JWKB surface-wave seismograms on a laterally heterogeneous earth

Abstract

A numerically efficient implementation of JWKB theory is developed to calculate synthetic surface-wave seismograms on a smooth, laterally heterogeneous earth model. We illustrate the method by applying it to model SH12/WM13; the phase and amplitude anomalies of long-period, fundamental-mode Love and Rayleigh waves are compared with the corresponding results obtained using the great-circle approximation and first-order ray-perturbation theory, with the perturbations to the local eigenfunctions ignored. As expected, the most serious discrepancies between these two approximations and full JWKB theory occur for paths with large transverse phase-velocity gradients, which lead to significant deviations away from the source-receiver great circle. The great-circle approximation consistently under-estimates the phase of the first-arriving G1 Love and R1 Rayleigh waves, in accordance with Fermat's principle of least time. The global average Fermat bias is greater for Love than for Rayleigh waves; this may be partly responsible for the observed Love-Rayleigh discrepancy currently attributed to upper mantle transverse isotropy.