Shear-Wave Velocity of the Ground Near Sixty California Strong Motion Recording Sites by the Spectral Analysis of Surface Waves (SASW) Method and Harmonic-Wave Sources

The seismic networks for California are coordinated through the Northern California Seismic Net (NCSN), the U.C. Berkeley Digital Seismic Net (BDSN), the southern California Seismic Network (SCSN), and the seismological laboratory of the University of Nevada, Reno (UNR). These networks are administered through collaborative efforts of the U.S. Geological Survey, California Institute of Technology (SCSN), U.C. San Diego and U.C. Berkeley, UNR, and the California Geological Survey (CSMIP). Many hundreds of these sites have little or no quantitative characterization for site amplification effects or natural site period of vibration. In this study, we investigate sixty strong motion recording (SMR) sites using an active-source approach that employs ultra-low frequency-controlled harmonic waves to measure the dispersive nature of surface waves in the ground. An inversion algorithm employing a non-linear least-squares best fit is used to invert shear-wave velocities for the upper 30 meters of the soil column.

The overall objective of this project is to visit unclassified California strong motion recording (SMR) sites and acquire spectral analysis of surface waves (SASW) data to characterize the site stiffness properties to a minimum depth of 30 meters. The principal products of this effort are the computation of shear-wave velocity logs, Vs-30 site values, and NEHRP site classification. This study was conducted at the request of the Pacific Earthquake Engineering Research (PEER) Center, Lifelines Program Task 1L10 to provide a quantitative basis for site classification of ground associated with recorded ground motions used in the Next Generation Attenuation (NGA) modeling project. Our preliminary findings were delivered in digital format on January 30, 2004 to PEER-NGA project participants through a password protected FTP site. An interim report with shear-wave velocity profiles was distributed to NGA members in digital form through the same web site on March 30, 2004. This was followed by an informal review process with several NGA participants that resulted in the computation of new shear-wave velocities using an inversion algorithm with reduced soil model layering, and clarification on the locations of several of the SASW test sites.