DOI: 10.17603/ds2-by4m-ed67
title: Microtremor Horizontal-to-Vertical Spectral Ratio (mHVSR) Data Collection at California Downhole Vertical Array Sites, 2022
title: in Microtremor Horizontal-to-Vertical Spectral Ratio (mHVSR) Site Characterization of California Vertical Arrays

description: The microtremor Horizontal-to-Vertical spectral ratio (mHVSR) method provides frequency-dependent ratios of Fourier amplitude spectra (FAS) for the horizontal to vertical components of a 3-component recording of ambient ground motions (typically velocities) from microtremors. This method can identify the frequencies associated with site resonances at sites with large impedance contrasts, and hence has potential to provide useful parameters for predicting seismic site response. The mHVSR method is applied by recording ground vibrations either from a temporarily-deployed seismometer, typically recording for a relatively short period of time (~1-2 hrs.), or from a permanently-installed broadband seismometer. It is also useful for studying the spatial, temporal, and azimuthal variation of ground vibrations at sites.

The overall goals of our HVSR research are to develop a publicly accessible database of HVSR data for use in research, to develop effective protocols for analysis and interpretation of HVSR data, and to develop methods for predicting site response conditioned on HVSR data. The database has been established and is available in the United States Community Shear-Wave Velocity (VS) Profile Database (PDB) (https://uclageo.com/VPDB/); the work presented in this doi is contributing to that database.
description: The State of California has multiple sites with vertical arrays consisting of surface and downhole sensors, which are used to investigate shallow site response under earthquake excitations. Characterization of these sites typically includes a boring log and seismic velocity profiles (Vs and Vp). We augment the site characterization using microtremor Horizontal-to-Vertical-Spectral-Ratios (mHVSR), where the ground vibrations are caused for example by wind, ocean waves, and anthropogenic sources. This information is useful to identify potential site resonances, which in some cases may be associated with impedance contrasts at depths beyond the limits of the array, and the consistency of the geology when multiple mHVSR are evaluated at different locations relative to the vertical array. Collaborative research between the University of California, Los Angeles (UCLA), University of Southern California (USC), and the University of Canterbury in New Zealand has performed mHVSR at 16 vertical array sites in California. At each site, 4 tests were performed in 4 concentric circles of increasing diameter to better understand the spatial and azimuthal variation of mHVSR for each of the 16 sites. The HVSR curves developed using this dataset may be accessed in the United States Community Shear-Wave Velocity (VS) Profile Database (PDB) (https://uclageo.com/VPDB/). This work was supported by Pacific Gas and Electric Company, Viterbi School of Engineering, University of Southern California Faculty Award, United States Geological Survey (USGS) EHP G23AP00066-00 Award 2023-0106, New Zealand Earthquake Commission(EQC) and QuakeCoRE.