On the wobbles of phase-velocity dispersion curves

Abstract

SUMMARY To calculate phase-velocity dispersion curves, we introduce a method which reflects both structural and dynamic effects of wave propagation and interference. Rayleigh-wave fundamental-mode surface waves from the South Atlantic Ocean earthquake of 19 August 2016, M = 7.4, observed at the AlpArray network in Europe are strongly influenced by the upper-mantle low-velocity zone under the Cameroon Volcanic Line in Central Africa. Predicting phase-delay times affected by diffraction from this heterogeneity for each station gives phase velocities as they would be determined using the classical two-station method as well as the advanced array-beamforming method. Synthetics from these two methods are thus compared with measurements. We show how the dynamic phase velocity differs from the structural phase velocity, how these differences evolve in space and how two-station and array measurements are affected. In principle, arrays are affected with the same uncertainty as the two-station measurements. The dynamic effects can be several times larger than the error caused by the unknown arrival angle in case of the two-station method. The non-planarity of the waves and its relation to the arrival angle and dynamic phase-velocity deviations is discussed. Our study is complemented by extensive review of literature related to the surface wave phase-velocity measurement of the last 120 years.