A New Set of Automated Methodologies for Estimating Site Fundamental Frequency and Its Uncertainty Using Horizontal-to-Vertical Spectral Ratio Curves

Abstract

This study proposes a new set of methodologies to estimate the site fundamental frequency using the horizontal-to-vertical spectral ratio (HVSR) of recorded surface ground motions. Because of the lack of consensus in HVSR calculation among researchers, a wide range of methods are practiced in this area, yielding different site fundamental frequencies at a given site due to analyst subjectivity. In this study, current practices for combining horizontal components—geometric mean and RotD50—are examined first, and results show that both methods provide comparable HVSR curves. However, RotD50 has the advantage of being orientation independent. Second, the application of Fourier amplitude spectrum (FAS) and 5% damped pseudospectral acceleration (PSA) in computing HVSR is studied, and results are presented for one case study in which PSA-based HVSR seems to suffer from scenario dependency, whereas the FAS-based results appear stable. Different values for Konno–Ohmachi smoothing parameter b were evaluated, and its effect on estimating the site fundamental frequency was considered; using b = 40 is our preference. In previous studies, average HVSR curves were used to estimate a single number representing the site fundamental frequency without incorporating uncertainty. This study proposes four methodologies that use events’ individual HVSR curves to estimate the site fundamental frequency and its associated uncertainty in a completely automated manner. Methods 1–3 use individual HVSR curves to find the maximum-likelihood estimate of the site fundamental frequency (fml), whereas method 4 uses both individual and average HVSR curves to estimate the first resonance frequency (f0). To evaluate the automated methods, a subset of the Next Generation Attenuation-West2 dataset is used to study 50 stations, and the results are compared with an independent study demonstrating good consistency. The proposed methods are further illustrated using data from the Garner Valley Downhole Array (GVDA), which highlights the pros and cons of the presented methods.

Reducing the uncertainties in the NGA-West2 ground motion models by incorporating the frequency and amplitude of the fundamental peak of the horizontal-to-vertical spectral ratio of surface ground motions

“…Figure 2 illustrates an example for estimating f ml , A ml , σ ln ( f ml ) , and σ ln ( A ml ) at Palmdale Fire Station using Method 1. The explanation, limitations, and application of the automated methods are discussed in detail by Yazdi et al (2022).…”

“…As a result, the amplitude of the peak in HVSR is likely to be less than the actual amplification factor. Yazdi et al (2022) further showed that the amplification factors obtained from the average HVSR of the site are generally smaller than the ones obtained from individual HVSR curves because the peak does not always occur at exactly the same frequency. Hence, using average HVSR curves results in smaller amplification factors.…”

“…In this study, a dataset of Next Generation Attenuation (NGA)-West2, including 1218 stations, is used to show that the site fundamental frequency can reduce the uncertainties at both deep and shallow soil sites. Maximum likelihood estimate of site fundamental frequency (f ml ) obtained from HVSR is assumed to represent the site frequency (Yazdi et al, 2022). First, the automated methods proposed by Yazdi et al (2022) are used to estimate the HVSR-based proxies (i.e.…”

Estimating epistemic uncertainty in soil parameters for nonlinear site response analyses: Introducing the Latin Hypercube Sampling technique

“…Note that prior researchers, such as Yee et al (2013) and Afacan et al (2019), have demonstrated the importance of shear strength in the modeling of site response at large strains and that researchers, such as Yee et al (2013) and Groholski et al (2016), have provided methods to adjust the backbone curve to account for it. Finally, another key aspect of this work is to quantify site response uncertainty using both the actual recordings at a geotechnical downhole array (Yazdi et al, 2022; Yee et al, 2013; Zogh et al, 2021) and the predicted median response spectra for different levels of intensity to assess the reliability of using the predicted median response spectra in case the actual recordings at the location of site are not available.…”

Uncertainty quantification of ground motion time series generated at uninstrumented sites

“…The intrinsic uncertainty of ground motions affects earthquake engineering disciplines, such as the performance-based post-earthquake assessment and decision-making (Aghababaei et al, 2021; Roohi and Hernandez, 2020; Weatherill et al, 2015). Several studies attempted to quantify and model this randomness in various seismic problems (Alamilla et al, 2001; Wen et al, 2003; Yazdi et al, 2022). In this study, we focus on quantifying the uncertainty and validity of the generated motions using the GPR model introduced by Tamhidi et al (2021).…”