Shear-Wave Velocity Profiling of Strong Motion Sites that Recorded the 2001 Nisqually, Washington, Earthquake

The 2001 M 6.8 Nisqually, Washington, earthquake was recorded by more than 70 strong motion sites in and around the Puget Sound region. We have characterized the shear-wave velocity (V_S) structure down to depths of 100 to 300 ft at the 32 permanent strong motion sites, which recorded the highest ground motions (peak horizontal ground accelerations [PGA] of 0.04 to 0.31 g), using the Spectral-Analysis-of-Surface-Waves (SASW) technique. Most of the surveyed sites are underlain by glacial till (Qvt) with the remaining sites on Holocene alluvium (Qal), glacial recessional (Qvr) and advance outwash deposits (Qva), or manmade fill/modified ground (m). V_S30 values for Qvt and Qvr range from 1,266 to 1,769 ft/sec and 1,139 to 1,826 ft/sec, respectively, corresponding to NEHRP site class C. In general, a pattern of higher PGAs with lower V_S30 was not observed suggesting that V_S30 cannot account for all site effects on the 2001 Nisqually ground motions.