Figure 1 Map of tectonic environment of the Sichuan-Yunnan and
surrounding regions with seismic stations, events and faults
superimposed. Red circles and green triangles show respectively event
and station locations selected for the P-wave velocity model inversion
in this study. The black dot and triangles show the locations of the
event and stations whose records are displayed in Figure 2. Major active
fault zones are shown by thick black lines with abbreviated names in
black, including LMSFZ (Longmenshan Fault Zone), XSHFZ (Xianshuihe Fault
Zone), ANH-ZMHFZ (Anninghe-Zemuhe Fault Zone), XJFZ (Xiaojiang Fault
Zone), LJ-XJHFZ (Lijiang-Xiaojinhe Fault Zone), JSJFZ (Jinshajiang Fault
Zone), RRFZ (Red River Fault Zone), LZJFZ (Luzhijiang Fault Zone), and
LCJFZ (Lancangjiang Fault Zone). Major tectonic blocks are indicated by
abbreviated names in blue, including SYRB (Sichuan-Yunnan Rhombic
Block), SB (Sichuan Basin), SGB (Songpan-Ganzi Block), QB (Qiangtang
Block), and YB (Yangtze Block). The yellow dashed lines with arrows show
the general directions of the velocity field of crustal deformation
(Wang and Shen, 2020). The plot in the lower-right corner shows the
P-wave ray paths (43,540) coverage used in this study.
The strong crustal deformation and relative motions between various
blocks make the Sichuan-Yunnan region one of the most tectonically
active zones with a high level of earthquake activities. Frequently
occurring small and moderate earthquakes, coupled with the deployment of
increasing number of broadband seismic stations in recent years provide
valuable data for us to improve the quality of regional structural
images. In this study, we develop a high-resolution P-wave velocity
model for the lower crust and uppermost mantle for the Sichuan-Yunnan
region by applying the fast marching Method (FMM) of Rawlinson and Urvoy
(2006) to a large dataset of P waves obtained by an automatic picking
algorithm based on deep learning (Wang et al., 2019). The true
resolution of the model has been estimated by the statistical resolution
matrix approach (An, 2012). Our results provide further support for the
existence of the lower crust flow and reveal more details on the complex
structure of the uppermost mantle of the Sichuan-Yunnan region.