S-wave velocity structure of the Sichuan-Yunnan region, China:
implications for extrusion of Tibet Plateau and seismic activities
Haoyu Tian1, Chuansong He11*Correspondence author: hechuansong@aliyun.com*, M.
Santosh2,3
1Institute of Geophysics, China Earthquake
Administration, Beijing 100081, China
2School of Earth Sciences and Resources, China
University of Geosciences Beijing, Beijing 100083, China
3Department of Earth Science, University of Adelaide,
Adelaide SA 5005, Australia
Abstract: The
Sichuan-Yunnan region is located at the intersection between the South
China Block, the Indian plate and the Tibet Plateau and is crisscrossed
with deep and large faults and is characterized by strong seismic
activities. Here we employ one-year continuous waveforms of the vertical
component of 89 broadband seismic stations in this region to evaluate
the velocity structure and its implications.
Through single station data preprocessing, cross-correlation
calculation, stacking, group velocity dispersion measurement and quality
evaluation, the group velocity dispersion curves of Rayleigh waves for
the different periods were obtained. We then use the surface wave
tomography method to obtain the Rayleigh wave group velocity
distribution of 9-40s in this area. Finally, the S-wave velocity
structure in the depth range of 0-60 km in the study area is obtained by
pure path dispersion inversion. The results show that the surface layer
or the top of the upper crust in the Sichuan Basin is characterized by
low velocity due to the influence of the sedimentary strata, whereas the
middle and lower crust of the Sichuan Basin shows high velocity
structure. The Sichuan-Yunnan diamond-shaped block (SYDB) shows a
high-velocity structure in the middle crust, , and a low velocity in the
lower crust. The seismic activities are mainly concentrated at the
western part of the region, with the earthquakes distributed at the
boundary between the low- and high-velocity structures, as well as the
adjacent region, which we correlate with the extrusion of the Tibet
Plateau.
Key words: Noise tomography, Rayleigh surface wave, S-wave velocity,
Crustal structure, Sichuan-Yunnan region.
Introduction
The Sichuan-Yunnan region of mainland China (99°E-109°E,20°N-33°N)
(Fig. 1) forms part of the southeastern margin of the Indo-Eurasian
plate collision zone, as well as a turning point of the
Tethyan-Himalayan orogenic system (Kan et al., 1977; Zhong et al., 1998;
Deng et al., 2002). The region is located at the intersection between
the South China Block, the Indian plate and the Tibet Plateau and is
crisscrossed with deep and large faults, as represented by the
Xiaojiang, Honghe, Lijiang-Ninglang, Anning River-Zemu River fault,
Longmen Mountain and Maitreya-Shizong fault (Xu et al., 2003). This
region also covers the central and southern domain of the North-South
Seismic Belt, and is characterized by strong seismic activities (Li,
1993; Su et al., 2004; Zheng et al., 2012).