STRUCTURAL HETEROGENEITY ALONG THE SAN ANDREAS FAULT AT PARKFIELD, CALIFORNIA, FROM SEISMIC IMAGING: VELOCITY, ANISOTROPY, AND ATTENUATION

Seismic imaging is an essential tool for extending surface and borehole observations of crustal heterogeneity into three dimensions. Borehole and surface site investigations carried out for the SAFOD project provide an unparalleled opportunity to examine the detailed structure of a fault zone, at multiple scales. Our long-term deployments of PASSCAL and USArray seismic instruments combined with data from the USGS Northern California and UC Berkeley HRSN networks, SAFOD borehole logs, borehole seismometers, and several active-source projects have yielded an extensive dataset for seismic imaging analysis, including seismic velocity, anisotropy, and attenuation. Not surprisingly, the three-dimensional heterogeneity of all three of these seismic parameters in the upper crustal volume surrounding SAFOD is extraordinarily complex. The widely variable seismic wave sampling of this volume motivated the development and application of an adaptive-mesh double-difference tomography technique to obtain higher-resolution structural information in the earthquake source regions. Key features are the high-velocity granitic rocks on the southwest side of the fault, a complex low-velocity zone beneath and southwest of the surface fault trace, and an extensive low-velocity zone overlying deeper bedrock on the northeast side. Properly accounting for the extreme seismic velocity heterogeneity at larger scales is crucial for unraveling the highly variable shear wave splitting observations. When projected to depth along the ray paths, average fast polarization directions fall mainly into either fault-parallel or fault-normal clusters, with localized transitions of oblique orientation. We also use the velocity model to back-project shear wave splitting delay times to crudely image zones responsible for the observed delays. Our attenuation (Q) tomography results provide further evidence for the strong heterogeneities surrounding SAFOD. The Qp and Qs models correlate reasonably well with the seismic velocity models, but they show no clear relationship to the strength of the inferred anisotropy.