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.
(RAPID grant) Virginia Tech Principal Investigators: John Hole and Martin Chapman Collaborating Institutions: Cornell University, U. S. Geological Survey An earthquake of magnitude 5.8 occurred on August 23, 2011 in Louisa County, Virginia. The earthquake caused damage in central Virginia, and was widely felt throughout the eastern USA. This event occurred within the Central Virginia Seismic Zone, which has a history of small to moderate events. Specific faults responsible for earthquakes in the eastern USA have not been identified due to relatively low strain rates and sparse seismic networks. The 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 aftershocks at sufficiently dense spacing to lower the event detection threshold and to allow the use of seismic wavefield imaging methods. This small RAPID grant funded fieldwork to deploy seismic networks to record aftershocks of the 2011 event. It did not include funds for subsequent data analysis. Beginning August 25, the Virginia Tech Seismological Observatory deployed seven high-quality earthquake seismometers. These stations complemented similar small networks deployed by the U. S. Geological Survey (USGS), IRIS (Incorporated Research Institutions for Seismology), University of Memphis, and Columbia University, for a total of ~30 stations. Data were recorded for several months by Virginia Tech, and made publicly available through the USGS and IRIS. To complement this traditional seismic network, beginning August 27, AIDA deployed 201 stations in three phases. Stations were deployed along lines at 200-400-m station spacing above the aftershock zone. A 60-km profile was deployed to quantify how shaking decays with distance. Due to the logistics of the large number of stations and limitations of the instruments, the stations were on the ground for about 12 days. Preliminary data analysis has focused on locating aftershocks in the subsurface and imaging local geologic structures using direct and reflected seismic waves from the aftershocks. The data are archived at IRIS. Aftershock locations define a best-fitting plane striking ~25° east of north and dipping 55° eastward, consistent with the main shock. Work in progress (not funded by this grant) includes locating hundreds of aftershocks, direct event imaging using waveforms to track fault slip, and applying innovative new methods to image earth structure surrounding the fault. The resulting high-resolution subsurface images and aftershock characterization are constraining the geologic model of the earthquake zone. These data also provide a template for future high-density aftershock deployments. Several graduate students were involved in the fieldwork and subsequent data analysis.
