S U M M A R YThe damage zone of a major fault can act as a low-velocity seismic waveguide. The fault-zone guided waves provide a potential method to constrain the in situ physical properties of the fault zone (FZ) at depth. Recently, there has been debate over the depth extent of observed fault waveguides and whether fault properties at seismogenic depth can be constrained by guided waves (GWs). To address these questions, elastic finite-difference synthetic seismograms were generated for fault-zone models that include an increase in seismic velocity with depth both inside and outside the FZ. Previous synthetic studies for a homogeneous fault showed that earthquakes off of the fault do not generate GWs unless the waveguide is restricted to a few kilometres depth. In contrast, earthquakes both inside and outside of a depth-varying fault waveguide generate strong GWs within the near-surface portion of the FZ. This is because the frequency-dependent trapping efficiency of the waveguide changes with depth. The nearsurface fault structure efficiently guides waves at lower frequencies than the deeper FZ. The low-frequency waves that are guided at the surface are not efficiently guided at greater depth, and therefore, travel as body waves. Fault structure at seismogenic depth requires the analysis of data at higher frequencies than the GWs that dominate at the surface and have been the subject of most previous investigations.

S U M M A R YSeismic guided wave dispersion can be used to image fault-zone structure at seismogenic depth. A two-station differential group velocity technique previously used for surface waves was adapted to solve for local fault-zone structure between two stations. This method was extended to solve for fault-zone structure between two earthquakes using differential group arrival times at a single station. The method was tested with finite-difference synthetic data for an inhomogeneous fault, as well as with a pair of shallow earthquakes recorded in the San Andreas Fault Observatory at Depth (SAFOD) borehole station. Results from a pair of deep earthquakes recorded in the SAFOD borehole station indicate that the low-velocity waveguide of the San Andreas Fault extends to >10 km depth. The waveguide at 10-12 km depth is 120-190 m wide and the velocity contrast is >20 per cent, similar to the values in the shallow subsurface. Multiple earthquakes and receivers could be used to map fault zone structure at seismogenic depth as a function of depth and strike.

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