H/V ratio and amplification factor: a numerical experiment using 2.5D modelling

Abstract

This paper presents the H/V ratio peak amplitude and frequency, their correlation with the site frequency and amplification factor, and limitations. The basin-edge and flat-layer models with different types of soils in the surficial layer were simulated using 2.5D (two and half dimensional) modelling. The effects of sediment bedrock velocity contrast on the H/V ratio peak frequency and amplitude, and the sensitivity of the H/V ratio peak to both the S-wave transfer function and the ellipticity of Rayleigh waves at the fundamental frequency were studied. Simulated results revealed that H/V ratio peak frequencies for both the basin-edge and flat-layer models were in good agreement with the soils fundamental frequencies, only when velocity contrast was larger than 3.5. The comparison of H/V ratio peak amplitude and amplification factors indicates that the use of amplitude of H/V ratio as amplification factor of soil/site is not justified because the obtained H/V ratio was not in accordance with the velocity contrast and can have any value independent of soil velocity and was not similar in magnitude for radial and transverse components. Analysis of the H/V ratio and spectral amplitudes of responses of basin-edge and flat-layer models revealed that it was the trapping of waves (body waves or surface waves) which was responsible for the spectral peak in the H/V ratio at the fundamental frequency. It was also concluded that surface waves were generated near the edge of the basin and propagated normal to the edge, towards the basin. The peak amplification factors were found in good agreement with the fundamental frequency of soil beneath respective recording stations.