Three-dimensional basin-mountain coupling effects on ground motion amplification in sedimentary basins

The seismic response of intermontane basins may differ markedly from that of isolated basins due to complex wave scattering arising from interactions between the basin structure and the surrounding topography. This study investigates three-dimensional seismic wave propagation and ground motion amplification of basin-mountain systems, using a multi-domain boundary element method. Parametric simulations are performed for basin-only, mountain-only, and coupled models, with systematic variations in mountain height and basin depth, and with corresponding 2D cross-sectional simulations for comparison. The results highlight a frequency-dependent basin-mountain coupling that significantly affects ground motion amplification within the basin. Although the sedimentary basin is the primary source of amplification, the surrounding topography can either suppress or intensify ground motion within the basin depending on frequency and geometry, while larger amplification tends to concentrate in the mountain area for higher frequencies and steeper mountains. Moreover, the results indicate a resonance-like response governed by wavelength-geometry commensurability, which produces exceptionally large amplification for a specific geometric configuration of the basin-mountain system, underscoring the importance of explicitly modelling the three-dimensional geometric coupling. Comparisons further suggest that 2D cross-sectional models may fail to reproduce essential three-dimensional wavefield features and consequently misestimate seismic demand in both amplitude and spatial distribution.