STRESS TRANSFER FROM THE AUGUST 2011 MINERAL, VIRGINIA, MW 5.8 EARTHQUAKE TO THE EVERONA FAULT – MOUNTAIN RUN FAULT ZONE

On August 23, 2011, a M_w 5.8 earthquake struck near Mineral, VA, drawing renewed attention to the occurrence of intraplate seismicity in the Mid-Atlantic region of the Central and Eastern United States (CEUS). The Everona fault (EF) - Mountain Run fault zone (MRFZ) is a system of northeast-striking faults located near Culpeper, VA, in the central Appalachians ~30 km from the epicenter of the Mineral earthquake. The MRFZ accommodated ductile deformation and retrograde metamorphism between the late Ordovician and Jurassic, however two northwest-facing scarps indicate that brittle reverse faulting events overprint ductile fabrics in the MRFZ. Although the MRFZ is thought to have been active primarily during the Paleozoic contraction and Mesozoic extension episodes, these scarps are Quaternary in age and probably tectonically influenced, according to analyses of the topography and eroded soil horizons. The MRFZ strikes northeast and dips 55° to the southeast, in accordance with the predominant dip of the regional primary foliation. Approximately 1 km to the west of the MRFZ, offset colluvial deposits identified in outcrop indicate a possibly related Tertiary or Quaternary structure that also strikes northeast but dips 20° to the northwest, known as the Everona fault (EF). The opposing dip of the EF makes it antithetic to the MRFZ. We evaluate to what extent slip from the Mineral earthquake may have increased the risk of future failure on the Everona fault - Mountain Run fault system by calculating regional and local Coulomb failure stress change (Δσ_CFS). Our models of Coulomb stress transfer indicate that the southern portion of the MRFZ and EF were loaded as a result of the Mineral earthquake (~+0.00353 bars). Stress also may have increased on possible continuations of the MRFZ or other Piedmont faults south of the currently mapped MRFZ and on faults antithetic to the MRFZ, like the EF. The EF and MRFZ are less than 50 km from the CVSZ. Although the stress changes due to the Mineral earthquake on the MRFZ and EF are too small to significantly affect seismic hazard on these faults, it is possible repeated events in Central Virginia Seismic Zone (CVSZ) may accumulate enough stress to trigger slip on the EF and MRFZ.