The difference between horizontal-to-vertical spectra ratio and empirical transfer function as revealed by vertical arrays.

Mianshui Rong,Hongguang Li,Yan Yu,Hao Sun

doi:10.1371/journal.pone.0210852

Abstract

The horizontal-to-vertical spectral ratio (HVSR) and empirical transfer function analyses were performed on the S-wave recordings from two vertical borehole strong motion arrays: the Garner Valley Downhole Array in southern California, and the KiK-net Ichinoseki-Nishi Vertical Array in West Ichinoseki, Japan. The results show that the discrepancy between the HVSR and the transfer function is mainly caused by the significant site response of the vertical component, thus, vertical incident P-waves are proposed to play an important role in the vertical amplification. The P-wave amplification is frequency-dependent. In the low-frequency range within f0 (the fundamental frequency of the site), the effect of the vertical P-wave amplification is slight, this is why HVSR and transfer function match in this frequency range. In the high-frequency range near 2 f0 or larger, the P-wave amplification is obvious, which maybe explain the discrepancy between the HVSR and the transfer function.

Highlights

The evaluation of site-effects due to local geology or topography has become a standard requirement in microzonation studies or site evaluation for important facilities [1]
The horizontal-to-vertical spectral ratio (HVSR) method was originally used to estimate the characteristics of the ground motion from the employment of the microtremor observation [5]. it has been applied to weak and strong motions from earthquakes [1,19,20].The HVSR is defined as the ratio between the horizontal Fourier amplitude of ground motion, Hs, and the vertical Fourier amplitude of ground motion, Vs, at the free surface: HVSR 1⁄4 HS
We compared HVSR and TF for the two sites (GVDA and IWTH25), and proved that their differences are based on vertical site responses, through theoretical solution of wave propagation in elastic layered site and synthetic accelerograms, which revealed that the vertical amplification caused by the vertical incident P-wave is probably the main reason for the vertical site responses

Summary

Introduction

The evaluation of site-effects due to local geology or topography has become a standard requirement in microzonation studies or site evaluation for important facilities [1]. Many empirical methods such as the standard spectral ratio method [2],the linear inversion method [3],the reference event method [4],and the HVSR method [5] are used to identify site characteristics.Among these methods, the HVSR is the spectral ratio technique using records of only one station. As Bonnefoy-Claudet and coworkers pointed out, ambient vibrations sources are (1) controlled by local surface sources, and (2) caused by the ellipticity of fundamental Rayleigh waves. The peak frequency of HVSR under ambient vibrations can be controlled

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