VELOCITY VARIATIONS IN THE UPPER CRUST NEAR THE 2011 MINERAL EARTHQUAKE, CENTRAL VIRGINIA SEISMIC ZONE, VIRGINIA, USA

A detailed tomography study in the vicinity of the 2011 Mineral earthquake is undertaken to understand the geological conditions of the upper crust and their relationship to aftershock earthquake activity. Local earthquake tomography is used to model 3D P- and S- wave velocity (Vp and Vs) structure using 324 aftershocks and 12 stations. Stations are located within ~15 km of the 2011 mainshock. A total of 5125 arrival times are used in the inversion (2465 P-wave arrivals and 2660 S-wave arrivals). The inversion volume is 22 x 20 x 16 km, with a block size of 1 x 1 x 1 km. The starting model is a halfspace with Vp=5.96 km/s and Vs=3.53 km/s and is based on previous studies. Good resolution is obtained in the depth range 1-5 km. We find that the majority of aftershocks located at or below 2 km are associated with a NE trending, SE dipping zone of negative Vp anomalies and positive Vs anomalies. Vp/Vs ratios in the zone reach a minimum of 1.54. We suggest that the zone is produced by the presence of quartz-rich rocks within the Chopawamsic formation. One possible interpretation is that quartz-rich rocks originated along the Paleozoic passive margin of Laurentia and were then incorporated into the Chopawamsic formation during island arc collision in the Taconic orogeny. Aftershocks shallower than 2 km are associated with a negative Vs anomaly and slightly negative Vp anomaly; this anomalous region is located NW of the proposed quartz-rich rocks and may correspond to the neck of the Ellisville Pluton. Another negative Vs anomaly located to the SE of the quartz-rich rocks may correspond to the Quantico formation.