SEISMOLOGY AND GEOLOGY ›› 2018, Vol. 40 ›› Issue (2): 310-322.DOI: 10.3969/j.issn.0253-4967.2018.02.002

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SPATIO-TEMPORAL STRESS FIELD VARIATION IN THE CENTRAL LONGMENSHAN FAULT ZONE AFTER THE 2008 WENCHUAN EARTHQUAKE FROM FOCAL MECHANISM SOLUTIONS OF SMALL EARTHQUAKES

JIANG Min1, CHEN Jiu-hui1, Yasuto Kuwahara2, Reiken Matsushita2   

  1. 1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China;
    2 Active Fault and Earthquake Research Center, AIST, Japan
  • Received:2018-02-13 Online:2018-04-20 Published:2018-06-01

利用小震震源机制解研究汶川地震后龙门山断裂带中段应力场时空演化

江敏1, 陈九辉1, Yasuto Kuwahara2, Reiken Matsushita2   

  1. 1 中国地震局地质研究所, 地震动力学国家重点实验室, 北京 100029;
    2 Active Fault and Earthquake Research Center, AIST, Japan
  • 通讯作者: 陈九辉,男,研究员,E-mail:chenjh@ies.ac.cn
  • 作者简介:江敏,男,1989年生,2015年于中国地震局地质研究所获地球物理专业硕士学位,主要研究方向为宽频带地震学和深部探测研究,电话:010-62009151,E-mail:min_jiang1989@163.com。
  • 基金资助:
    国家自然科学基金(41274060)与科技部国际科技合作项目(2011DFG23400)共同资助

Abstract: We determined the focal mechanism solutions(FMS)of aftershocks of the 2008 Wenchuan earthquake using the waveform data recorded by the Western Sichuan movable seismic array. We further obtained the spatio-temporal variation of the stress field by inverting the stress tensors from these data. The results show that the FMSs of the small earthquakes are primarily reverse faulting with considerable number of normal and strike-slip. The proportion of reverse type earthquakes clearly increases with time, and the spatial distribution of the FMSs is closely related with local geology and the characteristics of the deep faults. The stress tensor inversion results reveal that the orientations of the maximum horizontal compressive stress(SH)shortly after the mainshock in each area are mainly in EW to NWW-SEE directions, while in the area along Xiaoyudong-to-Lixian aftershock branch it is in NE-SW direction in shallow crust. This implies that the stress field at the early stage after the Wenchuan earthquake is mainly controlled by the dynamic stress change caused by the rupture propagation, and the conspicuous factor determining the SH direction is the characteristics of the rupture plane. Temporal variations of the stress tensors show that the stress regimes at depth changed from a mixture of reverse and strike-slip faulting to pure reverse, implying that local Coulomb stress caused by the main shock is released through strike-slip faulting and gradually recovers into the background stress field. The change of stress in the shallow subsurface follows that in the deep subsurface with observable time delay.

Key words: Longmenshan fault zone, focal mechanism, stress field, spatio-temporal variation

摘要: 利用川西流动地震台阵观测资料,求解汶川地震后发生在龙门山断裂带中段地区的余震震源机制解,反演了相应的应力场,得到了应力场的时空演化特征。结果显示,小震震源机制以逆冲为主并含有一定数量的走滑和正断型地震,逆冲型地震的比重随时间明显增加。震源机制类型的空间分布与断层深部展布及区域速度结构具有相关性。应力张量反演结果显示早期余震各分区水平最大主压应力SH方位主要分布在近EW—NWW范围内,沿小鱼洞—理县方向余震分支地壳浅部主压应力则沿NE-SW向,表明震后早期应力场主要受主震破裂传播引起的动态应力影响,SH方位与破裂面特征密切相关。应力张量随时间的变化显示深部应力结构由逆冲、走滑混合向纯逆冲转变,表明深部区域通过走滑型地震来平衡初期应力释放造成的局部库仑应力,并逐渐恢复到接近背景应力场,浅部应力变化追随深部变化特征,显示出在时间上的滞后。

关键词: 龙门山断裂带, 震源机制, 应力场, 时空演化

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