3D REFLECTION IMAGING OF CRUSTAL STRUCTURE OF THE HYPOCENTRAL VOLUME SURROUNDING THE AUGUST 23, 2011 VIRGINIA EARTHQUAKE FROM VSP PROCESSING OF AIDA AFTERSHOCK RECORDINGS

Here we report on our efforts to image fine structure in the hypocentral volume associated with the main shock and aftershocks of the August 23, 2011, Mineral Virginia, earthquake by reflection processing of the relatively dense seismic recordings obtained during the AIDA deployment following that event. Here we show how VSP (vertical seismic profiling ) techniques, a methodology widely by the oil exploration industry, can be applied by treating aftershocks as a 3D array of “downhole” sources with the AIDA instruments in the role of a reverse VSP surface array.The velocity models and event locations provided by the complementary analysis of AIDA recordings provide the auxiliary information needed for this type of imaging. After application of VSP normal-moveout corrections using the aforementioned epicentral information, the seismic traces were then transformed by VSP-to-Common Reflection Point (CRP) mapping into a stacked, 3D volume image of reflectivity. Although the quality of this 3D image is limited by the effectively 2.5 nature of this pilot AIDA array, key structural elements are revealed. The most prominent of these is a strong, moderately east-dipping reflector at a depth of approximately 6 to 8 km that directly underlies, and is continuous beneath, the more steeply dipping aftershock zone. We interpret this reflector as part of the complex, imbricate thrust sequence associated with Paleozoic convergence during the Appalachian orogen. Its apparent continuity beneath the fault zone implied by the aftershock hypocenters suggests that this inferred fault zone has little or no cumulative offset, supporting the speculation that this event represents a newly formed feature rather than reactivation of a longer lived tectonic structure. Perhaps more importantly, these pilot results clearly point to the kind of transformative imaging that can be expected from dense, true 3D acquisition deployments in the future