High-resolution imaging of the deep anisotropic structure of the San Andreas Fault system beneath southern California

Abstract

SUMMARY Geological and geodetic records show that motion is distributed over a broad and complex system of faults in southern California, with many large earthquakes occurring off the main San Andreas fault. This unusually complex plate boundary geometry is imposed by the big bend of the San Andreas Fault and the dynamics of the underlying lithosphere, but lithosphere rheology and distribution of applied forces are still poorly constrained. Geodynamic modelling is thus very uncertain, and distribution of lithospheric deformations highly controversial. Here, exploiting the property that deformation orients olivine crystals, we show that deep lithospheric deformations can be mapped directly with a new imaging approach of seismic anisotropy, with a resolution fine enough for detailed geodynamic interpretation. This method relies entirely on finite-frequency effects in the splitting of SKS waves. We build an extensive data set of more than 3400 SKS splitting measurements from the analysis of SKS waves recorded by all the broad-band stations in southern California. Resolution tests demonstrate that, using this data set, we are able to resolve anisotropic structures smaller than the size of the first Fresnel zone of SKS waves. The good vertical resolution in our tomographic images results from the short inter-station spacing in southern California. In our 3-D anisotropy model, we find no evidence for localized lithospheric shear deformation beneath the San Andreas Fault. Instead, the lithospheric plate boundary is localized along a broad shear zone beneath the East California Shear Zone. Therefore, surface and deep deformation patterns are poorly correlated and most likely decoupled. Active N–S convergence in the Transverse Ranges results in strongly coherent E–W alignment of olivine fast axes in the shallow lithosphere. At the top of the asthenosphere, the observed low level of anisotropy suggests weak lithospheric basal tractions and thus a strong decoupling between the lithosphere and the asthenosphere.