AIRBORNE MAGNETIC, GRAVITY AND RADIOMETRIC DATA ILLUMINATE SUBSURFACE GEOLOGY ASSOCIATED WITH THE M5.8 AUGUST 23, 2011 LOUISA COUNTY, VA EARTHQUAKE

Analyses of geologic features assists with understanding processes and hazards associated with intraplate earthquakes. Much of the bedrock in the M5.8 Louisa County, VA area is covered, but potential field data, which reflect geologic contrasts up to several km depth, and radiometric (gamma-ray spectroscopy for K, U and Th) data, which reflect the upper few cm of the ground, provide complimentary methods for imaging near-surface and deeper geologic features. We present results of airborne magnetic, gravity and radiometric surveys conducted in July 2012 over parts of Louisa, Fluvanna, and Goochland Counties. The surveys were flown with 200-m line spacing from a nominal altitude of 120 m above ground, providing geophysical maps with unprecedented resolution for this region.

Preliminary data show strong correlations between previously mapped geologic units and radiometric signatures, suggesting how contacts extend into areas lacking detailed mapping. Correlations include granitoid intrusive bodies (e.g. the Ellisville pluton, Falmouth Intrusive Suite and pegmatite-rich areas) which are associated with both gamma total count highs and also deep magnetic lows, and the Chopawamsic Formation and Ta River Metamorphic Suite, which show bands of radiometric and magnetic highs and lows likely representing different sub-units. The geophysical data suggest other features that have not been mapped such as abundant N to NNW-trending linear anomalies typical of Jurassic diabase dikes, broad magnetic lows reminiscent of subsurface granitoid bodies, and radiometric signatures along the South Anna River suggesting particular bedrock source contributions to downstream alluvial deposits.

Just southwest of the M5.8 earthquake, magnetic, gravity, and topographic data suggest a change in orientation of major geologic structures from ~N34E to ~N44E, while northeast of the earthquake previous data show straight magnetic lineations that are nearly continuous for over 40 km. The new data show complex magnetic anomalies near the orientation change with arcuate lineations bounding prominent lows. We combine geologic and geophysical data to consider features that may have been associated with the earthquake such as intrusive bodies, geologic contacts, fault relations, and possible zones of weakness.