Focal mechanisms are presented for 11 earthquakes from the Giles County, Virginia, seismic zone and for 12 earthquakes from the central Virginia seismic zone. These earthquakes (0 ≦ M ≦ 4) were monitored by local networks between January 1978 and October 1984. In Giles County, six single-event focal mechanisms and five composite-event focal mechanisms were determined. In central Virginia, eleven single-event focal mechanisms and four composite-event focal mechanisms were obtained. A computer program, FOCMEC, that systematically searches the focal sphere for valid mechanism solutions, based on both P-wave polarities and (SV/P)z amplitude ratios, was used to determine the focal mechanism solutions.
The results for the Giles County seismic zone show predominately strike-slip mechanisms on steeply dipping (73° ± 16°) NNE (right-lateral motion) and ESE (left-lateral motion) trending nodal planes. However, some (4/11) of the solutions show similar movement on nodal planes rotated 45° counterclockwise. The means and standard deviations for the trends and plunges of the P axes are N46°E ± 24° and 13.5° ± 20°, respectively, as computed from the six single-event focal mechanisms and two composite-event focal mechanisms.
Focal mechanisms from central Virginia exhibit much more scatter in mechanism types and nodal plane orientations than observed in Giles County. The P axes in central Virginia are generally northeast-trending for shallow earthquakes (<8 km) and northwest-trending for deeper ones (>8 km). The focal mechanisms exhibit a mixture of reverse and strike slip faulting on planes that dip 62° ± 16°.
The two Virginia seismic zones are separated by only 200 km, yet their seismogenic characteristics are very different. In Giles County, the earthquakes occur beneath the Appalachian decollement, and the faulting and inferred stress orientations are uniform. In central Virginia, however, the seismicity is occurring at and above the decollement and the associated fault planes and stress orientations are quite variable.