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 (V5 \ which are widely used for earthquake ground-motion site characterization studies, are now largely obtained using borehole methods. Drilling holes, however, is expensive. Surface methods are less expensive for obtaining V5 information, but not many comparisons with direct borehole measurements have been made. Because different assumptions are used in data interpretation of each surface method, and because 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 ten-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 V5 depth profile by calculating fundamental-mode Rayleigh-wave phase velocities from an earth model constructed from the borehole data. For wavelengths <~2 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 of 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 Rayleigh-wave phase velocities, these comparisons suggest that this non-intrusive method can provide Vs information adequate for ground motion estimation provided two conditions are met. These conditions are: (1) the site velocity structure can be approximated by a horizontally-layered structure at least on the size of the seismic array, and (2) when the surface \\ avelength is <~2 times of the array aperture.

Pre-loading effect on site response: Site amplification and soil properties mismatch

“…[10,20]). It has been previously shown that the two methods do not return the soil profile, in the same way as before preloading.…”

“…Statistical approaches (average and 71 standard deviation, Monte Carlo techniques, etc) are often used to handle V s data (e.g. [20][21][22][23][24][25]). Having this in hand, the statistically analyzed data are then imported into seismic response computations in order to get the transfer function or the design earthquake response spectrum and/or time history.…”

Characterization of the sedimentary cover at the Himalayan foothills using active and passive seismic techniques

“…In this case, a regular spacing was used as the numbers of available geophones were sufficient to prevent spatial aliasing. The length of the linear array is equivalent to the maximum wavelength that can be retrieved, which in turn determines the maximum depth of investigation of this experiment (Liu et al, 2000). The survey was performed along a linear array using roll along mode with a sampling interval of 0.500 ms with a total recording length of 1.024 s for each record.…”

S-wave velocity structures of the Taipei Basin, Taiwan, using microtremor array measurements

“…Moreover, it has been recognized that short-period microtremors (<1 s) are also useful for estimating shallow S-wave velocity structures (e.g., Chouet et al, 1998). Based on the success of such studies, the array method has been used to estimate both deep and shallow structures (e.g., Kawase et al, 1998;Liu et al, 2000). Satoh et al (2001a) estimated the phase velocities of microtremors in the frequency range $0.5-3 Hz using frequency-wavenumber (f-k) spectral analysis and then obtained the S-wave velocity structures for depths of up to 1 km at six sites using a Rayleigh-wave inversion technique in and around the Sendai Basin, Japan.…”