Full-3D Waveform Tomography for the Seismic Velocity and Attenuation Structure on the San Andreas Fault Zone in Parkfield Area

We are conducting time-lapse full-3D waveform tomography for the seismic velocity and attenuation structure on the San Andreas Fault zone in the Parkfield area. The seismic waveform data we are using come from several Parkfield seismic networks, including the borehole seismographs installed in the SAFOD mainhole and pilot hole, the HRSN stations, an array of 45 portable seismographs deployed across and along the surface trace of the San Andreas Fault near SAFOD site in fall 2003 as part of a site characterization program and an array of 45 portable seismographs deployed in 2002 and 2004 near the town of Parkfield. The tomography technique we are employing for this study is the full-3D waveform tomography (F3DT) method based on the scattering-integral (SI) formulation. In our F3DT algorithm, both the reference structural model and the derived model perturbations are all 3D in space and the full-wave sensitivity kernels are calculated from the full physics of 3D wave propagation, which is implemented using the finite-difference method. In this study, we are conducting time-lapse, joint inversions for both seismic velocity and attenuation structures of the San Andreas Fault Zone using waveform recordings from repeated explosions and earthquakes. Intrinsic attenuation estimates are usually derived from frequency-dependent amplitude measurements, which are also strongly dependent on elastic effects such as focusing/defocusing and 3D scattering. By jointly inverting for seismic velocity along with Q in a fully 3D setting, we can remove much of the uncertainty associated with focusing/defocusing and 3D scattering. About 60 percent of our recorded earthquakes are members of nearly 200 event clusters of repeating, virtually identical earthquakes exhibiting very small source dimensions. These widely distributed repeating micro-earthquakes provide us the opportunity to detect transient or systematic temporal changes of physical properties within the fault-zone through time-lapse tomography.