San Andreas Fault damage at SAFOD viewed with fault‐guided waves

Highly damaged rocks within the San Andreas fault zone at Parkfield form a low‐velocity waveguide for seismic waves, giving rise to fault‐guided waves. Prominent fault‐guided waves have been observed at the San Andreas Fault Observatory at Depth (SAFOD) site, including a surface array across the fault zone and a borehole seismograph placed in the SAFOD well at a depth of ∼2.7 km below ground. The resulting observations are modeled here using 3‐D finite‐difference methods. To fit the amplitude, frequency, and travel‐time characteristics of the data, the models require a downward tapering, 30–40‐m wide fault‐core embedded in a 100–200‐m wide jacket. Compared with the intact wall rocks, the core velocities are reduced by ∼40% and jacket velocities by ∼25%. Based on the depths of earthquakes generating guided waves, we estimate that the low‐velocity waveguide along the fault at SAFOD extends at least to depths of ∼7 km, more than twice the depth reported in pervious studies.