Ground motion at mountains and sedimentary basins with vertical seismic velocity gradient

Abstract

SUMMARY The boundary integral-Gaussian beam method (Benites & Aki 1989) is applied to study the ground motion in 2-D structures that exhibit irregular topography and interface, and whose shear wave velocity varies linearly with depth, for incident plane SH waves. In our first example of application the model is a half-space whose free-surface topography is a ridge of cosine shape, with vertical shear wave velocity gradient. In the second, the model is a semi-cylindrical sedimentary basin in a homogeneous half-space, in which the shear wave velocity of the sediments increases with depth. Our results for the case of the mountain show that the amplification on its top, predicted by the 2-D modelling when the velocity is constant, is enhanced when the velocity gradient is present, for all frequencies and by a factor up to 3. In the case of the basin, results show that the velocity gradient; (1) enhances the amplification at the edges of the valley, (2) makes the reverberations due to 2-D resonance have larger amplitudes and shorter intervals between arrivals, (3) shortens the total duration of the seismograms at all stations within the basin.