This RAPID supports post-seismic deployment of novel high density seismic arrays using instruments from the Earthscope Flexible Array provided by IRIS/PASSCAL to investigate the Mw 5.8 Virginia earthquake of August 23, 2011. 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. This event passes with ca. 15 km of a NW-SE oriented deep seismic reflection profile contracted in 1981 by the USGS, providing a rare opportunity to link seismicity to deep structure in the eastern U.S. The preliminary hypocentral estimate suggests that the earthquake occurred within a complex zone of east-dipping reflections that define an imbricated Paleozoic thrust sheet of the central Appalachian Piedmont that is underlain by Grenville basement at a depth of ca. 9 km.
By merging the technologies of earthquake location with high resolution structural imaging, this experiment seeks to demonstrate how aftershocks can be exploited by both passive and active seismic techniques to identify seismogenic structures at depth as well as energy propagation characteristics for a region where both types of information are either rare or non-existent. The recording experiment constitutes a prototype for imaging earthquake processes and associated structures with unprecedented resolution by exploiting a new generation recording technologies. If the data proves as rich as expected, this experiment may well establish a new protocol for future aftershocks studies.
Aftershocks are an important source of information on the geometry of faults that rupture during the main event, and on the nature of the rupture process itself. This experiment was designed to test whether new, highly portable seismic instruments which can be deployed rapidly in large numbers can provide new kinds of information about an earthquake fault and its surrounding region. In this case, seismic equipment normally used to study artificial seismic sources in resource exploration was deployed at close spacing near the epicenter of the August 23, 2011 Virginia earthquake to test new techniques for imaging crustal structure that may be related to the generation of the earthquake. The funds associated with this project were used to cover the expenses associated with the deployment of these "superdense" arrays and a preliminary analysis of the results. The experiment clearly demonstrated the feasibility of deploying dense aftershock arrays using large numbers of "off-the-shelf" equipment. The preliminary analyses confirm our expectations that such dense arrays can detect weaker events than conventional seismic aftershock arrays, provide more precise epicentral locations, and generate seismic images of the geological structure in the epicentral array with detail comparable to that associated withdirect observation of rocks exposed at the surface.
