Imaging geologically recent faults near Pembroke, Virginia using seismic reflection and ground penetrating radar

Five normal faults, forming two grabens and a half graben, were discovered in a road cut along U. S. Highway 460 near Pembroke, Virginia, in 1994. The faults are interesting because they lie in the central Appalachian Valley and Ridge geologic province where normal faulting is unusual, and because they are located near the epicenter of a magnitude 5.8 earthquake that occurred in 1897. The larger graben measures 11 m wide at the level of maximum exposure and is the main focus of interest because of its size. The faulted deposits, consisting of clay, sand, gravel, and some boulder-sized particles, rest unconformably on top of Knox Group (Cambro-Ordovician) dolomite. Seismic monitoring between 1978 and 1980 detected twelve small events that were located around the terrace deposit faults, raising the question of whether the faults were associated with the seismicity. An alternative explanation for the grabens is subsidence of the terrace deposits related to collapse during formation of sinkholes in the underlying bedrock. The faults were imaged along two profiles spaced 100 m apart, using seismic reflection and ground penetrating radar (GPR). The transmitting and receiving GPR antennas occupied the same positions as the seismic shot and geophone locations, in order to closely mimic the seismic survey. The GPR data supplemented the seismic data by providing shallower subsurface information. The preferred interpretation of the available data is that the grabens were caused by a thrust fault that emerged from bedrock to form a drape fold in the terrace deposits, although the connection between bedrock faulting and earthquakes in Giles County, including the 1897 event, remains to be established.