The scattering of SH waves by an alluvial valley of arbitrary shape in a layered half-space is modeled using the indirect boundary element method（IBEM） in the frequency domain.The Green’s functions of displacements and stresses are calculated by using the exact dynamicstiffness matrix of soil layers.Based on IBEM

the effects of width and thickness of the alluvial valley

and the incident angle and frequency on the surface displacements are studied.These results are compared with that of one-dimensional model.The numerical results indicate: The two-dimensional scattering effect of alluvial valley is significant and quite different from that of one-dimensional model.The influence of alluvial valley on determination of design earthquake ground motion parameters should be considered reasonably in evaluation of seismic safety for engineering site.