MAGNETIC AND GRAVITY DATA CONSTRAIN GEOLOGIC FEATURES ASSOCIATED WITH THE AUGUST 23, 2011 EARTHQUAKE IN LOUISA COUNTY, VIRGINIA

The M5.8 August 2011 earthquake and associated aftershocks in Louisa County, VA occurred in an area lacking detailed geologic map coverage but with magnetic and gravity data we can infer geologic features by extrapolating from mapped areas nearby. The earthquake epicenter lies within the Chopawamsic terrane of the Piedmont Province, a region interpreted as an island arc system that was accreted to North America during the Taconic orogeny and deformed during subsequent tectonic activity. Aeromagnetic data over this terrane show linear NE-trending highs over metavolcanics and metasedimentary rocks rich in magnetic minerals, and lows associated with metasedimentary rocks, felsic gneisses, and granitoids lacking in magnetic minerals. Magnetic anomalies delineate mapped segments of the Long Branch and Spotsylvania faults northeast of the epicenter, and the Lakeside and Little Fork Church faults south of the epicenter.

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.