ANALYSIS OF RAYLEIGH-WAVE REFRACTION FROM 3-COMPONENT SEISMIC SPECTRA

Lateral refraction of intermediate period surface waves is caused by h orizontal gradients in local phase velocity in the vicinity of the sou rce-receiver minor arc. For smooth structural perturbations, this is m anifested by an off-azimuth arrival (azimuth deviation) of the Airy ph ase. We present a method for estimating azimuth deviation as a functio n of frequency for Rayleigh waves observed on a three-component seismo gram. By taking advantage of the different dispersion characteristics of the Rayleigh and Love wave group arrivals, the method essentially c onstructs an optimal narrow-band taper for each frequency component, i n order to minimize the effect of Love wave contamination in the Rayle igh 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 obtaine d for (1) seismic records with a relatively high Rayleigh wave excitat ion, (2) ray paths which are confined to purely oceanic or purely cont inental lithosphere, and (3) ray paths of intermediate offset (30-degr ees-50-degrees).