Ray-Theory Based Analysis of the P/S Ratio Behavior Within Vertical Ground Motions

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Vertical ground motions have been traditionally neglected in the seismic-hazard analysis because they were believed to have minor effects on civil structures. Specifically, the vertical site-response and the physical parameters which affect it are still poorly understood. In recently-published vertical GMPEs, the site-response component is represented only by SV dependent parameter (i.e. Vs30). However, several studies have shown that - depending on magnitude, distance, and other site conditions, the P-wave contribution to the vertical component of ground motion, specifically at high frequencies, could be as large as that of the SV-wave or even larger. In this study, we hypothesize that the site-response component in vertical GMPEs can be better estimated by combining P- and S- related site parameters. However, such combination can only be obtained once their relative contribution to the vertical ground motion is properly understood and well-defined. This topic has not been fully explored before and hence represents a significant knowledge-gap in the current ability to characterize vertical site-response.

In order to examine the relative contribution of P- and SV- waves on the vertical ground motion component, an analytical algorithm was developed. It allows us to calculate and present all the possible direct wave-fronts that leave a point-source and meet at a single recording point. The calculation takes into account the geometrical setting as well as the frequency-dependent attenuation and is a function of a number of independent variables, such as the thickness of the overlying soil layer, the velocity ratio, and the source-to-site distance. The computed data allows us to establish the pure separated P and S amplitudes and their ratio in the frequency domain as a function of specific physical properties. This study is expected to facilitate an improved understanding of the relative contribution of P- and S- wave types to the vertical ground motion.

Presenting Author: Michael Frid

Authors