Recent observations have shown that shear waves trapped within low-velocity fault zones may be the most sensitive measure of fault-zone structure (Li et al., 1994a, 1994b). Finite-difference simulations demonstrate the effects of several types of complexity on observations of fault-zone trapped waves. Overlying sediments with a thickness more than one or two fault-zone widths and fault-zone step-overs more than one or two fault widths disrupt the wave guide. Fault kinks and changes in fault-zone width with depth leave readily observable trapped waves. We also demonstrate the effects of decreased trapped wave excitation with increasing hypocentral offset from the fault and the effects of varying the contrast between the velocity in the fault zone and surrounding hard rock. Careful field studies may provide dramatic improvements in our knowledge of fault-zone structure.