One-dimensional ground response analyses (GRA) can introduce model error to site response estimates when wave propagation is not dominated by vertically propagating shear waves. We identify sites suitable for GRA based on microtremor horizontal-to-vertical spectral ratios (mHVSRs). We analyzed 300 microtremor recordings from 17 vertical array sites in California, comparing mHVSRs at varying spatial separations. We find that low mHVSR spatial correlation, as measured using Longest Common Subsequence, tends to occur at vertical array sites that are poorly modeled by GRA. Conversely, stronger mHVSR correlations tend to occur at sites where GRA is relatively effective.

Traditional ergodic models are derived based on time-averaged shear-wave velocity in the upper 30 m of the site. These models are not able to account for site resonances, the presence and frequency of which can be established from microtremor HVSR surveys. Not all California sites exhibit such resonances, but knowledge that peaks are or are not present affects site response over a wide range of frequencies, with the former producing a response spectral peak near the HVSR peak. Research is underway to develop a model using microtremor HVSR data, which will be novel relative to previous models that are based on earthquake HVSR data. Our model is being formulated as modification to a global VS30 and z1.0 relationship. This paper explains the model development approach and findings of a systematic assessment of how HVSR curves relate to features of site-specific (or non-ergodic) response, which is informing model development.