Aftershock Sequence of the 2011 Virginia Earthquake Derived from the Dense AIDA Array and Backprojection

After the 2011 Mineral, Virginia, earthquake, a temporary dense array (aftershock imaging with dense arrays [AIDA]) consisting of similar to 200 stations was deployed at 200-400 m spacing near the epicenter for 12 days. Backprojection of the data was used to automatically detect and locate aftershocks. The co-deployment of a traditional aftershock network of 36 stations at similar to 2-10 km spacing enables a quantitative comparison. The AIDA backprojection aftershock catalog is complete to M -0.5 and includes 1673 events. For comparison, the traditional network was complete to M -0.1 with 813 events within the same time period and spatial volume. Only 494 of the traditional network events were of sufficient quality to compute improved double-difference locations, for a completeness of M +0.2. The AIDA backprojection catalog observes the same major patterns of seismicity in the epicentral region, but additional details are illuminated, and absolute uncertainty was reduced. The primary zone of seismicity is not a single fault but is a tabular zone of multiple small faults with no resolvable internal structures. This zone has a subtle concave shape along strike and with depth, and a broader zone of newly detected events is observed at shallow depth. In addition, a shallow cluster was detected and located to the east of the main aftershock zone. The addition of smaller events to the catalog did not change the b-value but illuminated spatial and temporal patterns. The b-value is different at less than about 3 km depth than at greater depth. Very low b-value, especially at greater depth, is consistent with observed very high stress drops. The results indicate the benefits of dense arrays and autodetection by backprojection for aftershock studies. The reduced detection threshold and higher spatial resolution enabled the study of earthquake mechanisms and strain transfer at a smaller scale.