Figure 4 Number of ray paths within each 0.2° × 0.2° cell at several depths in the range of 25–70 km. Map features are the same as in Figure 1.
Although we have collected seismic records for event-station pairs with separations up to 15°, over 90% of the picked onset times come from Pg and Pn waves with epicentral distances smaller than 10. This can be seen in the time-distance plot of all the data obtained in this study in Figure 5. The relatively high seismic attenuation of the Sichuan-Yunnan region (Wei and Zhao, 2019) may have prevented the P waves turning in the uppermost mantle from propagating efficiently over longer distances. The limited number of data at shorter epicentral distances suggest that regions above the lower crustal may not be very well sampled, whereas the vertical profiles of the ray paths in Figure 2 show that structures deeper than 70 km are poorly constrained by our dataset. Based on the data and ray path distributions shown in Figures 2, 4 and 5, we focus our attention on the lower crust of the Sichuan-Yunnan region as well as the topmost part of the upper mantle in Sichuan and northern Yunnan.