Seismic Amplification by Deep Subsurface and Proposal of a New Proxy

T Nagao

doi:10.48084/etasr.3276

Abstract

Codes of practice and ground motion prediction equations involve ground structure proxies to account for seismic amplification. Although the ground consists of both shallow and deep subsurface, proxies are mainly related to the shallow subsurface as it is shallow subsurface information that is mostly available. However, as deep subsurface seismic amplification is not negligible, it may not be appropriate to use shallow subsurface proxies. In this study, the relationship between shallow and deep subsurface seismic amplification factors is discussed on the basis of S-wave velocity profile data from Japanese KiK-net strong-motion observation system stations. The correlation between typical proxies such as the average S-wave velocity of the top 30m of the ground surface and the seismic amplification factor was examined. Although there was a negative correlation between the two, the degree of the correlation was weak. A new proxy showing stronger correlations with the seismic amplification factor is proposed and its effectiveness is demonstrated.

Highlights

Earthquake ground motions are amplified as they propagate through the ground, differences in ground characteristics are considered in design earthquake ground motions and ground motion prediction equations (GMPEs)
It was shown that the assessment of deep subsurface seismic amplification is crucial for properly evaluating earthquake ground motions and GMPEs
The deep subsurface to shallow subsurface seismic amplification ratios have a mean of 2.39 and a standard deviation of 0.74, and seismic amplification considering only the shallow subsurface underestimates the seismic amplification of the whole ground

Summary

Introduction

Earthquake ground motions are amplified as they propagate through the ground, differences in ground characteristics are considered in design earthquake ground motions and ground motion prediction equations (GMPEs). Vs30 can be calculated from ground data without seismic response analysis and a correlation between Vs30 and seismic amplification has been noted [2, 3]. For this reason, discussions on the appropriateness of using Vs30 as a proxy continue [4, 5], Vs30 is widely used at present and has been introduced as a proxy in National Earthquake Hazards Reduction program (NEHRP) [6], AASHTO [7], Eurocode8 [8], and various other models [9, 10] and GMPEs [11, 12]. Note that the ground’s natural period is used as a proxy in Japanese codes of practice [13, 14]

Methods

Results

Conclusion