Seismogenic structures in the central Virginia seismic zone

A correlation between earthquake hypocenters and seismic reflection data in central Virginia has become apparent on an "automatic line drawing" (ALD) display of seismic reflection data. With the reprocessed Virginia I-64 reflection Vibroseis data extended to 14 s,reflectors are imaged from the lower crust as well as from the upper crust. Specifically, the improved resolution and data quality of ALDs have produced an image of an antiformal structure bounded by mid-crustal reflections on the bottom and by major thrusts at the top. The reflections that define the roof of the antiform are most prominent from about 6 s (18 km)on the east near Richmond under the Coastal Plain sediments, to 1-1.3 s (3-4 km) between Richmond and Charlottesville, and to 3.5 s (10.5 km) on the west. Seismic signatures that can be followed downward from the surface between Charlottesville and Richmond appear to be truncated at the roof of the antiform. The dominant reflections that define the roof correlate with the seismic signature of the transported Taconic suture on the west flank and mylonites on the east flank.

The distribution of hypocenters in the area shows an excellent correlation with the westward-dipping reflections that form the roof of the antiform on its western flank. Earth-quake activity in this locale may be related to reactivation of the thrusts defining the roof and/or faults above the antiformal structure; however, distribution of the easternmost and deepest set of hypocenters appears to be related to an extensive near-vertical diabase dike swarm of Mesozoic age.