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Abstract

The geometry and material properties of sedimentary basins significantly enhance the amplitude and duration of seismic waves through frequency-dependent focussing, trapping and resonance. In addition, the regional surface topography can cause trapping and scattering of seismic waves, thus altering their amplitude and frequency content. Nevertheless, the influence of surrounding surface topography is hard to constrain, and mostly not included, in the Ground Motion Prediction Equations (GMPEs) for the basins. This oversight significantly impacts risk assessment in many densely populated intermontane basins, for example, Kathmandu, Kashmir and Dehradun basins in the Himalayan ranges. We use physics-based simulations to examine the wave propagation characteristics within synthetic sedimentary basins surrounded by realistic topographies, similar to the ones in the Himalayan region. For various thrust faulting seismic sources, the basin geometry (depth) and topographic features (width and average slope) are varied and the ground accelerations are calculated for the frequency range of 0.1 to 10 Hz. Results suggest, in the near-field, the proximity and dominant wavelength of the seismic source primarily control the amplification observed on the basin. In far-field, the topographic geometry plays a characteristic role by trapping higher frequencies, leading to significant damping in basin locations. A novel empirical proxy is developed using the ratio of basin depth to the surrounding topographic widths that enhance GMPEs. The residuals of the predicted intensities by GMPEs and observed data are smaller than those by the existing GMPEs, highlighting the significant role of the derived topographic factor that can be applied to intermontane basins worldwide.