Seismic Structure and Anisotropy of the Crust Around Major Fault Zones in Northern California from the Harmonic Decomposition of Receiver Functions

Teleseismic P-wave receiver functions are useful in the characterization of layered structure in a wide variety of tectonic and geological settings. In the presence of anisotropy and/or structural heterogeneity, however, the inversion of receiver function data using isotropic velocity models and methods are inadequate. The effects of anisotropy and dipping structures are manifested as strong azimuthal variations in both amplitude and timing of converted phases of receiver functions. Combined with effects due to vertical layering, the various signals are difficult to untangle. In northern California, visual inspection of receiver function data suggests that crustal structure is either strongly anisotropic or contains dipping reflectors with strong velocity contrasts, in particular near major fault zones. We examine the velocity structure of northern California by decomposing the azimuthal variations of 1-D migrated receiver functions into the first 3 harmonics: a constant term representing bulk isotropic structure and 180 and 90 degree periodic terms in both radial and transverse components. Using synthetic data we provide a semi-quantitative interpretation of receiver function harmonics in terms of either anisotropy and/or dipping structure, and use these findings to provide maps of crustal structure (Moho depth, orientation of anisotropy axes). A detailed study around the San Andreas Fault near Parkfield reveals the change in both isotropic and non-isotropic velocity structure along and across strike. These results may be used to infer important structural changes near major strike-slip faults and at depth.