Effect of a simple mountain range on underground seismic motion

Abstract

Summary. The influence of a simple mountain range on seismic ground motion is studied. A two-dimensional model of the medium and vertically incident plane SH-waves are considered. Attention is devoted not only lo the wavefield along the Earth’s surface, but also within the medium. The wavefield is computed in two steps: (I) the computation of the impulse response by the finite-difference method, (2) the computation of the response to a time variation of the incident wave. approximately corresponding to a ‘hundred-yeai-’ local earthquake a( the sitc. Numerical results (the impulse 1-esponse. the transfer function, the accelerograms, and their spectra) indicate strong spatial variability of the wavefield due to the topographical anomaly. The differences as large as 100 per cent in the peak amplitudes of the accelerograms, and of the order of 5 Hz in their predominant frequencies, at closely (- 200 m) spaccd internal points of the medium have been found. Attention is focused also on the effect of causal absorption. Even unrealistically strong absorption seems to be unable to reduce significantly the spatial variability of the ground niotion. caused by the topography. A variability like this. implying the occurrence of underground differential strains, might be of engineering inipoi-lance in the antiseismic desigti of underground structures (tunnels, for example) in mountainous seismic I-egions. The groundmotion variability along the surface of the mountain is considerably smaller than within the medium.