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, differingin apparentazimuthfrom the larger wave by an averageof about 13°a t 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.

Seismic structure of Iceland from Rayleigh wave inversions and geodynamic implications

“…The first step inverts for phase velocities from Rayleigh wave phase and amplitude data. We applied a two-plane-wave inversion technique [16,17], which can account for non-planar energy due to scattering or multipathing in the incoming wave field. We parameterized the study area using 315 grid nodes with irregular node spacing.…”

Lithospheric instability and the source of the Cameroon Volcanic Line: Evidence from Rayleigh wave phase velocity tomography

“…This approach, however, requires large numbers of parameters to describe the wavefield and can create artifacts within the resulting velocity model. This study employs a method developed by Forsyth and Li [2005] in which the incoming wave field is approximated by the interference of two plane waves. This approach better accounts for heterogeneities along the travel path outside of the study area than traditional plane wave methods but does not require the solution of many unknowns as is the case for the Friederich and Wielandt [1995] method.…”

“…Following the method developed by Forsyth and Li [2005], we solve for phase velocities in two steps. First, an iterative simulated downhill simplex annealing method [Press et al, 1992] is used to solve for the phase, amplitude, and propagation azimuth of the two waves, while velocity is constrained to a constant initial velocity model.…”

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

“…We invert the Fourier components (real and imaginary) of the bandpass filtered and windowed Rayleigh wave vertical and pressure records for phase velocities, station corrections, and incoming wavefield parameters using the method of Forsyth and Li (2005). The method accounts for deviations from a great circle path and scattering effects caused by heterogeneities between the source and receiver array with a two interfering plane wave approximation; this approximation has 6 wavefield parameters for each event.…”

“…For more details on the method see Forsyth and Li (2005) and Yang and Forsyth (2006). The nodes used to parameterize the model are shown in Fig.…”