SEARCHING FOR REACTIVATED GEOLOGIC STRUCTURE IN THE EPICENTRAL REGION OF THE AUGUST 2011 M5.8 EARTHQUAKE IN THE CENTRAL VIRGINIA SEISMIC ZONE

The August 23, 2011 M5.8 earthquake in the Central Virginia seismic zone was followed by many well-located aftershocks concentrated in a 2-km-wide cluster elongated N30E and dipping ~45˚ SE, similar to the slip surface determined for the M5.8 event. This cluster has a well-defined base at about 7-8 km depth that projects to the surface in the vicinity of a N30E-trending contact between Silurian granodiorite (~2 km-wide Ellisville ‘neck’) to NW and metavolcaniclastic rocks (upper greenschist facies) of the Ordovician Chopawamsic Fm to SE. Initial reconnaissance in the epicentral area found brittle deformation (post-Paleozoic) only along this contact, including silicified breccia where a 4-km-long J dike is offset ~200-250 m left-laterally across the contact. Our preliminary analysis of regional penetrative tectonic fabrics indicates a Paleozoic deformational history beginning with D1 producing NE-trending S1 schistosity and a map-scale N40E-trending, gently-N-plunging, steeply SE-dipping syncline in Ordovician Quantico Fm. D2 locally plicated S1 and was closely preceded or accompanied by intrusion of the Ellisville ‘neck’, which has internal S2 schistosity. D1 and D2 regional fabrics both indicate mostly horizontal compression with little evidence of large-scale translational shear. An exception to this is found along the SE contact of the Ellisville ‘neck’, where upright F2 folds in adjacent Chopawamsic rocks have been rotated into reclined folds with axes plunging downdip and a dip-slip sense of shear. New excavations along this contact in the Harris Creek drainage reveal abundant mylonitic fabric (D2 or later) with minor drag folds indicating left-lateral shear, broken quartz veins, gouge seams (2-4-cm), brecciated saprolite, striated clay-dike seams in saprolite, and paleo-scarps of interpreted tectonic origin in probable Quaternary surficial deposits at multiple locations. Based on these field data indicating repeated tectonic activity, we suggest the name Harris Creek fault zone for the SE contact of the Ellisville ‘neck’. In the absence of any surface-rupturing fault in the epicentral area from the M5.8 earthquake, our working hypothesis is that the Harris Creek fault zone at depth is the most likely candidate as the causative structure, with more field investigations to follow.