Ground-Motion Attenuation in the Atlantic Coastal Plain near Charleston, South Carolina

Charleston, South Carolina, lies on approximately 1 km of Cretaceous and Cenozoic sediments of the Atlantic Coastal Plain. Estimation of high-frequency absorption due to these sediments is important for strong motion prediction. We attempt the measurement using microearthquake data recorded at small distances by surface and shallow subsurface short-period stations in the Middleton Place––Summerville seismic zone. The problem is difficult because it involves potential bias due to the seismic source, propagation through basement, and strong site-specific spectral modulation. Previous studies involving drilling and seismic reflection profiling indicate a 775-m thickness of sediments, with average vertical P- and S-wave velocities of 2.14 and 0.700 km/sec for the network area. The attenuation parameter kappa ((kappa)s and (kappa)p) for S and P waves is estimated from spectral analysis of the direct S and converted Sp phases. The ratio of S to Sp provides a useful check for bias. Multiple linear regression using all stations yields (kappa)s = 0.049 and (kappa)p = 0.024. The regression results are interpreted as upper-bound estimates because they assume source corner frequencies in excess of 25 Hz. A similar analysis is carried out for a hard-rock environment using reservoir-induced microearthquakes at Lake Monticello, South Carolina. From that, we estimate a maximum potential bias of 0.014 sec^–1, yielding 0.035 < (kappa)s < 0.049 and 0.010 < (kappa)p < 0.024 as likely values near Charleston. We favor the lower limits of these ranges because they imply numerically similar values for the pathaverage quality factors (Os = 32, Op = 36), whereas the upper range values imply that Os is substantially larger than Op (Os = 22, Op = 15).