Depth Extent of the Fault Zone Wave Guide: Effects of Fault Variation With Depth

Deformation in major fault zones creates a low-velocity zone that can trap or guide seismic waves. Previous computer modeling has indicated that sources off the fault should not generate fault zone guided waves unless they are beyond the end of the wave guide. However, guided waves have recently been observed from off-fault earthquakes. In addition, guided wave delay times have not always systematically increased with distance between source and seismometer. These observations have been cited as evidence that the fault zone waveguide does not extend beyond a few kilometers in depth. Previous computer modeling has usually assumed a homogenous fault. However, stress-induced closure of porosity causes seismic velocity both outside and inside the fault to increase substantially with depth. This velocity increase will cause the dominant frequency of the waveguide to increase. Using reasonable geological models for major faults in California, the dominant frequency might change from ~5 Hz to perhaps greater than 20 Hz. Few studies have investigated guided waves with a broad range of frequencies on a single fault. The evidence for lack of a deep waveguide at low frequency may not indicate that the fault zone waveguide disappears at depth, but rather that its dominant frequency changes with depth. We have computed for sources in and out of the fault finite-difference synthetic seismic guided waves for a shallow fault waveguide and for a deep fault waveguide whose properties change realistically with depth. The results show a frequency dependence of guided waves caused by fault zone variation with depth. This may shed light on the current controversy over the depth extent of the low velocity fault zone.