EFFECTS OF IRREGULAR BOUNDARIES IN A LAYERED HALF-SPACE ON SEISMIC-WAVES

Seismic waves through a layered half-space with geometrically irregula r boundaries (surface and/or interfaces) are investigated. Wave scatte ring occurs at these irregular boundaries, resulting in coupling betwe en P-SV and SH waves, which is not the case in a perfectly layered hal f-space. A first order perturbation technique is applied to solve such a three-dimensional wave scattering problem. Specifically, the total wave field, generated by a seismic dislocation source buried in the la yered half-space, is decomposed into two wave fields. One is a mean wa ve field, which is response field in a perfectly layered half-space su bjected to a seismic dislocation source. This can be solved using a re flectivity method; i.e., an approach combining integral transform and wave propagation analysis in terms of reflection and transmission prop erties of portions of the layers. The other is a scattered wave field, which is due to the existence of irregular boundaries. The effects of the irregular boundaries on the scattered wave held are equivalent to those of fictitious discontinuity sources acting on the corresponding perfectly plane boundaries. The intensity of the fictitious discontin uity source depends on both the mean wave held and the irregularities. The solution for the scattered wave field is then obtained using the same approach as for the mean wave response held. The effects of the i rregular boundaries on the seismic wave responses, especially on the g round motion responses, are evaluated qualitatively and quantitatively . This is fundamentally important to the issue that whether the irregu lar boundaries can or cannot be approximately considered as flat ones, which is associated with the simplification of a complex model for th e real earth media subjected to the seismic waves. Numerical examples are presented for illustration. (C) 1996 Academic Press Limited