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.
Aftershocks following important earthquakes have traditional provided vital information on the nature of the associated main shock, in particular the geometry of rupture surfaces and the accumulation of strain in the days and month after the even. In this experiment we tested a new approach to recording these aftershocks, an approach with promises to provide greatly improved resolution on both the rupture process and nature of geological structures that may have played a role in the initiation of earthquakes. In particular, we deployed an unusually large and closely spaced array of seismographs in the vicinity of the August 23, 1911, M5.8 Mineral Springs, VA, earthquake using instrumentation originally designed for exploration of structure using artificial sources. These dense deployments, which we have termed AIDA for Aftershock Imaging with Dense Arrays, were expected to not only result in improved locations for aftershocks, but also allow us to use the aftershocks as sources for high resolution imaging of the geologic volume that contains the aftershocks. As it turned out, these arrays proved even more powerful than first suspected. The data recorded not only resulted in better event locations, it was found to detect and locate events at much smaller magnitudes that traditional aftershock deployments. Moreover the dense coverage facilitated a more detailed estimation of seismic velocity in the rock volume surrounding the main event and aftershock. Of particular importance is that 1) the AIDA array allowed for the computation of the rupture history of very small events and 2) the aftershock recordings could be used to generate seismic reflection images in 3D of structure near the aftershock zone. This later was accomplished by recognizing that the geometry of surface recording of subsurface microearthquakes can be treated with Vertical Seismic Profiling techniques to produce 3D volume reflection images. A similar AIDA deployment after the Maine Mw4.0 Waterboro, ME, earthquake of Oct 16, 2012 event confirmed these advantages of dense recording. Perhaps the most important aspect of both AIDA studies is the demonstration that dense surface deployments of large numbers of highly portable seismometers, like the ones used in oil and gas prospecting, can provide heretofore unavailable details of earthquakes and the structure that host them.
