The Mineral, Virginia (USA), earthquake of 23 August 2011 was an unusually strong seismic event in the eastern United States. It caused widespread structural damage to residential property near the epicenter. An analysis of residential property damage reports, in conjunction with visits to some damaged residences, reveals a 40 km 2 area of concentrated damage centered 11 km south of the town of Louisa. This area is west of the earthquake’s epicenter and may be in the immediate hanging wall of a northeast-striking, moderately southeast dipping causative fault suggested by seismic data. The degree of damage in this area is consistent with a maximum Modified Mercalli Intensity (MMI) of VIII. A surrounding area of ~550 km 2 reported damage that is consistent with an MMI intensity of VII. A statistical analysis of dwelling characteristics confirms that home age and condition were factors that influenced the frequency and severity of reported property damage. The median damage to homes constructed between 1900 and 1973, relative to assessed value, was approximately twice that of homes constructed after 1973 in Louisa County, and three times greater within areas of MMI intensity VI, VII, and VIII.

Deployment of temporary seismic stations after the 2011 Mineral, Virginia (USA), earthquake produced a well-recorded aftershock sequence. The majority of aftershocks are in a tabular cluster that delineates the previously unknown Quail fault zone. Quail fault zone aftershocks range from ~3 to 8 km in depth and are in a 1-km-thick zone striking ~036° and dipping ~50°SE, consistent with a 028°, 50°SE main-shock nodal plane having mostly reverse slip. This cluster extends ~10 km along strike. The Quail fault zone projects to the surface in gneiss of the Ordovician Chopawamsic Formation just southeast of the Ordovician–Silurian Ellisville Granodiorite pluton tail. The following three clusters of shallow (<3 km) aftershocks illuminate other faults. (1) An elongate cluster of early aftershocks, ~10 km east of the Quail fault zone, extends 8 km from Fredericks Hall, strikes ~035°–039°, and appears to be roughly vertical. The Fredericks Hall fault may be a strand or splay of the older Lakeside fault zone, which to the south spans a width of several kilometers. (2) A cluster of later aftershocks ~3 km northeast of Cuckoo delineates a fault near the eastern contact of the Ordovician Quantico Formation. (3) An elongate cluster of late aftershocks ~1 km northwest of the Quail fault zone aftershock cluster delineates the northwest fault (described herein), which is temporally distinct, dips more steeply, and has a more northeastward strike. Some aftershock-illuminated faults coincide with preexisting units or structures evident from radiometric anomalies, suggesting tectonic inheritance or reactivation.

Characterizing geologic features associated with major earthquakes provides insights into mechanisms contributing to fault slip and assists evaluation of seismic hazard. We use high-resolution airborne geophysical data combined with ground sample measurements to image subsurface geologic features associated with the 2011 moment magnitude (M w ) 5.8 central Virginia (USA) intraplate earthquake and its aftershocks. Geologic mapping and magnetic data analyses suggest that the earthquake occurred near a complex juncture of geologic contacts. These contacts also intersect a >60-km-long linear gravity gradient. Distal aftershocks occurred in tight, ~1-km-wide clusters near other obliquely oriented contacts that intersect gravity gradients, in contrast to more linearly distributed seismicity observed at other seismic zones. These data and corresponding models suggest that local density contrasts (manifested as gravity gradients) modified the nearby stress regime in a manner favoring failure. However, along those gradients seismic activity is localized near structural complexities, suggesting a significant contribution from variations in associated rock characteristics such as rheological weakness and/or rock permeability, which may be enhanced in those areas. Regional magnetic data show a broader bend in geologic structures within the Central Virginia seismic zone, suggesting that seismic activity may also be enhanced in other nearby areas with locally increased rheological weaknesses and/or rock permeability. In contrast, away from the M w 5.8 epicenter, geophysical lineaments are nearly continuous for tens of kilometers, especially toward the northeast. Continuity of associated geologic structures probably contributed to efficient propagation of seismic energy in that direction, consistent with moderate to high levels of damage from Louisa County to Washington, D.C., and neighboring communities.

Intraplate earthquakes, like the recent moment magnitude, M w 5.8 Mineral, Virginia (USA), earthquake of 23 August 2011, are sobering reminders of how little we know about the lithosphere and active tectonics of plate interiors, including passive continental margins. Unlike plate boundaries, plate interiors typically lack clear geologic evidence of the tectonic stresses that lead to earthquakes, the deformation of rocks, and the building of topography. Inspired by the Mineral earthquake, preliminary geophysical and geomorphic work in the Central Virginia seismic zone, centered in Louisa County, resolves apparent surface deformation expressed in geomorphic markers and river channel patterns. This deformation is consistent with the location and sense of slip of the reverse fault that ruptured in the Mineral earthquake. Surface deformation is recorded by the modern and ancient longitudinal profile of the South Anna River that flows above the epicenter of the earthquake and orthogonal to the structural grain of the Appalachian Piedmont. Sinuosity of the South Anna River channel is greatest in the uplifted hanging wall of the fault that ruptured in the Mineral earthquake. More dramatically, preliminary correlations of terrace deposits of the paleo–South Anna River suggest that they are several meters higher above the modern channel in the hanging wall with respect to the footwall of the Mineral fault rupture plane. The implication is that persistent faulting in the vicinity of the Mineral earthquake, over many millennia, has resulted in several meters of cumulative rock and surface uplift of the eastern hanging-wall block with respect to its western footwall. This is a rare example of crustal and surface deformation directly linked to observed seismicity in eastern North America, where earthquakes have long been observed to cluster spatially. If confirmed by ongoing mapping, these results serve as a surface deformation field constraint for geodynamic models of intraplate earthquake generation and associated hazards in a passive margin setting.