Depth extent of the fault-zone seismic waveguide: Effects of increasing velocity with depth

Abstract

The 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 near-surface 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. © 2008 The Authors Journal compilation © 2008 RAS.