Virginia Earthquake Aftershocks Identify Previously Unknown Fault Zone

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RESTON, Va.-- Aftershocks from the 2011 Virginia earthquake have helped scientists identify the previously unknown fault zone on which the earthquake occurred.

RESTON, Va.-- Aftershocks from the 2011 Virginia earthquake have helped scientists identify the previously unknown fault zone on which the earthquake occurred.The research marked one of the few times in the Eastern United States that a fault zone on which a magnitude-5-or-more earthquake occurred was clearly delineated by aftershocks, and is just one finding in a 23-chapter book with new information on the Virginia earthquake and eastern seismic hazards.

Research by the U.S. Geological Survey along with its partners and collaborators defined the newly recognized fault zone, which has been named the “Quail” fault zone. USGS and others worked cooperatively in an effort to capture the accurate locations of hundreds of aftershocks by deploying portable seismic instruments after the earthquake. Most of these aftershocks were in the Quail fault zone, and outlying clusters of shallow aftershocks helped researchers to identify and locate other active faults. Knowing where to look for the active faults helped to focus geologic mapping, geophysical imaging and other technologies to better understand earthquakes in the Central Virginia Seismic Zone and Eastern U.S.

The book includes contributions by Virginia Tech, the Virginia Department of Mines, Minerals, and Energy and the U.S. Nuclear Regulatory Commission among many others.

“Studies of the Virginia earthquake have improved our understanding of earthquakes and seismic hazards in Eastern North America,” said USGS geologist Wright Horton. “The Virginia earthquake served as a ‘wakeup call’ for many residents of the Eastern U.S., where the probability of major earthquakes is fairly low, but many buildings are vulnerable to damage during earthquakes.”  

The new book, “The 2011 Mineral, Virginia, Earthquake, and Its Significance for Seismic Hazards in Eastern North America”, is a collection of articles that covers a broad range of subjects relating to the 2011 earthquake. Highlights from the book include:

  • Earthquake shaking and its effects, such as widespread changes in groundwater levels, occurred at greater distances from the source in this and other Eastern U.S. earthquakes as opposed to those of comparable magnitude on the West Coast
  • Shaking intensities and related damage were more severe along the northeast trend of the Appalachians than in northwestward directions across this trend
  • Evidence that the earthquake ground motion was amplified in parts of D.C. and other areas around the Chesapeake Bay with thicker coastal plain sediments or artificial fill is stimulating further studies to determine how much seismic shaking is amplified by local geological conditions
  • Analysis of data on residential property damage in the epicentral area delineates a “bulls eye” distribution of shaking intensities and also confirms that damage is influenced by the age and construction of homes
  • Damage to unreinforced masonry buildings in D.C., as far as 80 miles from the epicenter, highlights the seismic risk to buildings in Eastern North American cities. Ground motions occur at farther distances from the epicenter on the East Coast than other parts of the U.S., and buildings are not as well designed to sustain these motions as in other locations 
  • Seismic reflection imaging—which is similar to medical sonograms—and geophysical flight surveys of the Earth’s magnetic and gravity fields were used to image geologic structures down to about 5 miles underground where the earthquake occurred
  • Airborne laser swath mapping using lidar, and radiometric flight surveys—which mapped radioactive elements in rocks and soils within a few feet of the land surface—identified and accurately located preexisting linear features including faults associated with aftershock clusters for detailed surface geologic mapping and trenching studies
  • New geologic mapping and trenching reveal previously unknown faults and evidence that the faults were active more than once in the past
  • Recorded ground motions from the Virginia earthquake were consistent with previous USGS estimates for the region, and they are helping to improve theassessments of potential earthquake ground motions used to design buildings that will be better able to withstand strong earthquakes

Earthquakes in Eastern North America are not as frequent or as well understood as those along Earth's tectonic plate boundaries, such as on the West Coast. The magnitude 5.8 Virginia earthquake was the largest to occur in the eastern U.S. since the 1886 earthquake near Charleston, South Carolina, and it may have been felt by more people than any other earthquake in U.S. history. It was felt over much of the Eastern U.S. and Southeastern Canada, triggered the automatic safe shutdown of a nuclear power plant and caused significant damage from Central Virginia to the National Capital Region. The earthquake provided a wealth of modern scientific and engineering data to better understand earthquakes and seismic hazards in Eastern North America.