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.