Velocity Models for the Crust Hosting the Main Aftershock Cluster of the 2011 Mineral, Virginia, Earthquake

Abstract

Abstract Three-dimensional P- and S-wave velocity (VP and VS) models are determined for the crust containing the main aftershock cluster of the 2011 Mineral, Virginia, earthquake using local earthquake tomography. The inversion uses a total of 5125 arrivals (2465 P- and 2660 S-wave arrivals) for 324 aftershocks recorded by 12 stations. The inversion volume (22 × 20 × 16 km) is completely contained within the Piedmont Chopawamsic metavolcanic terrane. The models are well resolved in the central portion of the inversion volume in the depth range 1–5 km; good resolution does not extend to the hypocenter depth of the mainshock. Most aftershocks are located within a northeast-trending, southeast-dipping region containing negative VP anomalies, positive VS anomalies, and VP/VS ratios as low as 1.53. These velocity results strongly argue for the presence of quartz-rich rocks, which we attribute to either the presence of a giant quartz vein system or metamorphosed orthoquarzite sandstones originally deposited on the Laurentian passive margin and subsequently incorporated into the Chopawamsic thrust sheets during island arc collision in the Taconic orogeny.