Evaluation of VS30 Near Strong Motion Recording Sites Using MASW and S-wave Refraction Tomography, San Bernardino County, California

Abstract:

We acquired 2-D active-source seismic surveys in the vicinity of strong motion recording stations in San Bernardino County in order to better understand the contributions of V_S30, the time-averaged shear-wave velocity in the upper 30 m of the subsurface, to observed ground shaking. We recorded both body and surface waves along linear profiles (up to 180 m long), and we used 2-D refraction tomography and Multichannel Analysis of Surface Waves (MASW) methods to evaluate shear-wave velocities, from which we calculated V_S30 at every meter along the seismic profiles. We determined that V_S30 varies from 360 to 538 m/s at sites of Quaternary deposits composed primarily of silt, sand, and gravel. Lateral variability in V_S30 along individual seismic profiles varies from about 20 m/s to more than 150 m/s. Importantly, we find that the observed V_S30 nearest to the strong motion stations and the average V_S30 calculated for the profiles differ by up to 11%, and the calculated V_S30 nearest to the strong motion stations is as much as 38% lower than the maximum V_S30and more than 20% higher than minimum V_S30 along the profiles. Our results suggest 1-D V_S30 values, such as those determined from seismic cone penetrometer surveys conducted at strong motion stations, may not accurately capture local site conditions due to lateral changes in subsurface materials and structures. V_S30 averaged over a few tens of meters may better account for the site response at strong motion recording stations that are in the vicinity of laterally varying surface or subsurface geology.