Surface Wave Propagation in the Western United States

Part I Multi-mode group velocity dispersion of both Rayleigh and Love waves was measured for a number of paths in the Western United States by means of a technique developed for separating the modes. Results for each region studied are interpreted in terms of a crustal structure which produces simultaneous agreement with all the modes observed, as well as available body wave data. Certain diagnostic features of group velocity dispersion curves were noted and used to advantage in this study. The analysis techniques developed have rather broad geophysical applications, for example in studies of source properties, interference phenomena, and noise properties. Part II The effects of the transition zone at the continental margin of Southern California on Rayleigh wave propagation have been investigated. Among the anomalous effects discovered are a minimum in phase velocity between 20 and 35 seconds period, different phase velocities on reversed paths across the same array, and systematic lateral re-fraction at the continental boundary. These anomalous effects can be attributed largely to the slope and curvature of the Mohorovičić discontinuity across this region. An ultrasonic model was constructed to aid in interpreting these results. Interpretation of the dispersion for periods below 20 seconds indicates that the crust thickens toward the continent with a slope of about 5° attaining a thickness of approximately 35 km under Southern California.