A PRELIMINARY ASSESSMENT OF NEOTECTONIC FEATURES IN THE CENTRAL VIRGINIA SEISMIC ZONE

Studies of post-Miocene crustal structures and structural history, or neotectonics, can provide valuable information on how faults have behaved (including seismically) under modern stresses, which in turn helps estimate future seismicity. Historical seismicity indicates that multiple faults across an area of ~10,000 km² in the Central VA Seismic Zone (CVSZ) have slipped and created 21 earthquakes >M3 in the past 300 yrs. The most recent and largest earthquake (2011 M-5.8) occurred on a reverse fault within a belt of early Paleozoic rocks of the Chopawamsic-Quantico Fms; epicenter of the penultimate earthquake (2003 M-4.5) plots ~30km to SW within the same rocks. Other historical earthquakes have occurred at >30 km from the 2011 event. Seismic-induced surface deformation (few cm of uplift along narrow NE linear zones) from the 2011 earthquake has been found at 2 locations at NE limit of aftershocks, suggesting a focusing of seismic energy in that direction in well-foliated schist of the Quantico Fm. An anomalously steep gradient along the profile of Beaver Creek, which runs NW-SE directly over the 2011 epicentral area and its projection to the surface, is suggestive of late Quaternary deformation. The Beaver Creek anomaly is similar to anomalies found on all major rivers crossing the CVSZ (R. Weems, USGS, personal com.), suggesting more-regional late Quaternary deformation. Also, an isolated Neogene alluvial terrace that is unique on the landscape directly over the 2011 epicentral area and is terminated downstream by the Sturgeon Creek /Freshwater Creek fault is suggestive of late Quaternary deformation. Indicative of strong ground shaking, paleoliquefaction dikes in Holocene alluvium have been found ~25-30 km SE of 2011 epicentral area along the James and S. Anna Rivers. Additional surface features in the CVSZ, such as anomalously elevated topography and pronounced topographic lineaments elongated NE-SW, could be of neotectonic origin, but such conclusions remain untested. In summary, there is much evidence suggesting previous neotectonic activity in the 2011 epicentral area, as well as elsewhere in the CVSZ, however, details are presently poorly known and await further study.