ANCIENT RIFTS, INTRAPLATE EARTHQUAKES AND GROUNDWATER FLOW IN THE ROANOKE VALLEY OF SW VIRGINIA

A magnitude 2.6 earthquake was strongly felt across the southwestern portion of Roanoke County on May 16, 2009. The Virginia Tech Seismic Database revealed two additional (M 1.6 to 3.1) seismic events that had occurred in the area since 2000, all of which are located in the vicinity of the Mowes Springs Fault. The MSF separates Precambrian Basement and Cambrian rift rocks on the SW (Blue Ridge Plateau) side of the valley from mildly metamorphosed Paleozoic platform rocks on the NE (Roanoke Valley side). The MSF has a steep NE dip and appears to have offset the basal Blue Ridge thrust by 5 miles across the basin, forming a prominent scarp along the SW side of the Roanoke Valley. Earthquakes along the MSF were calculated to have epicenters approximately 7 km below the valley floor near the base of proposed Alleghanian detachment. A NE-SW directed cross section using calculated earthquake movement solutions shows that periodic slip is in a reverse sense, uplifting Cambrian carbonates on the hanging wall over Precambrian rocks in the foot wall in agreement with slickensides and offsets measured in saprolite and bedrock along the MSF. This also is in agreement with the regional SW directed compressional stress field in the area. The MSF was initiated as detached Paleozoic rocks in the Pulaski sheet were transported across a displaced subthrust ramp (Harris 1970). NW directed subthrust offset described by Thomas as a relic of Neoproterozic transform faulting also localized Mesozoic dikes and transverse fracture zones along the MSF. New geologic surveys at the end of the 20th century recognized these fractures as aquifer zones and the Mowes Springs along the NW end of the MSF were replaced by a series of large diameter, high production wells drilled along the MSF for the City of Salem. Modern groundwater flux along these ancient fractures is suspected to have aided slip along the fault by reducing the angle of internal friction with respect to the operative stress field.