Weak motion data for constraining site-specific ground motion estimates

With the increasing recognition of the importance of non-ergodic components in seismic hazard analysis, it is becoming increasingly important to obtain site-specific amplification functions (AFs). The most common approach to obtain site-specific AFs is to perform site response analyses backed by detailed site characterization with measures of uncertainty. An alternative approach would be to utilize ground motions recorded at the site, if any, to constrain site effects. But this approach is only viable if ground motion recordings are available, which they generally are not. However, in a seismically active region (such as California), the rate of occurrence of small magnitude earthquakes (i.e., M<4.5) is sufficient for weak ground motions to be recorded using a temporarily deployed station, especially for the design of critical structures. This then raises the question: can weak ground motions resulting from small magnitude events recorded using temporary seismometers be used to constrain site effects? We explored the possibility of employing a Bayesian inversion approach for constraining site terms using weak ground motions. Towards this end, ground motion recordings associated with two dam sites in Northern California, Sites LB and SD, were used in a Bayesian inversion scheme to constrain source, path, and site terms with measures of uncertainty. At both sites, the empirical site transfer functions were observed to be similar to the analytical site transfer functions. The standard error of the transfer function estimate suggest that the results are stable when sufficient recordings are available. This work serves to highlight that Bayesian inversions can be used to recover site terms and outlines a procedure for how the results should be interpreted post-hoc.
Here, we present the Fourier Amplitude Spectra (FAS) of the ground motion recordings that was used in the Bayesian Inversions. Because the inversions were performed separately for the two sites, there are two sets of FAS data presented in the form of two CSV files. The FAS data presented here are obtained by uniformly processing the raw ground motion recordings using standard processing techniques. For more details on the data processing and filtering, please refer to Anbazhagan et al. (2025). The data files can be used to reproduce the empirical site terms obtained from the Bayesian inversions.