Fig. 1. Tectonic framework of the study area. The thick black lines show the boundaries of the various crustal blocks and basins. F1: Xiaojiang fault; F2: Honghe fault; F3: Lijiang-Ninglang fault; F4: Anning River-Zemu River fault; F5: Longmen Mountain fault; F6: Maitreya-Shizong fault. The black triangles are the locations of the seismic stations used in this study. Red lines: S-wave velocity profiles.

The unique geological and tectonic setting of this region, and earthquake-prone feature has attracted several studies to explore the deep features including seismic sounding（Hu et al., 1986；Wang et al., 2014），magnetotelluric sounding (Bai et al., 2010; Shen et al., 2015), body wave tomography (Liu et al., 1989; Wang et al., 2002; Huang et al., 2003; Xu et al., 2013; Lei et al., 2014), surface wave and noise tomography (Yao et al., 2006; Zhou et al., 2012; Zheng et al., 2015; Fan et al., 2015；Zheng et al., 2017), receiver function (Wu et al., 2001; Xu et al., 2007; Li et al., 2009; Xu et al, 2009; Wang et al., 2017; Hu et al., 2017) and surface wave and receiver function joint inversion (Bao et al., 2015).

Liu et al. (1989) noted that the velocity structure of the crust and upper mantle in the Sichuan-Yunnan region has obvious lateral heterogeneity based on teleseismic P-wave tomography. Wu et al. (2001) showed that the crust thickness in Yunnan area gradually decreases from northwest to southeast and the S-wave velocity structure is characterized by strong lateral inhomogeneity based on teleseismic receiver function inversion. Wang et al. (2002) employed P-wave and S-wave tomography and identified that the velocity anomalies of the lower crust and upper mantle are controlled by the faults. Huang et al. (2003) show that the velocity structure has obviously lateral heterogeneity in the Sichuan-Yunnan region by using the Pn tomography. Jiang et al. (2012) proposed that there is a significant difference in the crustal structure between the Sichuan Basin and the Songpan-Ganzi Block by using the Bouguer gravity data. Wang et al. (2014) used seismic sounding to obtain a two-dimensional crustal structure of the region where the eastern side of the Red River Fault shows low velocity structure. Based on Pn tomography, Lei et al. (2014) imaged a high velocity anomaly in the Sichuan Basin area and an obvious low velocity anomaly zone from the Songpan-Ganzi Block to the SYDB. Bao et al. (2015) employed a joint inversion of the surface wave and receiving function and suggested a dramatic lateral change in the crustal S-wave velocity.

On the other hand, advances in seismic tomography has enabled high-resolution seismic imaging, with a direct cross-correlation of the continuous background noise of two stations (Lobkis and Weaver, 2001; Campillo and Paul, 2003; Shapiro et al., 2005; Yao et al., 2006; Bensen et al., 2007; Fang et al., 2009). Compared with traditional tomographic technique, noise imaging technology does not depend on the azimuth distribution of natural earthquakes, and moreover, seismic ray coverage is denser and more reasonable due to the increase of broadband seismic stations (Lu et al., 2014). This approach greatly improves the resolution of shallow crust due to an increase in the short-period dispersion data. Using this technique, Zheng et al. (2015) indicated there is a low-velocity layer in the middle and lower crust of the southeastern margin of the Tibet Plateau. Fan et al. (2015) revealed the lateral variation of sedimentary layer thickness in the Sichuan Basin. Zheng et al. (2012) defined the lateral heterogeneity of the crust and the uplifted basement in the Sichuan Basin. Yao et al. (2006, 2008) suggested that the high and low velocity anomalies in this region are divided by some major fault zones.

In this study, using the data recorded by the China seismic network, we imaged the three-dimensional high-resolution velocity structure of the crust and upper mantle in the Sichuan-Yunnan area by using the noise tomographic technique. Our results provide new evidence on the terrane deformation, material migration, and seismic activities.

We collected the continuous vertical-component seismic data recorded by 89 stations from Data Management Centre of China National Seismic Network from December 2016 to December 2017 (Zheng et al., 2009).