Text S1. Resolution tests for isotropic
and anisotropic
models
Two checkerboard test models (Basu, 2019) were inverted at each period
with varying checkerboard sizes of 1.0, 1.5, and 2.0. The
first model (isotropic model) was perturbed by ±4% from the reference
velocity and no anisotropy variations. The second test model
(anisotropic model) has no velocity perturbations but contains
anisotropy with a magnitude of 0.04 km/s in alternating NS-EW
directions. The input a priori data and model errors are the same as
those used for the real data. Random noise with the same magnitude as\(\sigma_{d}\) (data error at each period) was added to the synthetic
models. The tradeoff between azimuthal anisotropy and heterogeneity was
minimal.
A final checkerboard resolution test was conducted with a checkerboard
test model containing ±4% velocity perturbations from the reference
velocity and anisotropy with a magnitude of 0.04 km/s in alternating
NS-EW directions. Resolution at each period is dependent on the ray-path
coverage. For periods of 20-60s, the best resolution was obtained with a
1.5×1.5 checkerboard size, and 2.0×2.0 checkerboards
give the best solution for the higher periods of 75-100s. Figure S1
shows the inversion results for the simultaneous phase velocity and
anisotropy test model for all periods for the chosen checkerboard sizes.
Once again, this result indicates that the tradeoff between azimuthal
anisotropy and phase velocity is minimal.