Nonergodic Path Effects from 3D Ground-Motion Simulations of an M 8 Ryukyu Earthquake in Eastern Taiwan

The Ryukyu subduction zone offshore eastern Taiwan possesses significant seismogenic potential, exemplified by the 1920 earthquake. However, the scarcity of near-field observations leaves the ground-motion characteristics of such mega-earthquakes poorly constrained, limiting the reliability of seismic hazard assessments. This study investigates long-period ground-motion characteristics of a potential M 8 Ryukyu earthquake using large-scale 3D ground-motion simulations. We simulated an M 8 scenario earthquake using characterized source models based on the “Recipe” procedure and conducted 3D finite-difference simulations for 192 source–velocity-model combinations, incorporating variations in hypocenter location, rupture velocity, asperity distribution, two tomographic velocity models, and topography. Varying the Gaussian source time function (STF) duration in postprocessing expands the dataset to 1728 scenarios. Synthetic waveforms were generated at 4950 virtual stations, producing more than 8.5 million synthetic ground-motion records, and analyzed using RotD50 spectral accelerations (SAs) at periods of 1.0, 3.0, and 5.0 s. Results indicate that rupture directivity and rupture velocity dominate the spatial variability of long-period ground motions, with parameter sensitivity exhibiting strong spatial nonuniformity across the study region. Calibration against the empirical CH20 ground-motion model (GMM) suggests Gaussian STFs of 2, 6, and 8 s provide the most consistent spectral representation for SA at 1.0, 3.0, and 5.0 s, respectively. The nonergodic path terms (δP2P) derived from two alternative velocity models, which capture the path effects of the deep 3D structure (VS≥2.4 km/s), reveal spatially consistent features, including a persistent amplification zone near Hualien linked to the Longitudinal Valley low-velocity structure, a deamplification belt along the Coastal Range and Central Range high-velocity material, and a frequency-dependent amplification in the Ilan Plain attributable to deeper crustal focusing or trapping rather than near-surface site effects. The workflow established here provides a transferable framework for refining GMMs and advancing nonergodic seismic hazard assessments in data-sparse subduction environments.