Site Amplification Formula Using Average V s 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 V s /surface V s ) improved much better if the surface V s 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 V s30. 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.