Love mode dispersion across subduction zones by finite element modelling

Phase velocity and mode conversion of Love waves with normal incidence across a subduction zone have been studied with the use of the finite element algorithm previously developed for seismic waves through anomalous Earth structures. Two rather different models are considered, representing subduction slabs under Japan and the North Island of New Zealand. Fundamental-mode Love-wave dispersion curves have been calculated for wave periods between 5 and 60s. Comparison with phase velocity curves for the adjacent ocean and island regions shows that the phase velocity for the subduction zones is, in general, slightly less than the mean of the phase velocities of the adjacent regions. For example, at a period of 20 s the phase velocity in the model of the subduction zone of Japan is 4.157 km s−1 and the mean of the phase velocities in the regions at the ends of the model is 4.245 km s−1; at the same period the phase velocity in the New Zealand subduction model is 3.895 km s−1 and the mean of the phase velocities at the ends of the model is 4.150 km s−1. At a period of 10 s, only 5 per cent of the energy of the incident fundamental Love mode is transmitted in the fundamental mode across the model of the subduction zone of Japan; at a period of 15 s, 18 per cent of the energy of the incident fundamental Love mode is transmitted in the fundamental mode across the New Zealand subduction zone. At these periods, a subduction zone significantly scatters the energy of the fundamental Love mode in the same way as a continental margin scatters it. The relevance of these results for the inversion of dispersion measurements of surface waves with circumglobal paths is that subduction zones have a negligible effect on the phase velocity of the fundamental Love mode and that scattering of the fundamental Love mode at subduction zones and at continental margins is important only at periods less than approximately 40 s.