3-D Seismic Wave Simulations for Non-Ergodic Ground Motion Modeling: Source and Path Variability in an M8 Ryukyu Subduction Scenario

The Ryukyu subduction zone offshore eastern Taiwan possesses significant seismogenic potential, exemplified by the 1920 M8 earthquake. However, even to date, the scarcity of near-field data leaves the ground motion characteristics of such mega-earthquakes poorly constrained, posing a threat to seismic hazard assessments. To estimate potential ground motions in inland eastern Taiwan from future mega-earthquakes, we simulated an M8 scenario earthquake using characterized source models (CSMs) based on the "Recipe" procedure (Irikura and Miyake, 2011). We employed a 3-D finite-difference method to conduct 1,728 full-waveform simulations, incorporating kinematic fault-rupture parameters, including rupture directivity, rupture speed, source time function, and asperity distribution, along with two recent tomographic velocity models and topography. Synthetic waveforms generated at 4,950 virtual stations (about 1.5 km spacing) were analyzed using RotD50 spectral accelerations (SA) at 1, 3, and 5 s. Detailed analysis highlights two notable characteristics of the dataset: first, rupture speed and directivity primarily govern the spatial variability and intensity of ground motions; second, tests demonstrate that utilizing a Gaussian source time function with periods of 2, 5, and 9 s yields optimal performance for assessing SA at 1.0, 3.0, and 5.0 s, respectively. We further calculated non-ergodic terms based on the CH20 GMM (Chao et al., 2020). The patterns clearly delineate northeastern Taiwan's geological domains: high values in the Ilan area (SA 1.0 s) and Longitudinal Valley (SA 1.0, 3.0, 5.0 s), and low values in the Coastal Range. These patterns mirror the crustal velocity structure, highlighting the dominance of path effects over relatively weak source effects. Consequently, our extensive simulation datasets provide a foundation for refining current GMMs and facilitate the transition toward non-ergodic seismic hazard assessments, thereby improving the accuracy of ground motion predictions for future mega-earthquake scenarios in the region.