3. Result
Our final dataset contains 43,540 Pg and Pn onset times from 822 events
to 284 stations. We use the FMM to invert this dataset in a two-stage
iterative process for the P-wave velocity structure in the lower crust
and uppermost mantle of the Sichuan-Yunnan region. The first stage
involves 6 inversion iterations starting from the
initial LITHO1.0 model. The
decrease of the root-mean-square (RMS) traveltime residuals with
iteration number is shown in Figure 7(a). The total RMS residual
decreases rapidly in the first few inversions but gradually levels off.
Therefore, at the end of the first stage after the 6th inversion, we
carry out a relocation of all events in the current model. Then, the
inversion continues to the second stage until the RMS residual levels
off again, and we choose to stop after the 10th iteration. Figures 7(b)
and (c) show respectively the histograms of the residuals before and
after the two-stage inversion. The residuals are distributed much more
narrowly around zero after the inversion, testifying to the
effectiveness of the tomography approach.