Rayleigh wave phase velocities in the Pacific with implications for azimuthal anisotropy and lateral heterogeneities

Abstract

The lateral distribution of fundamental-mode Rayleigh wave phase velocities in the Pacific has been calculated in order to determine the variation in velocity as a function of the age of the oceanic plate, the importance of azimuthal anisotropy, and the presence of secondary lateral heterogeneities. The data set used in this study consists of phase velocity measurements in the period range 20–125 s for 178 paths traversing the Pacific. Both the pure-path and spherical harmonic inversion techniques are used in this investigation with an emphasis placed on determining the effectiveness and resolving power of these methods in calculating the velocity distribution in an oceanic regime. The first-order velocity-age relation which dominantly characterizes surface wave dispersion in an oceanic environment can be adequately modelled by either the pure-path or spherical harmonic techniques. Azimuthal anisotropy is the dominant second-order effect and is best developed in regions less than 80 Myr in age. In this age zone, the faster direction of wave propagation is either in the direction of fossil seafloor spreading or in the direction of present-day absolute plate motion. In the western Pacific, there is a correlation with paleo-relative motion (fossil seafloor spreading) for periods less than 50 s. Lateral velocity heterogeneities which are not related to the age of the oceanic plate are found by pure-path inversions and are correlated with regions associated with shallow residual depth anomalies. This effect (slow velocities with respect to the velocity-age model) monotonically decreases as a function of period and is indicative of a shallow origin. Age independent anomalies were also modelled by the application of the pure-path and spherical harmonic methods in a sequential inversion.