Surface wave caustics

The geometrical theory of surface wave propagation on a laterally heterogeneous earth model diverges at caustics, where neighbouring rays cross. The caustics on a spherical earth are degenerate focal points at the source and its antipode; lateral heterogeneity removes this degeneracy and transforms the caustics into multiply cusped and folded curves. We investigate the geometric nature of these antipodal and source caustics using both linear ray-perturbation theory and exact ray tracing. The rcgions occupied by the R1-R2 and R2-R3 caustics extend as far as 20 " from the antipode and 30 " from source. respectively, even on a relatively smooth earth model such as M84A. Key words: caustics, lateral heterogeneity, surface waves.