Collaborative Research: Analysis of AIDA (Aftershock Imaging with Dense Arrays) data from the the 2011 M5.8 Virginia earthquake

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The August 23, 2011, magnitude 5.8 earthquake in Louisa Co., Virginia, provided an opportunity to test a novel type of high-density aftershock deployment utilizing EarthScope Flexible Array instruments. This event had a NE-striking reverse faulting focal mechanism, a hypocentral depth of 6 km (USGS), and occurred in a previously recognized seismic region known as the ¡§Central Virginia Seismic Zone¡¨. Beginning August 27, AIDA deployed 201 stations in three phases, including lines with a 200-m station spacing above the aftershock zone and a 60-km long regional profile. The survey was designed to record wavefields at sufficiently dense spacing to minimize spatial aliasing and lower the event detection threshold. This use of array methods will allow the location the aftershocks with high precision and the imaging of geologic structure with resolution on par with typical controlled-source crustal surveys. Preliminary work has focused on joint tomography for seismic velocity and hypocenter locations, synthesizing high resolution seismic reflection (CRP) profiles from Vertical Seismic Profile (VSP) processing of earthquake records and body wave and surface wave imaging using ambient noise techniques. In addition, this unique dataset will allow the investigators to test seismic interferometric methods for synthesizing virtual seismograph records at the physical location of the earthquake sources. Preliminary aftershock locations define a best-fitting plane striking ~25?a and dipping 55?a E, consistent with the moment tensor solution for the main shock. Initial application of seismic interferometry has recovered both virtual body waves and surface waves, and VSP to CRP techniques can be effectively used to produce reflection imagery of structure throughout the crust. Ongoing work includes locating more of the 1000 events with high signal-to-noise, event imaging using reverse-time waveform migration, and using seismic interferometry/CRP stacking to produce high quality images of earth structure in 3D . The resulting high-resolution subsurface images and aftershock characterization will not only constrain key geologic relationships at depth for this important, intraplate hypocentral region but should provide a template any future ¡§High Definition¡¨ aftershock deployments (e.g. 1000+ channels).The use of high density arrays the the one that is prototyped with AIDA represents a major advance in a) our ability to reconstruct in detail what happens at depth during an earthquake and b) to use the signals from earthquakes to provide imagery of the surrounding earth volume in unprecedented detail. Such ¡§high definition¡¨ imaging will greatly aid in probing/monitoring other important subsurface processes, from volcanism to hydraulic fracturing for energy and other resources. In both cases, these techniques represent an important new tool in detecting, understanding and mitigating natural hazards. These results should have a major impact on how we study earthquakes and related tectonic phenomena.