INVESTIGATING THE INFLUENCE OF PREEXISTING GEOLOGIC FEATURES ON OUTLYING SHALLOW AFTERSHOCK CLUSTERS TRIGGERED BY THE 2011 MINERAL, VIRGINIA, EARTHQUAKE

Intraplate earthquakes are commonly thought to occur on preexisting structures favorably oriented in the current local stress field, but specific source faults are elusive in the Central Virginia seismic zone, where earthquakes are not clearly linked to mapped Paleozoic or Mesozoic faults. We integrate geologic field studies and recent seismological observations to assess the influence of preexisting geologic features on outlying aftershock clusters triggered by the M_w5.8 Mineral, VA, earthquake of 2011. In addition to the NE-striking, SE-dipping aftershock cluster around the mainshock, 1666 well-located hypocenters affirm previously identified shallow aftershock clusters near Fredericks Hall (FHC), north of Cuckoo (CKC), and NW of the mainshock (NWC). Modeling shows that these outlying aftershocks are in regions of positive static Coulomb stress change <20 km from the mainshock. Focal mechanisms of aftershocks >6 km deep have NE-striking nodal planes and reverse motion, but those <4 km deep have NNW-striking nodal planes and reverse motion, suggesting a possible change in stress field with depth.

Aftershock hypocenters in the FHC are concentrated along and adjacent to a NE-striking belt of biotite metagranitoid, and deepen from SW to NE; but the nodal planes strike NNW, indicating slip on fault surfaces oblique to the NE-striking host unit. A NNW-trending diabase dike may perturb the stress field where it intersects the metagranitoid. Aftershocks in the CKC are near a NE-striking contact between the Quantico Fm. (mainly schists) and Chopawamsic Fm. (gneisses), but also have NNW-striking nodal planes.

We measure joint orientations at exposures in and near the aftershock clusters to test the hypothesis that some earthquakes are on preexisting joints, and to determine if NNW-striking joints match particular nodal planes. Common joint sets are moderately to steeply dipping and parallel to NE-striking foliation, orthogonal to foliation, or strike NNW, and are parallel to topographic lineaments imaged by LiDAR. Sheeting joints dipping <25° are also common. Initial results indicate that most NNW-striking joints dip >60°, while those dipping ≤60° NE or SW coexist and vary locally in relative abundance; those dipping NE are more abundant around Sherman Lake in the FHC, and those dipping SW are more abundant in the CKC.