Surface wave higher-mode phase velocity measurements using a roller-coaster-type algorithm
Abstract
In order to solve a highly non-linear problem by introducing the smallest a priori information, we present a new inverse technique called the `roller coaster' technique and apply it to measure surface wave mode-branch phase velocities. The fundamental mode and the first six overtone parameter vectors, defined over their own significant frequency ranges, are smoothed average phase velocity perturbations along the great circle epicentre-station path. These measurements explain well both Rayleigh and Love waveforms, within a maximum period range included between 40 and 500 s. The main idea of this technique is to first determine all possible configurations of the parameter vector, imposing large-scale correlations over the model space, and secondly to explore each of them locally in order to match the short-wavelength variations. The final solution which achieves the minimum misfit of all local optimizations, in the least-squares sense, is then hardly influenced by the reference model. Each mode-branch a posteriori reliability estimate turns out to be a very powerful instrument in assessing the phase velocity measurements. Our Rayleigh results for the Vanuatu-California path seem to agree correctly with previous ones.
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