Seismic response of alpine valley composite site under near-fault velocity pulse waves excitation

This study investigates the time-domain scattering characteristics of near-fault velocity pulse waves in alpine valley composite sites using the scaled boundary finite element method (SBFEM). A coupled near-field/far-field computational framework has been established, where the finite near-field region, characterized by semi-sinusoidal topography, is discretized using quadtree mesh generation. The unbounded far-field is accurately modeled through displacement unit-impulse response functions. Representative velocity pulses, extracted from the San Fernando, Northridge, and Chi-Chi earthquake records via the Hilbert-Huang transform, are converted into equivalent nodal loads at the computational interface to ensure physically consistent wave input. Parametric studies reveal that the depth of valley incision significantly influences velocity pulse wave scattering, reducing peak ground acceleration at the valley bottom by 85%–90% due to wave diffraction and shielding mechanisms. Variations in mountain height substantially amplify vertical displacements under non-velocity pulse waves, producing responses approximately 2.4 times greater than those induced by velocity pulses. The scattering of velocity pulse waves is particularly sensitive to incident angles, with peak ground acceleration amplification on the rear mountain slope exceeding that on the front slope by about 65%, highlighting directional amplification effects. These findings provide a theoretical foundation for evaluating and designing seismic safety in near-fault alpine valley terrain.