利用中国地震科学台阵研究青藏高原东南缘地壳各向异性:第一期观测资料的剪切波分裂特征

Crustal seismic anisotropy in the southeastern margin of Tibetan Plateau by ChinArray data:shear-wave splitting from temporary observations of the first phase

中国地震科学台阵第一期(2011—2013年)布设在南北地震带南段,本研究利用中国地震科学台阵布设在云南及相邻地区的部分流动台站记录到的2011年6月至2013年3月的数字地震波形资料,开展地壳各向异性分析.本文使用剪切波分裂系统分析方法(SAM方法),获得了研究区内67个台站的剪切波分裂参数.研究结果表明,受到云南及周边地区复杂的构造、应力环境和纵横交错的断裂分布的影响,该地区快剪切波偏振方向(PAZ)整体上显示出NNE向和NE向的优势取向,但在空间分布上比较复杂,虽然大部分台站的PAZ与构造应力场方向一致,但部分断裂附近台站的PAZ受到断裂的影响.结果显示,本研究区内不同区域的PAZ有一定差异性.本研究划分了5个子区,西部3个不同区域的PAZ从北到南分别为NNW向、近N-S向和NE向,有顺时针旋转的趋势,而东部的2个区域PAZ分别为NEE向和NNW向.研究证实,青藏东南缘地区的地壳各向异性空间分布虽然非常复杂,但大体上与区域内的主压应力的方向和断裂分布相关.

The southeastern margin of Tibetan Plateau has relatively complex tectonic movement and strong tectonic deformation,which grows deep fracture and shattered crust.In order to investigate the crustal deformation mechanism in this region,the study uses data obtained from temporary stations in the south part of North-South Seismic Belt set up under the project ChinArray Phase I from June 2011 to March 2013,to compute the seismic anisotropy applying Systematic Analysis Method of shear-wave splitting（SAM method）.SAM method is a comprehensive analysis technique for crustal shear-wave splitting,and it divides into three principal phases：calculation of cross-correlation function,time delay correction and polarization analysis check.Firstly,intercept two horizontal components of the waves,and rotate them to calculate the correlation function. When the correlation function reaches a maximum value,the corresponding rotation angle and time delay can be treated as the preliminary polarization of fast shear-wave（PAZ）and time delay of slow shear-wave（δt）.Then,according to the preliminary result,do the time delay correction and polarization analysis check.When the polarization pattern is close to the linear after the time delay correction,the calculation results are acceptable and will be the final result.We analyzed 826 data recorded from 67 stations of ChinArray.According to the regional distribution of stations,faults and the result of shear-wave splitting,we divided the study zone into 5sub areas,A,B,C,D and E.Area A locates at northwest of the study region,including21 stations.PAZ in this area are relatively complicated,which trend NW,NE,and NEE;area B is on the west of the study region,and also includes 21 stations,PAZ in this area strike to nearly N-S or NEE;area C locates at southwest of the study region,and includes 9stations,PAZ of most stations are in direction of NE;area D is on the northeast of the study region,and contains5 stations,PAZ in this area strike to NE or NW;area E locates at northeast of the study region,and includes 11 stations,PAZ of most stations are in direction of NNW.The PAZ in area A,B and C which locate at west of study region,are in direction of NNW,nearly N-S and NE from north to south,which show a trend of clockwise rotation.While D and E area at eastern part strike to NEE and NNW respectively,PAZ in area D may be influenced by the NE strike fault in the region,and PAZ in area E are consistent with the tectonic stress field,also may be affected by the mass faults strikes to N-S or NW.PAZ at most stations strike to the direction of NNE and NE,consistent with the direction of tectonic stress field,but at some stations near faults are affected by the strike of the faults.The result shows that PAZ is varied with the different areas.Complex tectonics and crisscrossed distributions of faults results in complex PAZ spatial distribution.PAZ at most stations are consistent with the direction of tectonic compressive stress field,while at other stations,they are consistent with the direction of fault strike.This study finally obtained more regional distribution details than the early results such as Shi et al.（2006;2012）,who used the permanent stations.However,due to the complicated geological features,characteristics of shear-wave splitting in the crust in this region still need more data and studies to sustain and verify.