Evaluation of P- and S- Wave Velocities at CGS Geotechnical Arrays Using Wave Propagation Analysis of Strong Ground-Motion Records

Abstract:

The average compressional and shear wave velocities of ground layers at the California Geological Survey’s instrumented geotechnical arrays are evaluated by analyzing the strong ground-motion records of the earthquakes. The Normalized Input-Output Minimization Method (NIOM) has been employed to correlate the ground motion waveforms recorded at various depths of the geotechnical arrays, and to calculate the P- and S- wave travel times and velocities between the depths at which the seismic sensors have been installed. As of August 2014, the California Geological Survey’s Strong-Motion Instrumentation Program (CGS CSMIP) operates 35 geotechnical arrays, a large number of them with cooperation of the California Department of Transportation (Caltrans). As of this date, 140 records from 68 earthquakes with magnitude 3.5 and larger have been recorded at 28 geotechnical arrays. The records are available for download through the Center for Engineering Strong Motion Data (CESMD), the joint center of the U.S. Geological Survey and the California Geological Survey. The velocities obtained in this study for the cases of weak and strong shaking are compared with the measured velocities at the geotechnical array sites obtained using geophysical methods such as SASW, PS suspension logging and downhole methods. The average shear wave velocities in the upper 30 m of the geotechnical arrays from this study are compared with the Vs30 values from geophysical investigations. Application of wave propagation analysis is especially important in studying the dynamic characteristics of ground layers during high stress ground shaking and yields average values for a larger area under a site as opposed to the area in the very close vicinity of the geotechnical array.