RELICT PALEOZOIC FAULTS IN THE EPICENTRAL AREA OF THE AUGUST 23, 2011 VIRGINIA EARTHQUAKE; GEOLOGY IN THE FERNCLIFF, VA QUADRANGLE

We mapped the bedrock geology of the northern half of the Ferncliff, VA, 7.5’ quadrangle in the summer of 2010; the quadrangle lies in the central Virginia seismic zone and is immediately west of the epicenter of the recent M 5.8 earthquake, one of the most intense shaking events recorded in the eastern United States. Our detailed mapping, funded by the USGS EDMAP program, is part of a broader, ongoing project focused on the understanding of the early Paleozoic evolution of the Laurentian Iapetan margin. Relict Paleozoic faults in the region represent likely zones of upper crustal weakness that offer avenues for slip in the modern stress regime.

In the Ferncliff area, the most prominent fault in the western Piedmont of Virginia, the Chopawamsic thrust fault, emplaces Middle-Late Ordovician magmatic arc rocks of the Chopawamsic terrane westward onto pre-Early Ordovician accretionary metaclastics of the Potomac terrane. Parallel to, and just east of the Chopawamsic fault, a second significant fault, the Long Branch thrust, displaces rocks within the magmatic arc terrane.

We have found that regional and state map compilations, although valid for broad assessment, are not accurate to the scale required to evaluate an event such as the 2011 Virginia earthquake. For example, previous maps show a tightly folded Chopawamsic fault directly to the northwest of the cluster of seismic activity; our detailed field work has shown that this complex fault geometry does not exist. In addition, we observed fault products of the Long Branch thrust south of its previous mapped extent. Imprecise depictions of the local geology could hamper or misconstrue any consequent seismic interpretations.

The adjacent I-64 seismic profile depicts a general southeastward dip to structures in the region and reinforces the possibility that remnant Paleozoic structures, either observed or inferred, to the northwest of the epicentral area may have been involved at depth in the August, 2011 event and subsequent aftershocks. Given the attitude of structures in the area, it is unlikely that any mapped faults with surface traces to the east of the seismic activity influenced this event. It remains unclear to us if the aftershocks mostly occurred along one fault plane or multiple surfaces that became loaded only after nearby seismicity.