Thursday, December 18, 2014

What Caused the Mineral Virginia Earthquake?

from USGS

On August 23, 2011 just outside of Mineral, Virginia a 5.8 earthquake occurred about five miles beneath the earth. The earthquake was felt by people from Georgia to Canada. The earthquake caused wells in my neighborhood to spew mud and foundations to crack and we are more than 60 miles northeast. The question is why did the earthquake occur near Mineral, Virginia and why was the earthquake felt here?

Those of us who took rocks for jocks (survey of geology) in college know that the Earth's crust is comprised of a series of continental and oceanic plates that are constantly moving. The plates ram into each other, sliding underneath or above each other, or pull apart. Most earthquakes arise along such fault zones and is triggered by the plate movement. The ground first bends and then snaps forming an earthquake to release energy along faults. There are no plate boundaries in Virginia, so, why did we have an earthquake?

The U.S. Geological Survey (USGS) scientists have been investigation why seismic events occur in certain parts of the central and eastern United States, like the Central Virginia seismic zone, since there are no plate boundaries there, unlike the San Andreas Fault in California, or the Aleutian Trench in Alaska.

In 2012 USGS scientists conducted low-altitude geophysical (gravity and magnetic) flight surveys over the epicenter of the earthquake, located about eight miles from Mineral, Virginia. Deep imaging tools were used because the earthquake occurred about five miles beneath the earth’s surface. Maps of the earth’s magnetic field and gravitational pull can show subtle variations that reflect the physical properties of the deeply buried rocks. From this information deep earth maps of the region were drawn.

According to Anji Shah, the lead author of the study: “These surveys unveiled not only one fault, which is roughly aligned with a fault defined by the earthquake’s aftershocks, but a second fault or contact between different rock types that comes in at an angle to the first one. This ... suggests that the earthquake occurred near a ‘crossroads,’ or junction, between the fault that caused the earthquake and another fault or geologic contact.”

The magnetic data obtained by the USGS showed a wide bend in the deeply buried rocks within the Central Virginia seismic zone. This anomaly suggests to the USGS scientists that seismic activity may be increased in other nearby areas with locally increased rock weakness or permeability. The primary fault line of Mineral earthquake (and its aftershocks) runs to the northeast almost continually for tens of miles practically to Haymarket. According to Dr. Shah the continuity of the associated geologic structures probably allowed the seismic energy to be carried in that direction, consistent with moderate to high levels of damage from Louisa County to Washington, D.C., and neighboring communities.

The gravity and magnetic data found that a fault seems to separate different types of rocks with varying densities and strengths. The scientists believe that the junction between the faults may be the origin of the earthquake and wonder if similar junctures exist elsewhere. There is still so much to learn about Earth and this is just one small step towards a deeper understanding the earth around us.

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