Nonergodic Ground-Motion Model for Vertical Response Spectra Using Offshore Ground Motions from Subduction Earthquakes near the Japan Trench Area

ABSTRACT An ergodic ground-motion model (EGMM) was developed for the vertical response spectra of offshore ground motions from subduction earthquakes near the Japan trench area. Subsequently, a nonergodic ground-motion model (NGMM) was created using the EGMM as a foundation to account for spatially varying source, site, and path effects and presented prediction values of NGMM in three directions under an assumed earthquake scenario. Both models are effectively applicable for predicting offshore vertical ground motions from subduction earthquakes with moment magnitudes ranging from 4.0 to 7.4, focal depths of less than 103 km, and rupture distances under 300 km. The EGMM visualization demonstrates that seawater significantly influences offshore vertical ground motions, especially for surface stations. The distribution patterns of these spatially varying effects align with the findings of Hu et al. (2023). This consistency indicates that spatially varying effects within a region remain relatively stable, regardless of whether they pertain to vertical or horizontal components. In addition, the spatially varying terms approach zero with large epistemic uncertainty in regions with sparse data. We compared the NGMM across three directions with the EGMM and onshore GMMs from Japan and found a significant difference between offshore and onshore GMMs, particularly for long-period and surface stations. The amplitude and attenuation of the NGMM in three directions varied depending on the propagation direction. The standard deviations of the NGMM in these three directions showed a high dependence on the availability of data for each direction. The total aleatory standard deviation of the NGMM was reduced by 22%–44% compared to the EGMM. This reduction in aleatory standard deviation, combined with the NGMM's predicted values, can have a significant impact on seismic hazard calculations for the Japan trench area.