Method and Theory

• After preparing the data, we employ a three step inversion scheme:

  1. Iterative, non-linear, heavily regularized, inversion to obtain very smooth models of phase velocity perturbation for each of the four period bands: 40-50 s, 50-66 s, 66-100 s, and 80 - 150 s (these correspond to frequency bands 5 mHz wide).

  2. A linearized "scattering" inversion, using the models from step one as starting points. Again, we obtain models of phase velocity perturbation in the four specified period bands (results for the 66-100 s band).

  3. A linear inversion of the four models produced in step two, combined with a crustal correction, to obtain a three-dimensional model of shear velocity perturbation.

• The key aspect of our inversion procedure is the use of linearized scattering theory. The scattering formalism allows the inversion to:

  • use both the phase and amplitude of the surface wave waveforms
  • accommodate off great-circle paths (multi-pathing)
  • model structure on the order of a single wavelength