EAST TENNESSEE GEOLOGICAL SOCIETY
JANUARY 2012


Monday, January 9, 2012
6:00 - 7:30 pm

Pellissippi State Technical Community College
10915 Hardin Valley Road, Knoxville
J.L Goins Administration Building, Cafeteria Annex


JANUARY PRESENTATION

August 23, 2011
Central Virginia Earthquake


By
Jeff Munsey
Tennessee Valley Authority
Knoxville, Tennessee

Abstract

On August 23, 2011 a magnitude 5.8 earthquake occurred in Louisa County, Virginia. This was the largest earthquake to occur in the southeastern U.S. since the 1897, magnitude 5.9 Giles County, Virginia earthquake. This earthquake was felt over much of the eastern U.S. and southeastern Canada, and according to the U.S. Geological Society was the most widely felt (felt by the most people) of any earthquake in U.S. history. Moderate damage occurred in Louisa County and adjoining counties in Virginia, and isolated cases of severe damage occurred south of Louisa, Virginia. Two schools in Louisa County were damaged so severely that they could not be used. Damage to most large industrial structures and infrastructure was light with some notable exceptions such as the Washington Monument. The North Anna nuclear plant recorded ground motion that exceeded the plant seismic design basis and caused some minor damage. The plant was offline for about 2 1/2 months performing inspections and testing. Some damage occurred at two small dams in Virginia, though neither resulted in dam failure. Although no surface faulting has been discovered, geomorphic effects of the earthquake included creation of liquefaction features (sand boils) and small rock falls. Water levels in wells as far away as New York were affected by the earthquake.

The August 23 earthquake occurred in a broad, diffuse region of seismicity known as the Central Virginia Seismic Zone. The Central Virginia Seismic Zone has produced small and moderate earthquakes since at least the 18th century. The previous largest historical shock from the Central Virginia Seismic Zone occurred in 1875. The 1875 shock had an estimated magnitude of about 4.8. The 1875 earthquake shook bricks from chimneys, broke plaster and windows, and overturned furniture at several locations. A magnitude 4.5 earthquake on 2003, December 9, also produced minor damage. Based on the rate of past earthquakes in this zone, the August 23 event was a 1 in 700 year event.

The August 23 earthquake occurred as reverse faulting on a north-northeast-striking plane that is well defined by aftershocks at a depth from about 8 km to essentially the surface. Previous seismicity in the Central Virginia Seismic Zone has not been causally associated with mapped geologic faults. At this time, the August 23 earthquake has also not been associated with mapped geologic faults; however, the trend of the faulting for the August 23 earthquake is similar to faults and ductile shear zones that have been mapped in the area. As of December 16th, approximately 700 aftershocks have occurred near the location of the main shock. Most of these earthquakes had very small magnitudes. The largest aftershock occurred about 1 1/2 days after the main shock and had a somewhat surprisingly low magnitude of 3.9.


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