SIMULATION OF SH-WAVE AND P-SV-WAVE PROPAGATION IN FAULT ZONES

Seismic fault-zone (FZ) trapped waves provide a potentially high-resol ution means for investigating FZ and earthquake properties. Seismic wa ves emitted within and travelling along low-velocity FZ layers may pro pagate many kilometres within the low-velocity structure associated wi th the fault. Waveform observation of FZ trapped waves can be modelled in terms of FZ layer velocities, thicknesses and attenuation coeffici ents. This can greatly improve the resolution of imaged FZ structure a nd microearthquake locations. At present, broad-band theoretical seism ograms are restricted to plane-parallel layers of uniform properties, However, it is not clear how realistic these models are compared with actual fault structures which could, for example, flare outwards near the surface, have irregular boundaries, interior heterogeneities, etc. To address these interpretational uncertainties, we perform finite-di fference simulations for irregular FZ geometries and non-uniform mater ial properties within the layers. The accuracy of the numerical soluti ons are verified by comparison with the analytical solution of Ben-Zio n & Aki (1990) for plane-parallel structures. Our main findings are: ( 1) FZs can widen at the crustal surface only slightly modifying the tr apped waves; (2) velocity variations with depth destroy trapped wave p ropagation at all wavelengths; (3) FZ trapped waves can be obscured by the presence of a low-velocity surface layer; (4) models with short-s cale random structures suggest that trapped waves average out irregula r FZ geometries, and hence can be effectively modelled by average-prop erty plane-layered media for the observed range of wavelengths.