Imaging the Fault Damage Zone of the San Jacinto Fault Near Anza With Ambient Noise Tomography Using a Dense Nodal Array

We apply the double-beamforming tomography to a monthlong temporary dense seismic array to obtain high-resolution images of the San Jacinto Fault's damage zone. We obtain Rayleigh waves between 0.3- and 0.8-s periods via vertical-vertical noise cross correlation, apply double beamforming to obtain phase velocities, and apply a piecewise 1-D least squares inversion to obtain shear velocities in the top 300 m. We observe a similar to 200-m-wide low-velocity zone that narrows with depth, which we interpret as the main damage zone in addition to two other similar to 100-m-wide subsidiary zones corresponding to secondary damaged structures, agreeing with the distribution of fault zone trapped waves produced by local earthquakes. The primary damage zone asymmetry indicates that materials on the northeast side of the fault are stiffer at seismogenic depth and suggests that large San Jacinto earthquakes tend to nucleate to the southeast and propagate to the northwest.