# Three-Dimensional Modeling of the Xichang Crust in Sichuan, China by Machine Learning
## 3 Velocity Structure Model  
Seismic tomography has now been proved very powerful tools for studying the three-dimensional velocity structure at different scales in the earth [44; 45; 46]. At present, the commonly used methods include body wave tomography, surface wave tomography, receiver function, joint inversion, etc. With the significant improvement of the density and quality of the regional seismic network, many velocity structure models have been produced [8; 9; 10; 11; 12; 13; 14; 15; 16]. However, due to the inherent uncertainty of the model and the different sensitivity of different data types and inversion methods, the inversion results also have some differences. The comparative analysis of different velocity models (Table 1) found that they are generally consistent in large-scale characteristics. It can study block structure, high-speed anomaly body, and Moho surface in a large area. In this study, the model with high horizontal/vertical resolution calculated by [PERSON] et al. (2016) [12] was selected. In order to prevent the non-uniformity of data points on the profile and distortion of interpolation result, we carried out layer interpolation based on different depths to obtain the distribution characteristics of velocity anomaly regions at different depths. Afterwards, the three-dimensional matrix data were superimposed to provide better constraints for the model, and the crustal medium frame model of this region was established for later geological modeling. \begin{table}
\begin{tabular}{c c c c c} \hline
**Regional Model** & **Inversion Method** & **H Resolution** & **V Resolution** & **Dataset** \\ \hline USTClitho2.0 & Joint Inversion & 0.5\({}^{\circ}\) & 5 km & [PERSON] et al., 2021 [16] \\ SWChinaCVM-1.0 & Joint Inversion & 0.5\({}^{\circ}\) & 5 km & [PERSON] et al., 2021 [15] \\ USTClitho1.0 & DD seismic tomography & 0.5\({}^{\circ}\)\(\times\) 0.5\({}^{\circ}\) & 5 km & [PERSON] et al., 2019 [14] \\ Xichang Region & Ambient noise tomography & 20–40 km & 20 km & [PERSON] et al., 2018 [13] \\ China & Surface wave dispersion & 0.5\({}^{\circ}\)\(\times\) 0.5\({}^{\circ}\) & 0.5 km & [PERSON] et al., 2016 [12] \\ Mainland China & P-wave anisotropic tomography & 0.7\({}^{\circ}\)\(\times\) 0.7\({}^{\circ}\) & 35 km & [PERSON] et al., 2016 [11] \\ Tibetan Plateau & Lg-wave Q tomography & 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) & 0.2 km & [PERSON] et al., 2013 [10] \\ Tibet & Ambient noise tomography & 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) & 0.2 km & [PERSON] et al., 2012 [9] \\ South China & Ambient noise tomography & 0.5\({}^{\circ}\)\(\times\) 0.5\({}^{\circ}\) & 0.2 km & [PERSON] et al., 2012 [8] \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of seismological velocity models. Figure 2: Workflow of modeling. Figure 3a shows the profile characteristics of the S wave velocity structure. By color standard processing, we enhanced the display of the crustal structure. We can conclude: (1) The shallow strata with wave velocity between 1.28 km/s and 2.8 km/s have great velocity variation and shallow burial depth, consistent with the relief of Xichang Basin and relate to the boundary of the basin. (2) The velocity of 2.8-3.2 km/s cover the whole region with the gentle change, reflecting the distribution characteristics of sedimentary cover on the shallow surface, which may be Precambrian metamorphic sedimentary rocks or igneous rocks. (3) Earthquucas mostly occur in the blue region where the wave velocity structure is 3.2-3.6 km/s. The geometric morphology of this region is consistent with the folded basement of geological history. We see the presence of magmatic rocks and shallow metamorphic rocks with brittle quality and low hardness. (4) The red region with wave velocity structure in the range of 3.6-3.8 km/s has a certain uplift feature along the N-S direction in the middle, which is relatively consistent with the exposed position of the Kangdian axis and the surface granites. It is an anomalous area with high wave velocity and a hard Crystalline basement.