Tuesday, August 23, 2011

August 23: Virginia Earthquake Might Be Tied To Deep, Ancient Fault Zone

At 1:51 pm EDT today, a moderately intense earthquake (magnitude 5.8) shook the ground for 30 seconds, centered on the town of Mineral, Virginia.  Mineral is located along the Piedmont about halfway between Richmond and Charlottesville.   The USGS estimates that the quake's epicenter was buried 3.7 miles deep.  The quake was felt from New York to North Carolina.  One thing to note about east coast earthquakes is that they are felt over much larger areas than west coast quakes, due to the unique geology of the east.  Additionally, the very shallow nature of the quake contributed to its widespread footprint.  The quake was felt in 22 states, from Maine to Florida, from Virginia to Illinois.

When you plot Mineral on a geological map of Virginia, you note that it sits within the metamorphic rock province of Virginia.   The region encompasses the Central Virginia Seismic Zone.  The rock units are extensively fractured and deformed, from ancient compressive forces that raised the Appalachian Mountains.  Mineral, Virginia sits exactly over an extensive fault zone, called the Spotsylvania Lineament, that cuts northeast to southwest across the state.  To geologists, the lineament is somewhat of a mystery.  Some believe that it developed hundreds of millions of years ago, during the Paleozoic Era, as part of tectonic collisions between small crustal plates called terranes.  

If the quake was indeed associated with this ancient, deeply buried fault zone, it is interesting to speculate that a geological feature this old still occasionally must relieve some strain (stored energy) - restless Earth at its best!

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