Ground Motion Prediction Equations for the Vertical Component from Subduction Interface Earthquakes in Japan Using Site Classes as the Site Term

ABSTRACT A large dataset from earthquakes up to the end of 2012 with a reliable earthquake tectonic category was used to develop a ground motion prediction equation (GMPE) for the vertical strong motions from the subduction interface earthquakes in Japan. Guided by the increase in the maximum log likelihood of the regression model, a minor extension on the maximum usable period was applied so that more records can be used at long spectral periods. A bilinear magnitude scaling function hinged at a moment magnitude (Mw) of 7.1 was adopted, and the scaling rate for events with Mw>7.1 is less than that for the other events. Site period-based site classes were used as the site parameter without a nonlinear site term. We used two datasets with the first dataset including strong-motion records from large events at stations with inferred site classes, and these records were removed in the second dataset so that all sites have a measured shear wave velocity profile down to the engineering bedrock. We found that excluding these records from sites with inferred site classes improved the model fit significantly. We modeled the effect of volcanic zones using the slant distance passing a set of volcanic zones. We also found that the between-site standard deviations for the vertical component in this study were generally less than those for the horizontal component at nearly all spectral periods. The total model standard deviations from this study are generally less than or similar to those from the GMPEs for the vertical ground motions for other earthquake categories. The distributions of between-site residuals with site period or TVS30 (120/VS30 in m/s) are not significantly biased, suggesting that the site term coefficients can be interpolated between that site class terms if a continuous predicted spectrum with site period or TVS30 is required.