Paper No. 4
Presentation Time: 9:00 AM-6:00 PM
HIGH-RESOLUTION SEISMIC TOMOGRAPHY FROM DENSE ARRAY RECORDINGS OF THE AUGUST 23, 2011 CENTRAL VIRGINIA EARTHQUAKE AFTERSHOCK SEQUENCE
DAVENPORT, Kathy1, HOLE, John A.1, CHAPMAN, Martin C.1, HAN, Liang1, QUIROS, Diego2, BROWN, Larry D.2 and MOONEY, Walter D.3, (1)Geosciences, Virginia Tech, Blacksburg, VA 24061, (2)Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, (3)Earthquake Hazards, U. S. Geological Survey, Menlo Park, CA 94025-3591, kdavenport@vt.edu
The M 5.8 earthquake on August 23, 2011 in Louisa Co. Virginia is the largest event recorded in the Central Virginia Seismic Zone. This intraplate seismic zone is comprised of many poorly-defined seismogenic structures. Specific faults responsible for earthquakes in the CVSZ and other eastern US seismic zones are usually not identified due to relatively low strain rates and sparse seismic networks. The August 23, 2011 event provided an opportunity to test a novel type of high-density aftershock deployment that will define the associated fault structures at higher resolution than traditional aftershock networks. The AIDA (Aftershock Imaging with Dense Arrays) 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 of aftershocks with high precision and the imaging of geologic structure with resolution on par with typical controlled-source crustal surveys.
Beginning August 27, AIDA deployed 201 stations in three phases, including lines with 200-m station spacing above the aftershock zone and a 60-km profile of three-component sensors to quantify regional attenuation characteristics. Preliminary work has focused on joint tomography for seismic velocity and hypocenter locations, vertical seismic profile event stacking, and ambient noise imaging. Preliminary aftershock locations define a best-fitting plane striking ~25° and dipping 55° E, consistent with the moment tensor solution for the main shock. Work in progress includes locating more of the >1000 events that have high signal-to-noise, event imaging using reverse-time waveform migration, and applying interferometry methods to image earth structure. The resulting high-resolution subsurface images and aftershock characterization will help constrain the geologic model of the region, and provide a template for future high-density aftershock deployments.
