Figure 7 Decrease in the traveltime residuals during the two-stage iterative inversion. (a) Decrease in RMS traveltime residual with the iteration number. Events are relocated after the 6th iteration, hence the drop in the RMS traveltime residual. The iteration is terminated after the 10th inversion. (b) Histogram of the traveltime residuals before the two-stage inversion. (c) Histogram of the traveltime residuals after the 10th iteration.
The P-wave velocity model obtained after the 10-iteration two-stage inversion is displayed in map views in Figure 8 for the lower crustal depths and in Figure 9 for uppermost mantle depths. Different color bars are used for different depth ranges to accommodate the difference in velocity values. Based on the resolution length estimation shown in Figure 6, we only present the portions of the model where the resolution lengths are shorter than 0.5°. Apart from the map views, velocities along 4 vertical profiles are shown in Figure 10.
The model slices in Figures 8 and 9 show that in most parts of the Sichuan-Yunnan region under study, the lower crust and the layer immediately below the Moho interface are characterized by relatively low-velocity zones, while the uppermost mantle below 55-km depth generally has average velocity. The overall low-velocity feature can be also seen in the apparent velocities of the Pg and Pn waves based on the linear regression of the traveltime measurements (Figure 5). The average apparent velocity of 4.96 km/s for the Pg arrivals is much smaller than the global average value of 6.7 km/s in model AK135 (Kennett et al., 1993). The average apparent velocity of 8.07 km/s for the Pn arrivals, which was also reported by other studies (e.g. Pei et al., 2007), is also relatively low as compared with the other regions.