Wave Diffraction In Multilayered Media With the Indirect Boundary Element Method: Application to 3-D Diffraction of Long-Period Surface Waves By 2-D Lithospheric Structures

Abstract

SUMMARY An indirect boundary-element method is presented for simulating seismic wave diffraction in layered media with irregular interfaces. Recursive formulae are developed to take into account an arbitrary number of layers without increasing the size of the problem in terms of computer memory. the interfaces between layers are of variable geometry, and interfaces can be superposed to introduce horizontally finite structures. such as lenses or sedimentary basins. In the present implementation, we study three-dimensional (3-D) diffraction of plane waves by two-dimensional (2-D) structures. the formulation is nevertheless sufficiently general to include the complete 3-D case. Even though the method can be used at all scales, the main purpose of the theoretical development is to simulate diffraction of long-period surface waves by heterogeneous lithospheric structures. A new approach to treat incident surface waves in multilayered media is therefore developed, but other wave types, such as body-waves and internal seismic sources, can easily be introduced. the method is verified by transparency tests and comparison with other simulation methods. the application on the 3-D diffraction of plane Rayleigh waves by a major lithospheric boundary shows that significant conversions between wave types are present, and that the diffracted waves influence the apparent phase velocities measured at the surface above the heterogeneity and several wavelengths behind it.