MAGNETIC AND GRAVITY DATA CONSTRAIN GEOLOGIC FEATURES ASSOCIATED WITH THE AUGUST 23, 2011 EARTHQUAKE IN LOUISA COUNTY, VIRGINIA
The magnetic anomaly associated with the Long Branch fault extends over 50 km in length but bends immediately north of the August 2011 epicenter to change orientation from N34E to N45E before crossing the aftershock area. The local topographic fabric similarly changes orientation. Limited resolution of existing magnetic data precludes imaging continuity at the bend and thus determining whether the Long Branch fault actually crosses the epicenter and aftershock area. At the bend are intersecting magnetic anomalies that trend N-S, parallel to Jurassic diabase dikes observed further south, although such a dike has not been mapped on the surface at this location. The N-S features and the N45E anomaly bound a triangular-shaped magnetic low where more detailed geologic mapping is required. The southern edge of this low is located near the termination of the Little Fork Church fault magnetic anomaly. Regional gravity data are sparse, but analyses of existing data and ~35 new stations collected in this area show variations suggesting compositional differences NW and SE of the magnetic low. The bending and/or termination of geophysical anomalies suggest that the August 2011 earthquake occurred where pre-existing contrasts in crustal structure and lithology may have been amenable to strain localization. Additional geophysical data collection and geologic mapping will help to constrain these possibilities.