Surface wave mode coupling – implications for surface wave tomography and for seismology in general
Surface wave tomography (SWT) is a powerful tool for studying the shear-velocity (Vs) structure of the Earth’s crust and upper mantle, typically using seismic surface waves in the period range 50-300 s. The quest for higher resolution Vs models requires using higher frequency (shorter period) as well as higher mode surface wave data, which are more challenging to incorporate in tomographic analyses. Such wave types are more susceptible to complex wave propagation effects arising from laterally heterogeneous Earth structure, which cannot be modelled by the ray-theoretical framework of conventional SWT methods. This talk focuses on one such effect, namely cross-branch mode coupling, or interconversion between surface wave modes. I will present a synthetic study that analyses surface wave mode coupling in detail, through 2-D numerical modelling, and also assesses its impact on multi-mode waveform inversion in two-step path-average SWT. The results of this study show significant mode conversion and, consequently, a breakdown of the path average approximation at periods of 30 s or less, where errors in the inferred group velocities can be as large as 7 per cent. This study serves to inform future generations of SWT methods. In the last part of this talk, I will describe how surface wave mode coupling is relevant for earthquake ground motion prediction, and therefore finds application in seismic hazard assessment.