Site Parameterisation/Classification

Testing the Regional Velocity Model J-SHIS

3D regional velocity models were often queried to establish depths to a certain velocity isosurface for sites without depth measurement. We (Zhu et al., 2019a) evaluated the accuracy of the depth parameters in the 3D velocity model from Japan Seismic Hazard Information Station [J-SHIS]) against their corresponding site-specific measurements from reliable boring data. The comparison indicates that the J-SHIS model underestimates site depths at shallow sites and overestimates depths at deep sites. The impact of the bias in depth on ground motion predictions has also been discussed. 

Optimal Site Parameterization

In the search for the optimal predictor(s) of site response, we investigated the effectiveness of various site characterization parameters, including average velocities to various depths, measured and inferred sediment thickness, and site resonant frequencies. We (Zhu et al., 2019b) demonstrated that site resonant frequency outperforms soil thickness in modeling amplification in Japan. Site resonant frequency can be readily derived from HVSR of micro tremors, and thus we recommend the application of site resonant frequency over soil thickness. 

Identification of Peak Frequencies from HVSR

Site resonant frequency is performant in modeling site effects. However, the identification of site resonant frequency from HVSR is not with uncertainty. We (Zhu at al., 2019c) thus examined the uncertainty using HVSR technique in detecting site resonant frequency at KiK-net sites in Japan. Our results show that the scenario dependence of response (pseudospectral acceleration) spectral ratio could bias the estimates of resonant frequencies for sites having multiple significant peaks with comparable amplitudes. Thus, the Fourier amplitude spectrum (FAS) should be preferred in computing HVSR. For more than 80% of the investigated sites, the first peak (in the frequency domain) on the average HVSR curve coincides with the highest peak. However, for sites with multiple peaks, the highest peak frequency (fp) is less susceptible to the selection criteria of significant peaks and the extent of smoothing to spectrum than the first peak frequency (f0). These results could help reduce the uncertainty in the estimation of site resonant frequency.