Differences Between Site Characteristics Obtained From Microtremors, S-waves, P-waves, and Codas

Abstract

We examine differences of empirical site characteristics among S waves, P waves, coda, and microtremors using records at 20 sites in and around the Sendai basin, Japan, and interpret the differences theoretically. At soft soil sites the horizontal-to-vertical spectral ratios (HVRs) for early P coda become different from HVRs for a P wave with increasing time and eventually converge on HVRs for microtremors. The HVRs for an S coda become similar to HVRs for microtremors with increasing time in the frequency range lower than 3 Hz at soft soil sites. By contrast, at a rock site and two hard soil sites, HVRs for S coda agree well with HVRs for an S wave. The soil-to-rock spectral ratios for horizontal (HHRs) and vertical (VVRs) components for early S coda are larger than those for an S wave at soft soil sites. When we use the deep sedimentary structures above the bedrock before Tertiary age, theoretical HVR for the fundamental mode of Rayleigh waves is consistent with observed HVR for microtremors and theoretical HVR for an obliquely incident SV wave is consistent with observed HVR for an S wave. Theoretical S-wave site amplification factor explains well observed HHR for S wave but does not coincide with HVR for microtremors. In general the frequencies of maximum peaks of HVRs for microtremors do not coincide with those of HVRs and HHRs for S wave. However, if we select HVRs with peak frequencies lower than 1 Hz and peak amplitudes larger than three, the peak frequencies of HVRs for microtremors roughly coincide with those of HVRs and HHRs for S wave. Even under these constraints, their amplitudes do not coincide with each other. Concerning coda, we conclude that the Rayleigh wave contamination in coda is significant in the frequency range lower than 3 Hz at soft soil sites.

Characterization of site conditions (soil class, VS30, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

“…While the array resolution for the FK method was fixed to the width of the main lobe of the array response at its mid-height (k min ), the HRFK technique usually provided improved resolution capabilities. Although numerous authors (e.g., Asten and Henstridge 1984;Tokimatsu 1997;Satoh et al 2001) have reported resolution improvements of a factor of three to six, we chose to limit the minimum resolvable wavenumber to k min /2 Wathelet et al 2008), which allowed for the retrieval of larger wavelengths compared to the FK approach. At high frequencies, all of the dispersion curves were picked as high as the aliasing features allowed.…”

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

“…wave (e.g., Yamanaka et al, 1994;Ishida et al, 1998;Satoh et al, 2001b). In a few basins the bedrock depth has been estimated to fit the calculated peak frequencies (Yamanaka et al, 1994;Satoh et al, 2001c) or trough frequencies (Ishida et al, 1998) of Rayleigh wave to the observed frequencies.…”

Variance Reduction and Signal-to-Noise Ratio: Reducing Uncertainty in Spectral Ratios

“…The lowest SNR cutoff value used is 2 (e.g., Borcherdt and Gibbs, 1976;Rogers et al, 1984;Darragh and Shakal, 1991;Satoh et al, 2001;Lozano et al, 2009). Most studies use the value of 3 (e.g., Field and Jacob, 1995;Theodulidis et al, 1996;Lachet et al, 1996;Bonilla et al, 1997Bonilla et al, , 2002Di Giacomo et al, 2005;Sawazaki et al, 2009).…”