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 Pand S-wave velocities of 2.14 and 0.700 km/sec for the network area. The attenuation parameter kappa (js and jp) 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 js 0.049 and jp 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 , yielding 0.035 js 0.049 and 0.010 jp 0.024 as likely values near Charleston. We favor the lower limits of these ranges because they imply numerically similar values for the path-average quality factors (Qs 32, Qp 36), whereas the upper range values imply that Qs is substantially larger than Qp (Qs 22, Qp 15).