基于密集台阵资料的S波分裂研究云南宾川上地壳各向异性特征

Analyzing the characteristics of the upper crustal anisotropy in Binchuan,Yunnan Province based on dense arrays and shear wave splitting

上地壳地震各向异性主要受区域应力引起的裂隙定向排列影响,也受局部构造单元,如断裂带等的影响.受地震台阵台间距的限制,系统性地研究断裂带等构造因素对各向异性影响的研究还较少.基于云南宾川地区密集台阵在两个月内记录到的9819个S波记录,我们使用MFAST(Multiple Filter Automatic Splitting Technique)方法获得了831个高质量的横波分裂结果.结果显示,(1)该区域内的平均快波偏振方向为NNW向,与区域最大水平主压应力方向一致;(2)空间分布上,断裂带附近台站的快波偏振方向呈现与断裂带走向相关的特征;(3)平均慢波时延为0.087±0.002 s,计算得到的各向异性强度为2.4%,高于云南地区的平均值.研究结果表明,宾川地区的上地壳各向异性主要受区域应力控制,局部特征受断裂带的改造作用,各向异性强度较强.本研究展示了密集台阵在系统性地研究上地壳各向异性的优势,即利用高密度的空间采样可以更好地分析上地壳各向异性的空间分布细节和影响机制.

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.