A 5.8M earthquake was felt in eastern Virgina, 40 miles from Richmond and 88 miles from Washington DC, on 8/23/2011 at 17:51 GMT (1:51 PM local time). The quake was felt over a wide area of the East Coast of the United States, as is typical of quakes in that region. It was one of the largest recorded quakes in that region. Because quakes are rare, there was a bigger reaction than there might have been to the same quake in another area. The energy released by the Japanese 9.0M quake was 60,000 times that released in this quake. There were aftershocks measuring M4.2 and M4.5 in the two days following the quake.
Many buildings were evacuated, including the White House, the Capitol and New York skyscrapers. Some cell phone service was disrupted and some public infrastructure, including the Holland tunnel,transit systems and airplanes were closed or delayed as a precautionary measure while they were checked for damage. It was felt as far away as Detroit. There have been no reports of deaths, although there were some injuries and some buildings sustained damage near the epicenter. Among the worst damaged buildings was the National Cathedral in Washington, DC, which had some of its pinnacles and statues broken. There was a crack in the Washington Monument. Bricks fell from some buildings and items were knocked off of store shelves.
This quake was typical of east coast quakes in that it was felt over a much wider area than a similar quake elsewhere. Earthquakes in this area are shallow and magnitudes are typically no greater than this one but the crust is denser than in the western US, so it propagates the earthquake waves better, over as much as 10 times the area as the same magnitude quake in the west. Earthquakes usually occur on the boundaries of tectonic plates that make up the Earth's crust. This area is not near a plate boundary. However, 300 - 500 million years ago, the plates were different than they are now. Faults in this area are not well understood but may be boundaries of ancient plates that have since been joined into the North American Plate. Eastern US faults are buried under newer geological formations and in places were covered by glacial till after the Ice Ages, making them difficult to study. The US Geological Survey (USGS) is an invaluable resource in understanding and tracking earthquakes. See the USGS summary of this quake. See also the US Geological Survey (USGS) summary of one of the aftershocks for a discussion of this issue.
See also the following news stories:
* US earthquake leads to evacuation of White House(The Guardian 8/23/11))
* Aftershock rattles US east coast(BBC 8/25/11))
* Why Virginia Quake Shook Entire Coast(Live Science.com 8/23/11))