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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