Azimuthal anisotropy from array analysis of Rayleigh waves in Southern California

Abstract

S U M M A R YDirect 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 2θ components dominate the 4θ components. Strength of anisotropy hovers around 1 per cent for the 2θ components with standard error (1σ ) of about 0.4 per cent. Fast axes orientations show stable orientation in the direction 110 • clockwise from north and its opposite direction 290 • . 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.

Azimuthal anisotropy of Rayleigh waves in the crust in southern Tohoku area, Japan

“…Unlike previous studies in Southern California [ Alvizuri and Tanimoto , ; Riahi and Saenger , ], the estimated azimuthal anisotropic patterns clearly depend on frequency. The frequency‐dependent anisotropy indicates depth variations of seismic anisotropy within the crust: the north‐south fast velocity direction around the upper crust and the trench‐normal fast velocity direction in the deeper crust.…”

“…In weak anisotropic media, the azimuthal dependence of phase velocities can be described as follows [ Smith and Dahlen , ; Alvizuri and Tanimoto , ]. where f is the frequency, θ is the azimuth, v ani is the phase velocity in an anisotropic medium, and v iso is the isotropic component of the phase velocity.…”

“…Such off‐great circle path propagation for high‐frequency surface waves [e.g., McGarr , , ] can be corrected by a beamforming technique [ Tanimoto and Prindle , ] or by the maximum likelihood method used in this study. Secondary peaks at higher frequencies, which can be recognized at the same radii as the first peaks, indicate multipath effects; this was also observed in previous studies in Southern California [ Tanimoto and Prindle , ; Alvizuri and Tanimoto , ] and Norway [ Maupin , ].…”

The relationship between upper mantle anisotropic structures beneath California, transpression, and absolute plate motions

“…Removal of a 4y term is justified for Southern California as a recent array analysis of Rayleigh waves demonstrated small amplitudes for 4y variation [Alvizuri and Tanimoto, 2011]. This set of data was inverted for depth variations of anisotropy, making a simplifying assumption on the form of anisotropy that the symmetry axes of P and S wave velocity align in the horizontal plane and the medium has hexagonal symmetry.…”

Arrival angle anomalies of Rayleigh waves observed at a broadband array: a systematic study based on earthquake data, full waveform simulations and noise correlations

“…Cotte et al 2000;Baumont et al 2002Baumont et al , 2004Tanimoto & Prindle 2002;Maupin 2011;Alvizuri & Tanimoto 2011;Foster et al 2014b). Such techniques are based on variants of time or frequency domain beamforming (for an overview see Rost & Thomas 2002), a technique which is used for a large range of applications in seismology.…”