Three-dimensional calculations of fault-zone-guided waves in various irregular structures

S U M M A R Y The detailed structure of fault zones (FZs) plays an important role in problems related to fault mechanics, earthquake rupture, wave propagation and seismic hazard. FZs are thought to consist of an O(10–100) m wide region of decreased seismic velocity but structural details such as their depth extent, lateral and vertical variations, etc. are elusive. The small spatial scales involved make such structures difficult to image with ray-theoretical methods such as tomography. However, seismic energy trapped inside FZ layers can provide dispersive wave trains that carry information on the FZ structure. These waves can travel many kilometres inside the FZ before reaching the surface and are therefore strongly altered by its properties. Candidate trapped waves have been observed above several active faults. Inversion algorithms exist that can model these observations in terms of planar fault zone structures. However, at present it is not clear how reliable these estimates are, as the effects of (even small) 3-D variations on trapped waves are not well understood. The goal of this study is to distinguish 3-D structures that do and do not significantly affect FZ waves. To achieve this, we perform numerical calculations of wave propagation in various FZ geometries and analyse the waveforms, spectra and envelopes of the synthetic seismograms. The main results are that (1) moderate changes of the shape of FZ or (2) small-scale heterogeneities or (3) depth-dependent properties do not strongly affect the observed FZ waves. In contrast, strong effects are to be expected from (4) breaks in the continuity of FZ structure (e.g. offsets), which may at some point allow imaging of such features at depth.