Abstract:While the fundamental theory behind the horizontal/vertical spectral ratio (HVSR) of microtremor Fourier amplitudes remains elusive, its application in estimating site condition parameters as well as in investigating seismic site effects is widely recognized. The microtremor HVSR was refined, and a novel empirical method for estimating the surface/borehole spectral acceleration ratio (SBSRa) of ground motion was developed. This methodology enabled the estimation of ground motion amplification factors by in-situ microtremor measurements, consequently enhancing foundational data for the seismic site effects researches. Utilizing a dataset comprising over 300,000 sets of strong motion acceleration records from 589 stations within Japan’s KiK-net network, the stable microtremor HVSR was derived from microtremor data recorded during the pre-event of the time history. The correlation between microtremor HVSR and ground motion SBSRa was calculated at each station under weak seismic motion. Interestingly, it was observed that microtremor HVSR and ground motion SBSRa exhibited analogous curves and approximate dominant frequencies. Building on this finding, empirical correction coefficients for each type of microtremor HVSR were determined using a frequency normalization approach. Subsequently, a methodology for predicting ground motion SBSRa based on microtremor HVSR was proposed. Reliability analyses demonstrated that the predicted SBSRa using this methodology achieved engineering acceptable accuracy. Furthermore, the Jaccard similarity coefficients between the predicted values and the measured values of ground motion SBSRa at 75% of stations exceeded 0.58, affirming the validity of the approximate estimation for ground motion SBSRa under weak seismic motion predicted by microtremor HVSR.
