Source effects on surface wave group travel times and group velocity maps

Abstract

In most seismic surface wave studies observed group travel times are interpreted as time delays due entirely to the wave propagation along the wave path, and source effects are considered as negligibly small. This is in contrast with observed phase times where correction for the source phase is generally acknowledged to be mandatory. An important, yet unanswered, question is how neglecting source group time (SGT) in broadband surface wave studies will affect the accuracy of the measured group velocity curves and the tomographic maps constructed from these measurements. We consider here the effect of SGT on group velocity measurements for fundamental Rayleigh waves and report on its dependence on period (10–200 s), source mechanism, and source depth. Varying these parameters strongly affects the magnitude and azimuthal pattern of SGT shifts and we present statistics of certain salient functionals that characterize this dependence. SGT is negligible for periods less than about 75 s and for earthquake shallower than about 25 km. At longer periods and for deeper events, average SGT corrections are greater than 10 s in magnitude, which for continental scale studies translates into group velocity perturbations of 1–2%. We estimate the bias caused by uncorrected SGT in inversions for Rayleigh wave group velocity maps across the Eurasian continent. The largest perturbations to these maps (up to 1–2% for the 50-s period and up to 5% for the 100-s period) are found near the periphery of the continent where ray coverage is poor. From these results, some statistical estimates for adjacent wave paths (clusters), and the fact that SGT corrections display considerable sensitivities to earthquake depths, we conclude that the effects of SGT on group velocity tomographic images may safely be ignored at periods less than about 75 s and for shallow sources. Although such corrections are appreciable at longer periods for events deeper than about 25 km and should in principle be applied, the inherent inaccuracy of present day CMT solutions and group velocity measurements make these corrections practically non-essential for current group velocity tomographic studies.