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

Department of Geological Sciences
4044 Derring Hall
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061
mcc{at}vt.edu
(M.C.C.)

Department of Geological Sciences
University of South Carolina
701 Sumter St.
Columbia, South Carolina 29208
talwani{at}geol.seis.sc.edu, cannon{at}geol.seis.sc.edu
(P.T., R.C.C.)

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 (_s and _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 _s = 0.049 and _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 < _s < 0.049 and 0.010 < _p < 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 (Q_s = 32, Q_p = 36), whereas the upper range values imply that Q_s is substantially larger than Q_p (Q_s = 22, Q_p = 15).

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