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.
Data and method
Data and processing
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).