长宁地区复杂地震活动的力学成因分析  

Analysis on the mechanical causes of the complex seismicity in Changning area,China

作　　者：胡幸平 崔效锋 张广伟 王甘娇 Arno Zang 史丙新[4] 姜大伟 HU XingPing;CUI XiaoFeng;ZHANG GuangWei;WANG GanJiao;Arno Zang;SHI BingXin;JIANG DaWei(National Institute of Natural Hazards,Ministry of Emergency Management of China,Beijing 100085,China;Jiangxi Earthquake Administration,Nanchang 330039,China;GFZ German Research Centre for Geosciences,Telegrafenberg,14473 Potsdam,Germany;Sichuan Earthquake Administration,Chengdu 610041,China)

机构地区：[1]应急管理部国家自然灾害防治研究院,北京100085 [2]江西省地震局,南昌330039 [3]GFZ German Research Centre for Geosciences,Telegrafenberg,14473 Potsdam,Germany [4]四川省地震局,成都610041

出　　处：《地球物理学报》2021年第1期1-17,共17页Chinese Journal of Geophysics

基　　金：国家自然科学基金(42004042);中国地震局地壳应力研究所中央级公益性科研院所基本科研业务专项资助项目(ZDJ2018-01)资助.

摘　　要：长宁地区地震活动复杂,发生在长宁背斜轴部的长宁MS6.0地震及其后续中强余震的震源机制与相距仅十几公里的南部向斜区内的中强震具有显著差异;这种差异仅仅是由于发震断层构造不同所造成的,还是孕育地震的应力场本身也存在局部变化?为了回答这一问题,本文在通过双差层析成像反演修正小震定位和波速结构的基础上,利用小震综合震源机制解方法获取了长宁地区地壳应力场的精细结构,并据此分析了其与中强震震源机制解的力学一致性.研究发现,长宁地区地壳应力场的最大主应力轴在整个区域内基本都处于近水平状态,其方位虽由北向南发生了一定角度的顺时针旋转,但也基本保持为近东西向;相比之下,应力类型在长宁背斜轴部和南部向斜区之间存在显著差异——前者为逆冲型,而后者为走滑型.同时,这两个局部区域的应力场与对应区域内的中强震震源机制解的吻合度较高,但与非对应区域内的中强震震源机制解的吻合度较低,甚至在力学上是相抵触的,表明区域应力场的局部改变是长宁地区复杂地震活动的必要力学基础.通过岩石力学估算,本文认为岩石泊松比的横向差异很可能是造成这种应力场局部改变的主要成因.此外,本文还对长宁MS6.0地震序列的发震构造进行了探讨,认为长宁背斜轴部在6~9 km深度内存在基底断层,而正是这种基底断层在区域应力场作用下发生错动导致了长宁MS6.0地震序列的发生.The Changning MS6.0 earthquake and its moderate to strong aftershocks,which occurred in the axial area of Changning anticline,exhibit different features with the moderate to strong earthquakes that occurred in the southern syncline area nearby,although these two areas are only a dozen kilometers away.Whether this difference is only caused by different fault structures,or caused by local change is still a matter of debate.In order to answer this question,basing on the correction of small earthquake location and seismic velocity structure through double-difference tomography inversion,this paper determined the fine structure of the crustal stress field by using comprehensive focal mechanism solutions,and analyzed its mechanical consistency with the focal mechanism solutions of moderate to strong earthquakes.It is found that for the different subareas of Changning area,the maximum principal stress remains horizontal,and its azimuth remains almost East-West oriented although it has a small clockwise rotation from north to south;meanwhile,the stress regime exhibit a significant local change,which is thrust stress regime for the axial area of Changning anticline,against strike-slip stress regime for the southern syncline area.The stress fields in these two subareas highly accord with the focal mechanism solutions of moderate to strong earthquakes in the corresponding area,but poorly accord with,or even contradict that in the non-corresponding area,which indicates that the local stress change of stress field is the necessary mechanical basis for the complex seismicity behavior in Changning area.Additional rock mechanics analysis points out that lateral difference of Poisson‘s ratio of rock is probably the main cause of this local change of stress field.This paper also discusses the seismogenic fault structure of Changning MS6.0 earthquake sequence,and suggests the activation of the basement fault(s)in the depth range of 6 to 9 km in the axial area of Changning anticline driven by regional stress field is the potential

关 键 词：长宁地区 复杂地震活动 应力场局部变化 岩石泊松比 基底断层 

分 类 号：P315[天文地球—地震学]