MAGNETIC AND GRAVITY ANOMALIES REVEAL GEOLOGIC FEATURES ASSOCIATED WITH THE 2011 MW5.8 MINERAL, VIRGINIA EARTHQUAKE

We use magnetic and gravity anomaly maps to interpret subsurface geologic features in the vicinity of the 2011 Mineral, Virginia Mw5.8 earthquake. While intraplate earthquakes are generally thought to occur along pre-existing faults, the presence of numerous ancient faults in areas such as Central Virginia seismic zone (CVSZ) makes an estimation of seismic hazard very challenging. The evaluation of geologic characteristics which cause fault slip to be favored in a given area are thus of strong interest. For this study, we use high-resolution airborne magnetic, gravity, and radiometric surveys flown in the Mw5.8 epicentral area to resolve shallow and subsurface geologic features. We also use legacy magnetic and gravity data to provide a regional context. The magnetic maps show prominent linear anomalies that generally correspond to folded and faulted metavolcanic and metasedimentary layers. In some areas these are deformed about felsic intrusions, which appear as broader magnetic lows. Thinner 15-40 nT anomalies striking N-NNW are interpreted as Jurassic diabase dikes. The Mw5.8 earthquake occurred near a complex junction of NE-SW and E-W striking magnetic lineaments, and is associated with a broad bend in NE-striking geologic structures. Gravity data show a prominent ~2 mgal/km gradient extending to the NE and SW over 20 km; corresponding models strongly suggest low density material in the Mw5.8 hanging wall. Several aftershock clusters were observed near intersections of Mesozoic dikes with NE-SW structures that also reside near gravity gradients. Intersections of obliquely oriented features may influence seismicity via local rheologic weakness, higher permeabilities, and/or favorable orientation of fault planes relative to regional stresses. Gravity gradients may indicate changes in lithostatic pressure that modify the local stress field. At a regional scale, filtered magnetic and gravity data show that the CVSZ lies within a broader change in strike of anomalies. We speculate whether historical CVSZ earthquakes are associated with other intersections between oblique structures and/or local changes in structural orientation.