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.