NEW INSIGHTS ON THE SEISMOTECTONIC SETTING OF SEISMICITY IN THE CHARLESTON REGION, AND THE ORIGIN OF THE 1886 EARTHQUAKE

A revised regional structural and tectonic interpretation of the buried Mesozoic basins of southeastern North America suggests that seismicity in the Charleston region of South Carolina can be related to specific and predictable fault geometries linked to the South Georgia Rift (SGR). In particular, the regional basin structure can be characterized by a system of NE-striking, NW- and SE-dipping normal faults which define asymmetric half-grabens, broken periodically by NW-striking sub-vertical (primarily dextral) strike-slip faults that define transfer zones within the basin structure. Reactivation of these basin structures under a compressional stress regime would predictably result in reverse motion on the relatively steeply-dipping (>45 degrees) normal faults, and sinistral strike-slip movement on the NW-striking faults. The Middleton Place-Summerville Seismic Zone (MPSSZ) is a seismically active system of faults located 30 km to the northwest of Charleston, South Carolina. Examination of the recorded seismicity in this area indicates a pattern consistent with seismic activity on NW-striking high-angle faults and NE-striking, NW-dipping reverse faults. In particular, historic seismicity in the region around Summerville, South Carolina appears to fit this pattern well. More than 15 events, ranging in magnitude from 0.5 M to 3 M, define a linear trend ~12 km long striking ~N60W. Two focal mechanisms determined for this population of events indicate sinistral strike-slip movement on a high-angle NW-striking fault system, which we here define as the Lincolnville Fault after Talwani et al. Hypocenters (3 to 15 km) of all events are consistent with the known depth range of SGR basin faults. Previous studies have related seismicity in the MPSSZ to reactivation of Mesozoic extensional structures, but this study is first to place these within a well-constrained structural and tectonic framework for the South Georgia Rift, and may provide a better model for understanding intraplate seismicity in general. The 1886 event may have occurred on this NW-striking strike-slip fault, which parallels and lies adjacent to U.S. Interstate 26.