A 5.9 magnitude earthquake might not seem like much to people from the Nation's West Coast, who are more accustomed to quakes of similar magnitude — but geologists say that a 5.9 quake in New England is decidedly different from a 5.9 quake along the Pacific Coast.


According to the USGS, earthquakes in the Eastern U.S. tend to be felt over a much broader range of land, with quakes in regions east of the Rockies sometimes affecting areas as much as ten times larger than an earthquake of similar magnitude on the West Coast. The USGS finds:

A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).


The far-reaching effects of an earthquake with an epicenter in Virginia therefore have serious implications for places nearby (like Washington, D.C.) with historic monuments, and other buildings not necessarily designed for the frequency of quakes experienced in other parts of the country.

Speaking to the tendency for East Coast quakes to affect a wider area, and the potential impact of a quake as large as today's on historical buildings, seismologist Graham Kent — over at The Washington Post's Live Seismologist Chat — said that as you move closer to the epicenter of a quake like today's, the potential affect on buildings' structural integrity obviously increases. He continued:

There will be people inspecting various buildings. I think the most important point in that regard, is that the Earth's crust is a lot colder there, so energy travels much further without being dissipated. Even though it's a 5.9, it's a lot bigger deal than a 5.9 would be in California or Nevada. You might see damage further away from the epicenter than you might expect.


But what sort of infrastructure does the East Coast have in place to accommodate for earthquakes? How will existing infrastructure be affected? So far, limited damage has been reported from the earthquake, and is believed to be limited primarily to the quake's epicenter in Virginia.

The state is home to a nuclear power plant, located just miles from the earthquake's epicenter, that was immediately taken offline (there have been no reports of damage to the power plant); Amtrak trains were halted; and cellphone services failed to stay afloat as networks became bogged down by an excess of phone calls.