2.4. Inversion
We inverted the average dispersion curve of each region to obtain one-dimensional shear-wave velocity profiles, also using an implementation of Herrmann and Ammon (2002). The earth model was parameterized with layer thicknesses of 1 km in the upper 6 km, 2 km from 6 to 50 km depth, and 5 km from 50 to 100 km depth. The inversion ran to a depth of 100 km, and although we lack resolution at that depth, doing so avoided forcing structure to artificially shallow depths. The majority of the dispersion curves could be picked to 50 s,so we interpreted only the upper 75 km of the inversion results. This corresponds to the approximate crustal thickness at the northern eight stations (Rai et al., 2006); for this reason we did not force a velocity jump at a Moho discontinuity in any of our inversions