INFLUENCE OF BEDROCK STRUCTURE ON SHALLOW AFTERSHOCKS IN THE CENTRAL VIRGINIA SEISMIC ZONE

In addition to the SE-dipping cluster of aftershocks around the mainshock, the 2011 Mineral earthquake triggered outlying shallow aftershocks in the Fredericks Hall cluster (FHC) and a cluster north of Cuckoo (CKC), both <20 km NE of the mainshock, and in a northwest cluster (NWC) ~5 km NW of the mainshock. FHC hypocenters extend along and beneath a NE-striking, steeply SE-dipping belt of Paleozoic biotite metagranitoid and metadiorite-metagabbro, and are concentrated near the intersection of this belt with a NNW-striking Jurassic diabase dike. CKC hypocenters are near a NE-striking, steeply SE-dipping contact between Paleozoic schists (Quantico Fm.) and metavolcanic gneisses (Chopawamsic Fm.). NWC hypocenters are in the NE-striking Chopawamsic Fm. However, focal mechanisms in the FHC, CKC, and NWC indicate mainly NNW-striking reverse faults oblique to the dominant NE-striking structural fabric.

Regional bedrock structures parallel to the NNW-striking nodal planes of shallow aftershocks include sparse Jurassic diabase dikes and widely distributed joints (tensile fractures) into which the dikes were emplaced. These joints are also parallel to topographic lineaments evident from 1-meter LiDAR data and appear to be favorably oriented for slip in the modern stress field. Thus, evidence suggests that the 2011 mainshock triggered reverse slip on preexisting NNW-striking joints oblique to the NE-striking host units and gneissic foliation.

The NNW-striking nodal planes and joints dip both NE and SW. Coulomb stress modeling shows positive stress change for hypothetical receiver faults that dip NE in the FHC and CKC, and those that dip either NE or SW in the NWC. Coulomb stress change resolved on individual measured joints in FHC and CKC areas is mostly positive for NE dips and negative for SW dips, consistent with nodal planes that dip NE; but in the NWC is positive for most NE dips and some SW dips, consistent with nodal planes that dip either NE or SW. Distributions and focal mechanisms of outlying aftershocks from the Mineral earthquake, bedrock structures, and stress modeling thus show that relatively small perturbations of stresses in the Central Virginia seismic zone can trigger shallow aftershocks on preexisting joints that have favorable orientations.