Measuring surface-wave overtone phase velocities using a mode-branch stripping technique

We present a method for the retrieval of the phase velocities of surface-wave overtones. The 'single-station' method is successful for several Love and Rayleigh overtone branches (up to at least four) in mode-specific period ranges between 40 and 200 s. It uses mode-branch cross-correlation functions and relies on adjusting the phase and amplitude of the mode branches one at a time. A standard statistical optimization technique is used. We discuss in detail the a priori information that is added to stabilize the retrieval procedure. In addition, we present a technique to estimate the reliability of individual phase and amplitude measurements. The retrieval method and the technique to estimate reliabilities can be used together in a highly automated way, making the methods especially suited for studying the large volume of digital data now available.
We include several applications to synthetic and recorded waveforms. We will discuss in detail an experiment with 90 waveforms that have travelled along very similar paths from Vanuatu to California. For this path, we will present average overtone phase velocities and an average 1-D velocity structure.