FINDING FAULTS IN THE CHARLESTON AREA, SOUTH CAROLINA

The inference of multiple faults in the Summerville area, SC, goes back to the studies carried out after the 1886 Charleston earthquake. Many of these accounts describe a SW-NE direction of motion in Charleston while chronicles in the Summerville area describe mainly vertical motions. Additionally, different focal mechanisms and sounds emanating from a small hypocentral volume suggest the presence of multiple faults. As the faults in this area do not have a surface expression, discerning their seismotectonic framework becomes difficult. Therefore, in search of the seismogenic faults, the seismicity data have been analyzed using HypoDD as well as first motion data recorded at different seismic stations. As a result, I suggest an improved seismogenic framework to explain the 1886 and ongoing seismic activity (depths of 3 to 12 km) near Charleston, South Carolina. The revised framework lying at depths greater than 3 km, consists of ~50 km long ~N30°E striking, NW dipping Woodstock fault associated with oblique right lateral strike slip motion. It has a ~6 km antidilational left step near the Middleton Place-Summerville area associated with steeply dipping, ~N30° to 40°W trending Sawmill Branch, Lincolnville and Charleston reverse faults. The southernmost Sawmill Branch fault is currently the most active and is connected with the N65°W trending Ashley River fault. This framework is consistent with a large variety of data that includes constraints from macroscopic observations following the 1886 earthquake, surface (geomorphic) data, shallow stratigraphic (z ≤ 150 m), seismic reflection (z ≤ 1 km), refraction (z ≤ 2,500 m) and potential field data.