Observations of Surface Wave Azimuthal Anisotropy in Southern California by Direct Application of the Beamforming Method

We use the Southern California Seismic Network as an array and apply a beamforming method for 190 teleseismic events to determine their Rayleigh surface wave phase velocities and their arrival directions. Our results indicate that the array aperture has excellent resolution for periods 16-100 seconds but above 100 seconds phase velocities begin to show large error bars because of its spatial size. We recover the ubiquitous pattern of azimuthal phase velocity variation of Rayleigh waves for periods 16-50 seconds; the amplitude variations are about 1-1.5%, consistent with an assumption of a weakly anisotropic medium. Our results also confirm an assumption in most studies, of negligible dependence on the 4-theta component. Though these results apply only in Southern California, direct confirmation of small 4-theta variations seems to be rare. Our analysis shows that anisotropy is present in the period range 15-50 seconds which correspond approximately to 50-200 km wavelengths, and suggests anisotropy extends to the upper mantle with an prevalent anisotropic fast axis that is oriented along the direction SE-NW (110/290 degrees clockwise from North). This direction is interesting in that it is sub-parallel with the Mohave section of the San Andreas Fault. It is this section of the fault that is misaligned with the general direction of the North America-Pacific Plate boundary. Thus we speculate that the fast axis alignment may be associated with shear and compression of the Transverse Ranges.