Comparison of Phase Velocities from Array Measurements of Rayleigh Waves Associated with Microtremor and Results Calculated from Borehole Shear-Wave Velocity Profiles

Abstract

Shear-wave velocities (V S ) are widely used for earthquake groundmotion site characterization. V S data are now largely obtained using borehole methods. Drilling holes, however, is expensive. Nonintrusive surface methods are inexpensive for obtaining V S information, but not many comparisons with direct borehole measurements have been published. Because different assumptions are used in data interpretation of each surface method and public safety is involved in site characterization for engineering structures, it is important to validate the surface methods by additional comparisons with borehole measurements. We compare results obtained from a particular surface method (array measurement of surface waves associated with microtremor) with results obtained from borehole 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-saturated sediment overlies decomposed granite on top of granite bedrock. Our array 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-surface sediment consists of clay, sand, and gravel, we determined phase velocities using an array located close to a 60-m deep borehole where downhole velocity logs already exist. Because we want to assess the measurements uncomplicated by uncertainties introduced by the inversion process, we compare our phase-velocity results with the borehole V S depth profile by calculating fundamental-mode Rayleigh-wave phase velocities from an earth model constructed from the borehole data. For wavelengths less than ϳ2 times of the array aperture at Garner Valley, phase-velocity results from array measurements agree with the calculated Rayleigh-wave velocities to better than 11%. Measurement errors become larger for wavelengths 2 times greater than 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 the calculated Rayleigh-wave velocity. Because shear-wave velocity is the predominant factor controlling Rayleighwave phase velocities, the comparisons suggest that this nonintrusive method can provide V S information adequate for ground-motion estimation.

Low‐frequency passive seismic experiments in Abu Dhabi, United Arab Emirates: implications for hydrocarbon detection

“…For example, patterns of ground motion amplification observed during earthquakes are comparable to amplification patterns modelled from microtremor data (Horike, Zhao and Kawase 2001; Rodriguez and Midorikawa 2003; Cara et al 2008; Haghshenas 2008). Similarly shallow shear‐wave velocities determined from the analyses of microtremor data have been verified through application of downhole measurements and seismic refraction profiles (Liu et al 2000; Louie 2001; Di Giulio et al 2006).…”

Shear‐wave velocity profiling at sites with high stiffness contrasts: a comparison between invasive and non‐invasive methods

“…In Asten et al () a blind comparison was reported between passive surface wave recordings, seismic CPT and seismic refraction at a site with about 20 m of soft soil , overlying firm glacial till . Liu et al () reported the case of alluvial sediments (about 100 m thick) overlying granite bedrock with shear‐wave velocity slightly higher than 1500 m/s.…”

Thunder‐induced ground motions: 2. Site characterization

“…Rayleigh wave dispersion is typically considered as a good constraint on the S wave structure [Lai and Rix, 1998;Liu et al, 2000;Louie, 2001]. A systematic search of model parameters based on the ranges determined from the refraction results (Table 1) was performed to empirically examine the effects of seismic velocities and thicknesses on the measured Rayleigh wave phase velocity dispersion.…”