Development of a simple offshore ground motion prediction equation based on S-net data and residual analysis to reveal site effects

Abstract

Ground motion prediction equations (GMPEs) in offshore regions are important for evaluating the durability of subsea structures and tsunami risk associated with seafloor slope failures. Since the ground conditions differ between onshore and offshore areas, it is desirable to develop a GMPE specific to the seafloor. Previous models have some problems, such as the influence of buried observation equipment and whether physical property that substitutes for seafloor conditions can take site effects. In this study, to predict the distribution of seafloor seismic acceleration, a new GMPE was regressed on the peak ground acceleration (PGA) data of S-net using minimum necessary seismic parameters as explanatory variables. The obtained model was compared with conventional onshore and offshore GMPEs and successfully predicted PGA with smaller errors. The residuals between the observed and predicted PGA were used to examine the factors responsible for the site effect of the S-net sites. The areas overestimated by GMPE correspond to areas where semi-to-consolidated sediments are exposed and do not correlate with sediment thickness obtained from seismic reflection surveys. The new GMPE can obtain PGA within 300 km of the epicenter from the moment magnitude (Mw 5.4–7.4), focal depth, and source distance considering the marine geology.