On 8 October 2001 a small felt earthquake (mbLg2.60) occurred near Upper Three Runs Creek in the north central area of the Savannah River Site, South Carolina. It was located at 33.32°N, 81.67°W with a focal depth of 3.9 ± 0.8 km and an origin time of 00:23:01.12 UTC. Seven very small aftershocks (MD ≤ 1.4) followed the main event with the last one occurring 6 March 2002. All activity occurred within a small area of 6.0-6.5 km2. Further analysis of collected data indicates a correlation of this low-level seismic activity with a small northwest-trending structure observed in detailed gravity and magnetic data. Both single-event and composite focal mechanisms were derived using local and regional stations. Results indicated predominantly dip-slip motion along a fault striking north-northwest at 335° and dipping 41° to the southwest. A 3D plot of the eight hypocenters clearly defines a fault plane nearly analogous to that obtained from the focal solutions.
The Upper Three Runs series of events is another example of a separate class of earthquakes that occur within the central Piedmont and upper Coastal Plain of South Carolina. The Upper Three Runs sequence of events demonstrates that shallow intersections of structures interpreted from potential field data can be the foci for localized stress concentrations where microearthquake activity (M ≤ 3.0) can occur. These earthquakes are attributable to small-scale faults associated with pockets of relatively high stress concentrations and are generally accompanied by loud noises. Their shallow depths and small epicentral areas suggest that these earthquakes are extremely localized and are not attributable to any large-scale regional features.