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.