Finding Faults in the Charleston Area, South Carolina: 1. Seismological Data

Macroscopic observations following the 1886 Charleston, South Carolina, earthquake and analyses of instrumentally recorded seismicity between 1974 and 2004 suggest the presence of two or more active faults. In order to more clearly define the active faults and determine their seismotectonic framework, instrumentally located hypocenters were relocated using the double-difference algorithm HypoDD. The revised hypocentral locations were associated with different faults based on the first motions recorded at different locations. The result is a plausible framework that shows several important changes from earlier interpretations. This framework defines a localized stressed volume, which consists of the similar to 50-km-long similar to N30 degrees E striking, NW dipping Woodstock fault associated with oblique right-lateral strike-slip motion with a similar to 6-km-long antidilatational left step near Middleton Place. Three similar to NW-SE striking reverse faults, two NE dipping and one SW dipping, were recognized within this left step; of these, the NE dipping Sawmill Branch fault zone lying between Middleton Place and Summerville is the most active. Minor activity was observed on the NE dipping Lincolnville and the SW dipping Charleston faults. The southernmost Sawmill Branch fault zone also shows evidence of left-lateral strike-slip motion. The similar to N55 degrees W trending Ashley River fault lying between Middleton Place and the Magnolia Plantation appears to be currently inactive.