Estimation of Ground Motion at Free Surface of Seismic Bedrock Using Empirical Site Amplification Factors by Spectral Inversion Considering Heterogeneous Attenuation

<p>We proposed a new approach to estimate empirical site amplification factors through spectral inversion considering heterogeneous attenuation along the propagation path, and to inversely calculate response spectra at the free surface on seismic bedrock (Vs 3200 m/s) by dividing downhole records on hard soil by these amplification factors. In our spectral inversion process, we enhanced the existing methodology to model the simultaneous analysis of horizontal and vertical ground motions. We also selected two reference stations for constraint conditions based on quantitative indexes. Our amplification factors estimated by this method were compared with those estimated by an existing method based on a network of two adjacent points. Although results from both methods were generally consistent, our approach appeared more reliable in the high-frequency range (∼5-20 Hz) due to the optimal selection of reference stations and consideration of heterogeneous attenuation. Our response spectra at free surface on seismic bedrock were compared with those derived from a conventional method that integrates outcropping analysis based on 1-D multiple reflection theory with soil-velocity adjustment using an empirical equation. Both methods yielded generally consistent results at downhole stations on seismic bedrock equivalent (hard rock), where soil-velocity adjustment was unnecessary. On the other hand, larger discrepancies were appeared, particularly for periods longer than 0.2 s, at downhole stations on soft rock with significant depth difference from seismic bedrock. This seems to be the effect of our method, which estimates amplification factors considering not only surface but also deeper subsurface structures for each station.</p>