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A 3.6 magnitude earthquake shook Washington D.C. On July 16, rattling some nerves and raising the question: “how can this happen here? We're not in California!” While it is true that the majority of earthquakes in the U.S. do occur in California, there are multiple seismic zones and faults throughout the country, some more active than others, and some in surprising places.

Many of the faults, zones and volcanoes (active and dormant) up the West Coast and into Alaska are well-known—think the San Andreas fault in California, Comstock fault in Nevada and Mount Saint Helens in Washington. In addition many mountainous areas, like the Colorado Rockies (Sangre de Cristo), Wyoming Tetons (Teton fault) and Wasatch Range in Utah (Wasatch fault), are assumed to have some degree of seismic activity—to say nothing of Yellowstone!

But earthquakes do happen periodically elsewhere in the U.S., sometimes with devastating results, and there are certainly more to come in the future. Below is a brief overview of other seismic zones in the continental United States.

Central Virginia Seismic Zone This area lies just west of Richmond, VA. Activity here is not frequent, and not particularly energetic, producing a a 4.5 temblor in December 2003.

South Carolina Coastal Seismic Zone This zone runs from the Georgia border up to Wilmington, NC. There was a big quake along this fault in 1886 in Charleston, SC--a 7.3, which was felt as far away as New York City and Bermuda.

A cluster of zones and faults lies west of the Blue Ridge Mountains, including:

The Giles County Seismic Zone west of Blacksburg, VA on the West Virginia border. This area has seen some activity historically, but none especially large since a 5.9 temblor in 1897.

west of Blacksburg, VA on the West Virginia border. This area has seen some activity historically, but none especially large since a 5.9 temblor in 1897. The nearby Pembroke Fault was discovered in the early 1990s; it is currently quiescent.

was discovered in the early 1990s; it is currently quiescent. The East Tennessee Seismic Zone runs south of the faults above, through Knoxville, TN. It is active but fairly low-energy, with a 4.6 quake reported in 1993 in northeastern Alabama.

There is also a series of Gulf Coast Faults from the Texas-Mexico border through Mobile, AL to the western tip of the Florida panhandle. This is a relatively low activity zone, but it does occasionally spawn a mid-size quake, like a 6.0 in 2006 out in the east-central Gulf itself.

The most dangerous region, however, centers in the area from Indianapolis, IN heading downriver to Memphis, TN, consisting largely of the New Madrid Fault, Fluorspar Zone and Wabash Valley Seismic Zone. This zone also affects portions of western Kentucky and Tennessee, northern Arkansas, and large swathes of southern and eastern Illinois.

The most famous event along the New Madrid Fault (named for a small southern Missouri town) was a series of large quakes (from 7.0 to nearly 8.0) that shook the region from Dec. 16, 1811 through Feb. 7, 1812. Witnesses reported uplifts, temporary waterfalls on the Mississippi River (which appeared to even run backwards for a time in some areas), fissures, landslides and sand blows. Some of these quakes were felt in Boston and Toronto.

In modern times (1974) monitors were installed along this fault to track the numerous small quakes that plague this area. This vigilance is warranted, as a 7.7 slip on the New Madrid today would kill thousands and devastate tens of thousands of unreinforced structures, especially in Memphis.

So when the news reports a small quake in an odd location, like Oklahoma, upstate New York, or even the nation's capital, viewers should bear in mind that it's not just the West that sleeps uneasily—the Earth can slip and slide underfoot anywhere.

http://gldims.cr.usgs.gov/qfault/viewer.htm

http://www.hsv.com/genlintr/newmadrd/