GEOLOGIC FRAMEWORK OF THE CENTRAL VIRGINIA SEISMIC ZONE

The Central Virginia Seismic Zone (CVSZ) is defined by a cluster of historic earthquake epicenters centered on the James River between Richmond and Charlottesville. Records of seismicity in the CVSZ extend from the eastern Blue Ridge basement and its metasedimentary/metavolcanic cover sequence eastward across the western Piedmont mélange, the Chopawamsic volcanic-plutonic arc terrane, the gneiss domes and cover sequence of the Goochland terrane, and the pluton-dominated Southeastern Piedmont terrane. The CVSZ, therefore, spans the boundaries between the ancient Laurentian margin and terranes accreted during the Paleozoic. Each of the accreted terranes is bounded by through-going, northeast-striking Paleozoic ductile faults that were reactivated as brittle extensional faults during the Mesozoic. Existing geologic mapping at various scales is adequate for defining regional trends and major structures, but lacks sufficient detail to assign individual earthquakes to particular faults. This is especially true for the August 2011 Mw 5.8 earthquake and its aftershocks. Existing mapping along regional strike to the northeast and southwest indicates that the main quake and its aftershocks occurred within the Chopawamsic terrane, which is bounded on the northwest by the Chopawamsic Fault and on the southeast by the Spotsylvania High-strain Zone. The recent seismic events do not, however, appear to be related to either of these major faults. Structures within the terrane that may contribute to seismicity include the Long Branch Fault, the Quantico-Columbia Syncline, and other unnamed, poorly-mapped structures. Additional mapping is needed to define the details of bedrock structure within the epicentral region and identify possible seismogenic structures.