Linear and Nonlinear Valley Amplification Effects on Seismic Ground Motion

Abstract

Records and analyses have shown that, apart from soil stratigraphy, the geomorphic conditions (such as those characterising an alluvial valley) tend to modify the amplitude, the frequency content, the duration, and the spatial variability of seismic ground shaking. As most of the related records and studies to date refer to weak motions (and thereby to linear soil response), the question that has been raised is whether and by how much the unavoidably nonlinear soil behaviour during strong shaking may reduce the unavoidable “valley amplification” effects. The paper aims at shedding some light on this important issue by analysing numerically the effects of the sub-surface geomorphic conditions of a valley on its ground surface seismic motion, with emphasis on the influence of soil nonlinearity. Two-dimensional linear and equivalent-linear ground response analyses are performed to study an alluvial valley in Japan, the behaviour of which had been monitored during many earthquakes in the early 1980's. Then, using the geometry of this valley as a basis, a parametric investigation is performed on the effects of potential soil nonlinearity arising from the increased intensity of base excitation and/or decreased “plasticity” index* of the clayey soil material. It is shown that strong soil nonlinearity may depress the amplitude of the multiply-reflected and, especially of the horizontally propagating Rayleigh waves, leading to substantially lower valley amplification.