wpostServer: http://css.washingtonpost.com/wpost2

Most Read: Local

Posted at 01:59 PM ET, 08/23/2011

D.C. earthquake: Powerful tremor shakes region

Strongest quake to hit Virginia since 1897

Live chat transcript with Seismologist

The U.S. Geological Survey (USGS) has confirmed an earthquake struck central Virginia at 1:51 p.m ten miles south-southeast of Louisa, Va., near Mineral, Va. It was felt throughout the D.C. metro region and over a large part of the eastern U.S. USGS reports its magnitude measured 5.8.

Minutes after the quake, Marcia McNutt, USGS director -- who watched objects falling from the shelves in her office -- concerned about aftershocks, cautioned that the shaking might not be over.

“When something like this happens, remember what to do in the case of a seismic event. Duck, get under something sturdy like a desk or a doorway, get away from falling glass. Make sure that you are not in the way of falling objects like pictures, bookshelves, books, anything that’s not firmly connected the wall.”


(U.S. Geological Survey)
An earthquake also occurred in the D.C. metro region July 16, 2010. A 3.6 magnitude quake centered near Gaithersburg shook the area. Today’s 5.8 magnitude quake was about 160 times bigger than that quake and almost 2,000 times as powerful (USGS How Much Bigger Calculator).

Virginiaplaces.org reports:

Virginia is classified as a “moderate” seismic risk, and has a 10-20% chance to experience a 4.75 quake every century or so. In quakes above 4.5 on the Richter scale, buildings begin to fall...

Since 1977, Virginia has experienced 160 earthquakes, of which just 16% were felt according to Virginia Tech.

Experts say there are two active earthquake areas in Virginia: The one apparently responsible for Tuesday’s quake runs along the James River between Charlottesville and Richmond and is known as the Central Virginia Seismic Zone. The other is an area centered in Giles County in southwest Virginia, which had a 5.8-magnitude quake more than a century ago.

The Virginia Tech Seismological Observatory(VTSO) offers the following on earthquake history in the central Virginia seismic zone:

Since at least 1774, people in central Virginia have felt small earthquakes and suffered damage from infrequent larger ones. The largest damaging earthquake (magnitude 4.8) in the seismic zone occurred in 1875. Smaller earthquakes that cause little or no damage are felt each year or two.

It would appear today’s quake is the largest on record in that region. USGS said it was the strongest quake to hit the entire state since the 5.8 magnitude tremor in 1897.

VTSO adds the following:

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).

Reports on the USGS website indicate that the quake, 3.7 miles deep, was felt from Georgia to southeast Canada (see neat interactive map from Detroit Free Press).

The energy from the quake propagates farther in this region compared to a similar quake in the Southwest U.S. as the crust is colder and there are fewer cracks to dissipate the energy.

“The rocks are old and cold and they carry the seismic energy very far. Even a magnitude 6 or less earthquake can be felt over a considerably large area, unlike California where the shaking is more concentrated,” said Mike Blanpied, associate coordinator for the USGS earthquakes hazards program.

Useful Links: Virginia’s Largest Earthquakes
Virginia Earthquake history
More Virginia Earthquake resources

Aftershock risk?

USGS’s Blanpied cautioned aftershocks are possible:

“Aftershocks could go on for days, weeks, or even months. They’re most likely to be felt under the next three or four days.”

An aftershock of 2.8 on the Richter scale occurred at 2:46 p.m., 5 miles south-southwest of Mineral and a 2.2 magnitude aftershock occurred at 3:20 p.m. 8 miles south of Louisa.

Eyewitness account at epicenter from CWG’s Steve Tracton

Purely coincidentally myself, wife, and grand daughter were in a Food Lion within a mile of the center of the quake, Mineral ,Va. The first indication of something unusual was a the sound, louder than a loudest thunderclap I’ve ever heard, but not thunder like - more like an explosion. I knew of course from weather conditions it could not have been thunder and concluded immediately it had to be an earthquake (I’ve only felt one relatively minor earthquake before in when in Israel.)

Immediately after, the store floor shook violently, lights went out and everything - and I mean everything - came crashing down from shelves. Luckily we were not in an aisle so were not hit by falling cans and glass, although, of course, quite scared at first.

Everyone in the store was ordered out right after the building stopped shaking. I attempted to take some pix with phone camera but was blocked from doing so. I did hear that there was some structural damage to the store - cracks in the floor and possibly foundation.

Fortunately, it appeared no one was seriously injured, except possibly one woman who appeared to have been hit on the head by something. Certainly could have been worse but, nevertheless, an experience to be remembered.


USGS map indicating earthquake epicenter, where it was felt, and the degree of shaking. (See latest zoomed in map)

More Washington Post Earthquake coverage:

Earthquake rattles Washington area

Earthquake videos

Post Now

BlogPost

Dr. Gridlock

By  |  01:59 PM ET, 08/23/2011

Categories:  Latest, Environment

 
Read what others are saying
     

    © 2011 The Washington Post Company