Azimuthal anisotropy from array analysis of Rayleigh waves in Southern California

Direct detection of Rayleigh-wave azimuthal anisotropy is reported by applying an array analysis to broad-band seismic data in Southern California, USA. Our approach has excellent resolution for frequencies between 30 and 60 mHz and good resolution between 10 and 30 mHz. Limitation from array size limits accuracy below 10 mHz and complicated wave propagation effects lead to difficulty above 60 mHz. Between 30 and 60 mHz, azimuthal anisotropy of Rayleigh-wave phase velocity is detected cleanly. Phase velocity results show that the 2 theta components dominate the 4 theta components. Strength of anisotropy hovers around 1 per cent for the 2 theta components with standard error (1 sigma) of about 0.4 per cent. Fast axes orientations show stable orientation in the direction 110 degrees clockwise from north and its opposite direction 290 degrees. Depth sensitivity kernels for P-wave and S-wave anisotropy indicate that anisotropy is confined to depths in the upper 100 km and may even be to shallower depths. We speculate that the fast axes alignment may be associated with a strong shearing process over the thickness of lithosphere due to obliquely collisional motions between the Pacific Plate and the North America Plate in the region.