Ground‐Motion Models Incorporating Nonergodic Effects from 3D Numerical Simulations in Japan

We analyzed scenario ground‐motion simulations from the Miyagi, Kanto, Nobi, and Osaka regions to understand the spatial distribution of nonergodic site terms for ground motions of crustal events. We selected one of the Japan regional ground‐motion models (GMMs) as the reference ergodic model and estimated adjustment terms based on the 3D simulation results to develop a nonergodic ground‐motion model (NGMM). First, a $Z2.1$ basin depth term was constructed from the 3D simulations, denoted $fbasin3d$⁠, and this term was applied to the Morikawa and Fujiwara (2013; hereafter, MF13v1) GMM. The basin terms from the simulations show stronger scaling with the basin depth compared to empirical ground‐motion data from K‐NET and KiK‐net. Second, the spatial distribution of the nonergodic site term from the simulations, denoted $δS2SNE$⁠, was estimated by applying the varying coefficient model within the region covered by the simulations. The net adjustment to the MF13v1 GMM is the sum of $fbasin3d$ and $δS2SNE$⁠. The nonergodic aleatory variability for the NGMM is reduced by 10%–20% compared to the ergodic GMM. The NGMM not only changes the average level of ground motion compared to the ergodic GMM but also modifies the spatial distribution of the nonergodic term and the aleatory variability. In all study regions, the nonergodic site terms derived from the 3D simulation are found to be within the epistemic uncertainty range of the site terms obtained from empirical data. This indicates that integrating simulation results into the GMM can potentially constrain the prediction in regions where empirical data are insufficient. In addition, lacking simulation results for a suite of alternative 3D velocity models to address epistemic uncertainty, we assume it is half the standard deviation of the nonergodic site terms in the region (0.5 $ϕS2S$⁠).