Recurrence rates of large earthquakes in the South Carolina Coastal Plain based on paleoliquefaction data

We present a reanalysis of results of 15 years of paleoliquefaction investigations in the South Carolina Coastal Plain. All earlier radiocarbon age data and locations of organic material collected by various investigators were reviewed and recalibrated to obtain a uniform data set. The calibrated dates and the spatial extent of the sandblows having similar dates were used to estimate ages and magnitudes of prehistoric earthquake episodes. The results of this analysis suggest seven episodes (episodes A‐G) of prehistoric liquefaction in the past 6000 years and two possible scenarios for their occurrence. In the first scenario, three seismic sources exist within the Coastal Plain of South Carolina; at Charleston (A, B, E, and G) with magnitudes M 7+, Georgetown (C and F), and Bluffton (D) with magnitudes M ∼6. In the second scenario, episodes C and D are combined into one episode, episode C′. In this scenario all earthquakes occurred at Charleston and with M 7+. Episodes A and B seem to be more representative of the earthquake cycle and suggest a recurrence time of 500–600 years for M 7+ earthquakes at Charleston. The recurrence times and magnitudes for episodes C and D are estimated at ≥2000 years and ∼6.0, respectively. The older episodes are less frequent, a fact that may be attributable to times of low ground water table. Before ∼6000 years B.P., the ground water table was too low to permit observable liquefaction features to develop at the surface.