Geomorphic evidence for persistent, cumulative deformation of the Virginia Piedmont in the vicinity of the 23 August 2011 Mineral earthquake

Intraplate earthquakes, like the recent moment magnitude, M (sub 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.