PROPAGATION EFFECTS FROM THE 23 AUGUST 2011 M 5.8 VIRGINIA EARTHQUAKE AND IMPLICATIONS FOR SHAKING HAZARDS

The 23 August 2011 M 5.8 Virginia earthquake was not recorded by a dense network of seismographs in the epicentral area. As a result, the causative fault and propagation effects are not well understood. To better understand source and propagation effects, we use USGS “Did You Feel It” (DYFI) intensity data for the Virginia earthquake as a proxy for peak ground velocity (PGV) instrumental measurements. The large number of observations provides reasonable data from which PGV can be inferred. Numerous studies elsewhere have shown that fault zones are lower in seismic velocity relative to adjacent rocks that are not faulted. The low-velocity fault zones act as a waveguides, whereby high-amplitude seismic energy propagates long distances along continuous faults or along discontinuous but connected faults. The epicenter of the Virginia earthquake was located in the vicinity of the Long Branch fault (LBF), Chopawamsic fault (CF), and Spotsylvania fault (SF), which, together with the Stafford fault system (SFS), have been mapped as a nearly continuous series of fault zones from central Virginia to Washington, DC. Our analysis of the DYFI data from the Virginia earthquake shows that strong shaking was concentrated in a NE-SW-trending zone, approximately aligned with the mapped faults. Within 50 km NE of the epicenter, the highest intensities approximately coincide with mapped traces of the LBF and CF (with significant energy along the SFS at greater distances), suggesting either the LBF or CF as the causative fault, with the SFS connected to it. However, there were significantly lower intensities northwest and southeast of the fault zones, suggesting that amplification from near-surface sediments was less a factor. Our analysis of the DYFI data also suggests that strong shaking in the Washington, DC area was enhanced by Moho reflections and by guided waves along the SFS. Because critical facilities, including nuclear, military, and governmental facilities, are located along and near the trend of the fault zone and because Washington is located at critical distances for Moho reflections, moderate- to large-magnitude earthquakes within the central Virginia seismic zone can pose a hazard. At regional distances to the northeast, the highest intensities coincide with multiples of the Moho reflection.