Seismic Response of 20-Valleys: Local Site Effects

Layering and geometry effects on ground response of two-dimensional sedimentary valleys under oblique seismic excitation are investigated. The seismic wave has the shape of a half cycle incoming pulse with SV-wave characteristics. Vertical and oblique incoming signals of varying duration are used. The analysis is performed using finite element techniques, an equivalent effective force method to prescribe the free field motion within the domain of computation, and an artificial boundary to absorb the scattered motion. Simple examples are presented confirming the validity of this methodology. It is shown that surface waves generated at the valley edges propagate through the basin producing an increased ground response. Horizontal and vertical displacements are affected significantly by valley geometry, particularly by the inclination of the valley sides. Layering and inclined waves contribute to producing amplification and very long duration of ground motion