Analysis of Rayleigh wave refraction from three-component seismic spectra

Lateral refraction of intermediate period surface waves is caused by horizontal gradients in local phase velocity in the vicinity of the source-receiver minor arc. For smooth structural perturbations, this is manifested by an off-azimuth arrival (azimuth deviation) of the Airy phase. We present a method for estimating azimuth deviation as a function of frequency for Rayleigh waves observed on a three-component seismogram. By taking advantage of the different dispersion characteristics of the Rayleigh and Love wave group arrivals, the method essentially constructs an optimal narrow-band taper for each frequency component, in order to minimize the effect of Love wave contamination in the Rayleigh wave azimuth estimation. We have measured the frequency-dependent azimuth deviation of fundamental mode Rayleigh waves of period 20–100 s under a variety of conditions. The most coherent results are obtained for (1) seismic records with a relatively high Rayleigh wave excitation, (2) ray paths which are confined to purely oceanic or purely continental lithosphere, and (3) ray paths of intermediate offset (30°–50°).