Magnitudes of Prehistoric Earthquakes in the South Carolina Coastal Plain from Geotechnical Data

Abstract

In-situ geotechnical data were collected near paleoliquefaction features in the South Carolina Coastal Plain (SCCP) to assess the magnitudes and peak ground accelerations of prehistoric earthquakes. Currently used paleoseismic analysis methods were used to back-calculate the magnitudes of prehistoric earthquakes and peak ground accelerations that can cause paleoliquefaction features in the SCCP. The results from multiple methods yielded similar results and indicated that the prehistoric earthquakes in the SCCP that occurred during the past 6,000 years and caused paleoliquefaction features have magnitudes ranging from 5.3 to 7.8. The peak ground accelerations needed to cause liquefaction are also consistent among the different methods and ranged between about 0.1 and 0.3 g for M 7.5 and between about 0.1 and 0.5 g for M 6.0 earthquakes. These results were used together with those of earlier paleoseismological investigations to estimate the magnitudes and peak ground accelerations associated with prehistoric earthquake episodes. For earthquake episodes centered at Charleston, the estimated magnitudes and peak ground accelerations range from 6.8 to 7.8 and 0.16 to 0.24 g, respectively. For episodes centered near Georgetown, the estimated magnitudes and peak ground accelerations range from 5.5 to 7.0 and 0.21 to 0.42 g, respectively.