Array analysis of two-dimensional variations in surface wave phase velocity and azimuthal anisotropy in the presence of multipathing interference

Abstract

Multipath propagation of surface waves introduces distortions in waveforms that can bias array measurements of phase velocities. We present a method for array analysis of laterally and azimuthally varying phase velocities that represents the incoming wavefield from each earthquake as the sum of two interfering plane waves. This simple approximation successfully represents the amplitude and phase variations for most earthquakes recorded in the MELT Experiment on the East Pacific Rise in the period range from 16 to 67 s. The inversion for velocities automatically reduces the importance of data from earthquakes or periods that are not described well by this approximation. Each iteration in the inversion involves two stages: a simulated annealing inversion for the best wave parameter description of each event, and a linearized inversion for velocities and changes in the wave parameters. At 29 s period, the two-plane-wave solutions indicate that nearly every signal is significantly affected by multipathing. The larger of the two plane waves typically has an apparent azimuth of propagation that is within a few degrees of the great circle path. The smaller wave is more scattered, differing in apparent azimuth from the larger wave by an average of about 13° at 29 s. Both lateral and azimuthal variations in Rayleigh wave phase velocity in the study area are significant, although it is possible to trade off azimuthal anisotropy with rapid and probably unrealistic lateral variations in velocity. Apparent azimuthal anisotropy reaches 5 to 6%, with the fast direction approximately perpendicular to the ridge.