Adjoint?State Traveltime Tomography for Azimuthally Anisotropic Media and Insight Into the Crustal Structure of Central California Near Parkfield

Seismic anisotropy provides crucial information on the stress state and geodynamic processes inside Earth. We develop a novel adjoint-state traveltime tomography method using P-wave data to simultaneously determine velocity heterogeneity azimuthal of subsurface. First, an anisotropic eikonal equation is derived model first-arrival traveltimes in azimuthally media. Traveltime then formulated as optimization problem constrained by equation, which subsequently solved method. Ray tracing not required. Its high accuracy achieved solving associated adjoint with efficient numerical solvers. In addition, equation-based earthquake location for media developed solve coupled hypocenter-velocity problem. The methods are applied central California near Parkfield test their performance practice. A total 1,068,850 first clearly maps upper middle crust. average shows striking contrast across San Andreas Fault (SAF). crust, we find structural SAF zone stress-induced off zone. fast directions generally fault-parallel due decreased effect maximum horizontal compressive stress. all, real-data application suggests that new can be reliably used investigate seismic structures.