DOU Hui,
WANG BaoShan,
XU YiHe et al
.2021.Analyzing the characteristics of the upper crustal anisotropy in Binchuan, Yunnan Province based on dense arrays and shear wave splitting Chinese Journal of Geophysics(in Chinese),64(12): 4292-4307,doi: 10.6038/cjg2021P0478
Analyzing the characteristics of the upper crustal anisotropy in Binchuan, Yunnan Province based on dense arrays and shear wave splitting
DOU Hui1, WANG BaoShan1,2, XU YiHe1,3,4, WANG WeiTao1, ZHANG Bo1
1. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China 2. School of Earth and Space Science, University of Science and Technology of China, Hefei 230026, China 3. Department of Earth Sciences, University of Cambridge, Cambridge CB30EZ, United Kingdom 4. Geophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, Dublin D02Y006, Ireland
Abstract：Seismic anisotropy in the upper crust is mainly controlled by the aligned fractures due to regional stress and is also influenced by the regional structure units, like fault zones. However, rare studies are applied to systematically analyse the regional effects due to the limited station sampling. This study presented a shear wave splitting analysis based on a dense array (381 stations over two-months records, March 25-May 25, 2017) in Binchuan Basin, Yunnan Province, which provides sufficient spatial resolution to resolve the problem. Binchuan Basin is situated northwest of Yunnan Province, close to the city of Dali and the Red River fault. Subject to the strong regional stress caused by the eastern extrusion of the Tibetan Plateau, a set of faults formed in and around the basin, intersecting with each other. Some major faults are nearly perpendicular to the direction of the maximum horizontal principal stress. The misalignment between the strike of the faults and the regional stress provides a unique opportunity to differentiate the contribution of the two mechanisms. Additionally, frequent local earthquakes offer abundant data for shear wave splitting analysis. We manually picked 9819 S-wave arrivals and measured the shear wave splitting parameters using MFAST (Multiple Filter Automatic Splitting Technique), a shear wave splitting method that can automatically assess the quality of the measurements using cluster analysis and a series of quality control criteria. We obtained 831 high-quality shear wave splitting measurements that spread across 173 stations. Our results show that (1) the average polarization of the fast shear waves is NNW (N17.17°±4.64°W), which is consistent with the direction of the maximum horizontal principal stress that is obtained from analysis of GPS data and focal mechanism of earthquakes in a broader region (Yunnan or Sichuan-Yunnan); (2) in spatial, the polarization of the fast shear wave on stations nearby the faults shows patterns related to the faults' strike, indicating a strong regional influence from the faults; (3) the average delay time of the slow shear wave is 0.087±0.002 s, which corresponds to the amplitude of anisotropy of 2.4%, higher than the crustal average in Yunnan Province, suggesting a higher level of anisotropy in the Binchuan area. In summary, the anisotropy of the upper crust beneath the Binchuan area is mainly controlled by its regional stress field, with detailed features influenced by local faults, and the amplitude of anisotropy is relatively stronger than the average in Yunnan Province. The study shows the advantage of using dense arrays on systematically study the upper crustal anisotropy; namely, the high-spatial-resolution can help to analyze and differentiate different mechanisms of anisotropy in the upper crust.
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