Influence of the V_S profiles beyond 30 m depth on linear site effects: Assessment from the KiK-net Data

Site effects may be assessed using a standard soil classification parameter, V_S30 (the harmonic average shear-wave velocity in the first 30 m); however, this index does not account for the complexity of the velocity profile, especially its variability at depth. In the present study, in addition to V_S30, we propose consideration of the gradient of the V_S profile from 0 to 30 m depth, denoted B₃₀. A lower gradient value means low velocity increases with depth; a higher gradient indicates a rapid velocity increase with depth in the shallow layers. In addition, we consider the fundamental resonance frequency of the soil (f₀), which has been shown to be a relevant parameter for site-effect assessment and which is obtained from the empirical site response. Using the Japanese KiK-net database, we analyze the variability of the V_S profiles and the empirical borehole site responses of selected sites through the V_S30, the velocity gradient B₃₀, and f₀. We select 289 sites for which the 1D linear numerical modeling is close to the empirical site response and a V_S at the downhole station is greater than 1000 m/s. For a given V_S30 class, B₃₀, and f₀ can be used to distinguish between two types of sites: deep sedimentary sites and sites with high velocity contrast at shallow depths. We find that, even if the gradient is calculated using shallow information, its use improves the site amplification characterization, compared to using only V_S30, by reducing the intersite site-response variability. As expected, however, this improvement is limited for deep sedimentary sites. On the other hand, f₀ is able to reduce the intersite response variability for deep sedimentary sites even though it is limited to specific V_S30 classes. Thus, the combined use of V_S30, B₃₀, and f₀ improves the assessment of linear site amplification.