Tuesday, 23 August 2011

Virginia Earthquake shakes Washington, New York. 23 August 2011.

A few minutes before 2.00 pm on 23 August 2011 the eastern coast of the United States was shaken by a rare earthquake. The quake was centered 14 km south of Mineral (141 km southwest of Washington DC), Virginia, and was felt as far south as South Carolina, as far north as New Hampshire and as far east as Oregon (this is slightly dubious). The quake was measured as 5.9 on the Richter Scale by the US Geological Survey, and was at a depth of about 0.1 km - very shallow and therefore potentially very dangerous, particularly in an area like the eastern US where buildings have limited earthquake protection. As yet there have been no reports of any casualties, but there has apparently been considerable damage to property in and around Mineral and there are reports of damage to the central spire of the National Cathedral in Washington DC. In addition the North Anna Nuclear Power Plant in Virginia was taken off line while it was checked for any damage, and buildings were briefly evacuated in several East Coast cities.

A magnitude 2.8 aftershock was felt locally at 2.46 pm. This was also a very shallow quake (~0.1 km) to the south of Mineral, although with slightly less than one thousandth of the energy this is much less likely to have caused any significant damage.

Virginia is a long way from any active tectonic margin, and is not known for its earthquakes. However the state does have two potentially hazardous faults, associated with the origins of the Appalachian Mountains during the Palaeozoic. This occurred when the ancient Iapetus Ocean closed, causing a collision between North America and Europe. When this happened the rocks that form the modern Blue Ridge Mountains were pushed up over the older rocks to the east. This lead to the formation of a fault on the eastern margin of the Blue Ridge Mountains, between the Blue Ridge rocks and the older sediments they overlay. In the West the rocks of the overlying Piedmont Province were also forced upwards, but have been slipping back, away from the Blue Ridge Province (this is a simplified explanation), creating a second major fault, between the Piedmont and Blue Ridge Provinces. It is on this fault that the earthquake occurred.

A cross section through the rocks of Virginia.

Earthquakes were reported in Virginia in 1774, 1811, 1812, 1828, 1833, 1852, 1861, 1875, 1897, 1898, 1908, 1910, 1918, 1919, 1929, 1959, 1977, 1978, 1980, 1981, 1983, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1991, 1992, 1993, 1994, 1994, 1995, 1996, 1997, 1998, 2000, 2001, 2003, 2004, 2006, 2007, 2009 and 2010. While many of these caused damage and alarm, only two fatalities have ever been attributed to an earthquake in Virginia, when a quake in 1833 caused a mine collapse at Brown's Coal Pits, near Dover Mills in the north of the state.

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