云南地区现今应力场特征及成因分析

Characteristics and cause analysis of the present stress field in Yunnan

基于国际开放地震数据库(Global CMT),统计了云南地区1976—2015年期间66组震级(M_w)≥4. 8的震源机制数据。通过TENSOR程序反演震源机制的简化应力张量发现:云南地区应力机制以走滑型(SS)为主,占66. 7%;其次为正断型(NF),占22. 7%,且主要集中分布在云南西北断裂附近。将66组震源机制数据按区域分为4个次级区块分别联合反演,其结果显示:云南西北地区拉张作用显著,最大主应力方向为WNW—ESE和NNW—SSE;云南西南、东北地区以挤压作用为主,最大主应力方向分别为NNE—SSW和NW—SE;云南东南地区板块相对稳定,基本没有中强震发生。以震源机制反演结果为约束,利用有限元数值模拟得到了研究区连续应力场变化特征,即具有顺时针旋转的趋势,其主要原因是喜马拉雅东部构造特殊的下地壳和上地幔物质使得云南地区成为印度板块向北推进的优势通道,进而挤压应力向东南偏转形成旋转的应力场。

Based on the Global Centroid Moment Tensor（Global CMT）,a total of 66 focal mechanism data with their magnitudes over 4.8 during the years of 1976--2015 have been obtained and analyzed.The reduced stress tensors of focal mechanism that inverted through TENSOR program indicated that the stress regime is given priority to with the type of strike-slip（SS） faulting,accounting for 66.7%,and followed by the normal faulting（NF） stress regime which distributed in near faults of northwestern Yunnan,accounting for about 22.7%.We grouped the 66 focal mechanism data in 4 distinct regions（boxes） based on their distribution and then conducted joint inversion on them.The results of inversion suggested that the extensional forces are remarkable in northwestern Yunnan and the orientations of the maximum principal stresses are WNW-ESE and NNW-SSE.In southwestern and northeastern Yunnan,tectonic stresses are mainly performed as compression,and the orientations of maximum principal stresses are NNE-SSW and NW-SE respectively.Nevertheless,the plate in southeastern Yunnan is relatively steadier than other areas,hence there are no intermediate and high magnitude earthquakes happened.Restricted by the inverting result of focal mechanism,we got the characteristics of continuous stress field in the Yunnan according to the finite element numerical simulation.The result shows that the present stress field is characterized by clockwise rotation.The reason was that the special material of the upper mantle and the lower crust in eastern Himalaya structure made the area of Yunnan as a preferential path when the Indian Plate was pushing towards North,and then the compressive stress shifted towards southeast and formed a rotating stress field.