Regional assessment of prehistoric earthquake magnitudes in the South Carolina Coastal Plain

The magnitudes of prehistoric earthquakes that created liquefaction features (i.e., sandblows) in the South Carolina Coastal Plain (SCCP) are not exactly known. In this study, in situ geotechnical data, including cone penetration test (CPT) data with pore water pressure measurements, in the vicinity of discovered sandblows at Hollywood, Sampit, Gapway, and Fort Dorchester sites located in the SCCP were used with the cyclic stress method and Ground Motion Prediction Equations (GMPEs) to back-calculate the minimum earthquake magnitude (M) and peak ground acceleration (a_max) to form a regional assessment of minimum a_max-M in the Charleston area. Results indicate that the minimum earthquake magnitude of the prehistoric earthquakes associated with the Charleston Source ranges from 6.6 to 7.2 for the earthquake that occurred about 546 ± 17 years before present (B.P.) and from 6.2 to 6.7 for the earthquake that occurred about 5038 ± 166 years B.P. Results also show that the minimum earthquake magnitude for the prehistoric earthquakes with a source inferred to the Sawmill Branch fault and occurred at least 3500 years ago, ranges from 5.1 to 5.7.