S-WAVE VELOCITY MODEL OF NEAR-SURFACE SEDIMENTS IN CHARLESTON, SOUTH CAROLINA

FAIRBANKS, C.D., and ANDRUS, R.D, Department of Civil Engineering, Clemson University, Clemson, SC 29634, ZHANG, J., Fugro Consultants LP, Houston, TX 77801; CAMP, W.M. and CLEARY, T.J., S&ME Inc, Mt. Pleasant, SC 29464; and CASEY, T.J. and WRIGHT, W.B., WPC, Mt. Pleasant, SC 29464, cfairba@clemson.edu.

The 1886 Charleston, South Carolina earthquake (moment magnitude of about 7.3) resulted in widespread damage and 60 deaths. The extent of major damage caused by the earthquake in the city district of Charleston appears to have depended, in part, on thickness and stiffness of the Quaternary sediments. This study characterizes the stiffness of near-surface sediments in the area, and refines an earlier structure contour map of the base of Quaternary (or top of Tertiary) beneath the Charleston peninsula.

The stiffness of six major geologic units is characterized using in situ shear (S)-wave velocity measurements conducted by various investigators at 91 sites. The six major geologic units are: man-made fills, Holocene and late Pleistocene deposits, the Wando Formation, the Ten Mile Hill beds, the Penholoway Formation and Daniel Island beds, and Tertiary sediments. Calculated mean S-wave velocities for these units in the top 25 m are 141 m/s, 108 m/s, 190 m/s, 178 m/s, 309 m/s and 393 m/s, respectively, assuming data are log-normally distributed. Corresponding one standard deviation ranges are 93-205 m/s, 72-162 m/s, 148-242 m/s, 131-238 m/s, 202-453 m/s and 175-660 m/s, respectively. These results can be used to estimate profiles of S-wave velocity at locations where only the geologic profiles are known. The earlier structure contour map of the top of Tertiary on the peninsula was developed by Weems and Lemon using about 30 boreholes. Their contour map is refined using the logs of over 300 borings and cone soundings. The revised contour map reveals that the top of Tertiary is a more complex erosional surface than previously shown, with elevations ranging from -6 m to -24 m. Based on the revised contour map and the surficial geology map by Weems and Lemon, the top of the Tertiary beneath the peninsula is incised by paleochannels that primarily pre-date the 70,000 to 130,000-year-old Wando Formation.