Site amplification formula using average vs in equivalent surface layer based on vertical array strong motion records

Abstract

Seismic site amplification formula was developed for virtual surface arrays to be consistent with vertical arrays using a number of KiK-net records during recent 8 destructive earthquakes. A correlation between peak spectrum amplification and S-wave velocity ratio (base Vs/surface Vs) improved much better if the surface Vs was evaluated from fundamental mode frequency combined with a thickness of equivalent surface layer in which peak amplification is exerted, rather than using the conventional Vs30. Also found was that soil nonlinearity effect during strong earthquakes has only a marginal effect on the surface array amplification observed and evaluated by the formula using small-strain Vs-values. The theoretical basis for the minor nonlinear effect has been discussed by conducting a simple study on a two-layered system with strain-dependent soil nonlinearity. The proposed amplification formula seems also applicable with a satisfactory accuracy to sites where soil liquefaction is involved.