FAULTS DELINEATED BY AFTERSHOCKS ASSOCIATED WITH THE 2011 CENTRAL VIRGINIA EARTHQUAKE AND THEIR TECTONIC SETTING

The 23 August 2011 earthquake and aftershocks in the central Virginia seismic zone occurred in polydeformed metamorphic rocks of the Appalachian Piedmont. They were at depths of ~8 km or less and within thrust sheets as imaged on the nearby I-64 seismic-reflection profile. The majority of aftershock hypocenters, including the 3 largest, Mw3.9, Mw3.8, Mw3.2, define a ~1-km-thick SE-dipping tabular cluster from 7.5 to 1 km depth with slight concave-up curvature. This cluster spans a lateral distance of ~10 km and is centered beneath Yanceyville on the South Anna River. The best-fit plane to this cluster, the Quail fault (QF), strikes N30°E and dips 46°SE. The mainshock relocated in this cluster has a focal mechanism determined from waveform inversion that shows a nodal plane striking N28°E and dipping 50°SE with rake angle 113° and mostly reverse motion. The QF underlies the NW side of a NE-trending outcrop belt of foliated metasedimentary-metavolcanic rocks of the Chopawamsic Fm. and schist of the Quantico Fm. It projects to the surface just SE of a 2-km-wide, NE-striking, foliated biotite granodiorite in the “tail” of the lower Silurian Ellisville pluton, and close to a previously mapped albeit questioned trace of the Paleozoic-age Chopawamsic fault. The QF and foliations in the Chopawamsic Fm. have similar orientations, suggesting control on seismicity by earlier penetrative fabric. The downdip SE margin of the QF cluster terminates near subsurface projections of the steeper NE-striking Paleozoic-age Long Branch and Sturgeon Creek/Freshwater Creek faults.

A cluster of late hypocenters ~1 km NW of the QF cluster strikes NE but dips steeply NW. The inferred subsidiary fault and QF converge updip.

Several km E of the QF cluster, a tight group of ~10 hypocenters <2 km deep occurs 3 km NE of Cuckoo. This group lies near the mapped trace of the steep Paleozoic-age Ebenezer Church fault at the SE contact of Quantico Fm. schist.

~10 km E of the QF cluster, 12-13 early hypocenters 4.5 to ~0 km deep are aligned near a vertical plane that strikes N39°E. They span a lateral distance of 8 km and underlie the communities of Fredericks Hall and Threemile Corner. The inferred fault, the Fredericks Hall fault, lies near a NE projection of the Paleozoic-age Little Fork Church fault, which is the boundary between the Chopawamsic Fm. and a pegmatite belt to the E.