The Aftershock Sequence of the 2011 Mineral, Virginia, Earthquake: Temporal and Spatial Distribution, Focal Mechanisms, Regional Stress, and the Role of Coulomb Stress Transfer

The aftershocks of the 23 August 2011 M-w 5.7 Mineral, Virginia, earthquake were recorded by 36 temporary stations installed by several institutions. We located 3960 aftershocks from 25 August 2011 through 31 December 2011. A subset of 1666 aftershocks resolves details of the hypocenter distribution. We determined 393 focal mechanism solutions. Aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane; other aftershocks occurred 10-20 km to the northeast. A large percentage of the aftershocks occurred in regions of positive Coulomb static stress change, and similar to 80% of the focal mechanism nodal planes were brought closer to failure. However, the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity. Aftershocks at depths less than 4 km exhibit reverse mechanisms with north-northwest-trending nodal planes. Most focal mechanisms at depths greater than 6 km are similar to the mainshock, with north-northeast-trending nodal planes. A concentration of aftershocks in the 4-6 km depth range near the mainshock are mostly of reverse type but display a 90 degrees range of nodal-plane trend. Those events appear to outline the periphery of mainshock rupture, where positive Coulomb stress transfer is largest. The focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km, along with the mainshock, point to the possibility of a depth-dependent stress field prior to the occurrence of the mainshock. Analysis of earthquake occurrence using a new magnitude scale (M-D*) indicates a Gutenberg-Richer law b-value of 0.864 and an Omori law p-value of 1.085, indicative of a typical aftershock sequence.