Spatial distribution of average shear wave velocity and seismic amplification from a DEM-derived topographic slope: A case study from Nepal

Nepal is one of the most seismically active nations worldwide. However, systematic characterization of near-surface seismic site conditions in this region remains limited. In this study, seismic site responses were assessed across Nepal using a DEM-derived slope to estimate the average shear wave velocity of the upper 30 m (v_S30) and associated soil amplification. DEM data were downsampled to optimize computational efficiency while preserving physiographic variability. v_S30 was derived using slope-based empirical correlations and classified into NEHRP site classes (B–E). Soil amplification factors were also estimated from v_S30, and uncertainty was quantified using Monte Carlo simulations and bootstrap resampling. The results show a clear N–S gradient in near-surface stiffness, with the mountain region exhibiting the highest v_S30 (≈ 281 m/s), followed by the Himalayan regions (183–213 m/s), and the Terai Plains (≈187 m/s). NEHRP classes D and E dominate, highlighting the prevalence of soft soils and their high amplification potential, particularly in the Terai and intermontane basins. A strong inverse relationship between soil amplification and v_S30 was observed, with valleys and low slopes consistently showing elevated amplification (r = 0.802–0.973). Uncertainty analysis indicates regional contrasts, with the Himalayan regions showing low v_S30 errors but high amplification uncertainty, whereas the Terai Plains exhibits higher uncertainty but relatively stable amplification estimates. These findings underscore the importance of incorporating slope-derived v_S30 and soil amplification into regional seismic hazard assessments, microzonation, and urban planning, as well as the need to collect site-specific geophysical measurements in complex terrains and urban areas to refine seismic hazard evaluations.