THE 2011 MINERAL, VIRGINIA, EARTHQUAKE TRIGGERED SHALLOW AFTERSHOCKS ON FAVORABLY ORIENTED STRUCTURES IN AREAS OF POSITIVE STATIC COULOMB STRESS CHANGE

Aftershock focal mechanisms from the M_w 5.8 Mineral, Virginia, earthquake of 2011 suggest a depth-dependent stress field, where most aftershocks >6 km deep show reverse slip, NE-striking nodal planes, and ~85° to ~120° P-axis trends similar to the main shock (103°), and those <4 km deep show reverse slip, NNW-striking nodal planes, and ~70° to ~75° P-axis trends. Distal shallow aftershock clusters were triggered <20 km NE of the main shock at Fredericks Hall (FHC) and north of Cuckoo (CKC), and also NW of the main shock (NWC). Modeling indicates that these outlying clusters occurred in areas of positive static Coulomb stress change.

Hypocenters in the FHC concentrate along and beneath a NE-striking, steeply SE-dipping belt of biotite metagranitoid and metadiorite-metagabbro, primarily near the intersection of this belt with a NNW-striking Jurassic diabase dike and its magnetic anomaly. Hypocenters in the CKC are near a NE-striking, steeply SE-dipping contact between the Quantico Fm. (mainly schists) and Chopawamsic Fm. (gneissic metavolcanic rocks) on the SE limb of a synclinorium. Hypocenters in the NWC are in NE-striking Chopawamsic Fm. However, focal mechanisms in all three clusters indicate mainly NNW-striking reverse faults oblique to the dominant NE-striking structural fabric.

Analyses of preexisting joint orientations in and near these aftershock clusters show moderately to steeply dipping sets parallel, orthogonal, and oblique (NNW-striking) to the NE-striking foliation. These joint sets are parallel to topographic lineaments identified on LiDAR imagery. NNW-striking joints parallel to the strike of aftershock nodal planes dip NE and SW as do the nodal planes. Jurassic diabase dikes are the only regional structures that have similar strike, suggesting that the NNW-striking joints originated as tension fractures from rift-related paleostress during or before dike emplacement.

Thus, evidence suggests that the 2011 main shock triggered shallow aftershocks having reverse fault slip on preexisting NNW-striking joints parallel to Jurassic dikes in areas of positive static Coulomb stress change. This hypothesis is being tested by Coulomb modeling of shear stresses on receiver faults consistent with measured dip angles and directions of joint sets in each aftershock cluster.