Comparison of phase velocities from array measurements of Rayleigh waves associated with microtremor and results calculated from borehole shear-wave velocity profiles

Shear-wave velocities (V-S) are widely used for earthquake groundmotion sit e characterization. V-S data are now largely obtained using borehole method s. Drilling holes, however, is expensive. Nonintrusive surface methods are inexpensive for obtaining V-S information, but not many comparisons with di rect borehole measurements have been published. Because different assumptio ns are used in data interpretation of each surface method and public safety is involved in site characterization for engineering structures, it is imp ortant to validate the surface methods by additional comparisons with boreh ole measurements. We compare results obtained from a particular surface met hod (array measurement of surface waves associated with microtremor) with r esults obtained from borchole methods. Using a 10-element nested-triangular array of 100-m aperture, we measured surface-wave phase velocities at two California sites, Garner Valley near Hemet and Hollister Municipal Airport. The Garner Valley site is located at an ancient lake bed where water-satur ated sediment overlies decomposed granite on top of granite bedrock. Our ar ray was deployed at a location where seismic velocities had been determined to a depth of 500 m by borehole methods. At Hollister, where the near-surf ace sediment consists of clay, sand, and gravel, we determined phase veloci ties using an array located close to a 60-m deep borehole where downhole ve locity logs already exist. Because we want to assess the measurements uncom plicated by uncertainties introduced by the inversion process, we compare o ur phase-velocity results with the borehole V-S depth profile by calculatin g fundamental-mode Rayleigh-wave phase velocities from an earth model const ructed from the borehole data. For wavelengths less than similar to 2 times of the array aperture at Garner Valley, phase-velocity results from array measurements agree with the calculated Rayleigh-wave velocities to better t han 11%. Measurement errors become larger for wavelengths 2 times greater t han the array aperture. At Hollister, the measured phase velocity at 3.9 Hz (near the upper edge of the microtremor frequency band) is within 20% of t he calculated Rayleigh-wave velocity. Because shear-wave velocity is the pr edominant factor controlling Rayleigh-wave phase velocities, the comparison s suggest that this nonintrusive method can provide V-S information adequat e for ground-motion estimation.