Three-dimensional Scattering Of Obliquely Incident Plane Waves By An Alluvial Valley In A Layered Half-space

It has been fully verified that a depositional valley has significant effects on the seismic ground motion as a local site. Site response analysis of valley, which is one of the most important local sites, has been an attractive research topic, and has an important theoretical meaning.The indirect boundary element method (IBME) is used to solve the frequency domain solution of 3D scattering of incident plane waves by a depositional valley in a layered half-space, analyze the differences between the case of 2D wave scattering and that of 3D, and discuss the effect of the incidence angle on surface displacement and the effects of the stiffness and the thickness of the layer on the displacement by taking an alluvial valley in one layer over half-space as an example. The paper also presents the scattering of incident Ricker wave by an depositional valley in time domain by the inverse Fourier transform on the basis of frequency domain, and analyses the time domain scattering effect of horizontal incidence angle, stiffness and thickness of soil .It is shown that there is essential difference between the case of 3D wave scattering and that of 2D. So we can’t simply make the incidence plane waves into the decomposition of inplane and antiplane in the cross-section of the valley, then calculate and superpose respectively. The amplification depends on horizontal incidence angle, stiffness and thickness of soil greatly. As the angle between incident direction and axis of the valley increases gradually, displacement of horizontal and vertical direction changes obviously. Stiffness ratio of the rock and soil has a great influence on the displacement amplitude near the valley, but affects the spatial distribution less. Thickness of soil not only affects the surface displacement amplitudes, but changes the surface displacement of the spatial distribution. In the time domain scattering effect, vibration duration within the deposition is longer than the deposition outside, and horizontal incident angle, stiffness and thickness of soil layer have obvious effect on the seismic ground motion.