RELICT PALEOZOIC FAULTS IN THE EPICENTRAL AREA OF THE AUGUST 23, 2011 VIRGINIA EARTHQUAKE: ASSESSING POTENTIAL SOURCES OF REACTIVATION WITH FIELD OBSERVATIONS

Field observations made during 1:24,000 scale mapping prior to the M 5.8 Virginia earthquake of 2011 have implications for understanding the event. Two prominent relict Paleozoic faults in the vicinity of the epicenter represent the most suspect zones of upper crustal weakness that offer potential avenues for modern slip. To the west of the epicentral area, the most significant fault in the western Piedmont of Virginia, the Chopawamsic fault (Cf), dips to the east and emplaces Middle-Late Ordovician magmatic arc rocks of the Chopawamsic terrane westward onto pre-Ordovician accretionary metaclastics of the Potomac terrane. To the east of the Chopawamsic fault, the east-dipping Long Branch fault (LBf) displaces rocks within the magmatic arc terrane. Existing regional maps lack the resolution necessary to evaluate these faults with respect to an event such as the 2011 Virginia earthquake.

Our detailed field mapping in the area has shown that the Cf is not folded at map scale in the epicentral area, as shown on previous maps, and its surface trace lies substantially west of the epicentral area. Additionally, in this area, the Cf has been stitched by the c. 444 Ma Ellisville pluton, a locking feature that likely continues to impede motion along the structure. Consequently, the Cf is eliminated as a candidate for reactivation during the 2011 event.

The LBf has been mapped as terminating just to the NNE of the epicentral area. However, we have observed localized L-tectonites in the immediate area of the epicenter of the 2011 earthquake, which is to the SSW of, and along strike with, the previously mapped terminus of the LBf. The pronounced L fabric (070°/35°) in this area lies within the rupture plane (035°/53°) of the 2011 event, as indicated by analysis of tightly constrained aftershocks that occurred in the week following the mainshock. Considering their location and attitude, these tectonites likely represent finite strain within a previously unrecognized footwall splay or termination zone of the LBf.

On the basis of these field observations as well as the geographic position of the primary slip surface determined by an analysis of the aftershock sequence, we interpret the principal rupture of the 2011 event and most aftershocks to be associated with reactivation along a previously unrecognized structural zone related to the LBf.