THE SEISMOGENIC ENVIRONMENT OF THE 2013 MINXIAN-ZHANGXIAN MS6.6 EARTHQUAKE BASED ON THE DEEP ELECTRICAL STRUCTURE

doi:10.3969/j.issn.0253-4967.2015.02.016

Abstract

Abstract:

On July 22, 2013, an M_S6.6 earthquake occurred at the junction of Minxian and Zhangxian. After the earthquake, magnetotelluric(MT)measurement was carried out at 45 sites along the NE-oriented profile across the West Qinling orogen(the west segment)and the earthquake area. Remote reference, "robust", and phase tensor decomposition techniques were used to process the MT data, and the NLCG two-dimensional inversion method was adopted to get the deep electrical structures. The deep electrical structure images indicate that there exists an inverted trapezoidal high-resistivity layer in the West Qinling orogenic belt(west segment)at the depth from the surface to about 20km deep, which is shallow in the northeast and southwest and deep in the middle. Under the high-resistivity layer is a low-resistivity layer, and they conjoin each other. There is a low-resistivity layer in the Songpan-Ganzi block(north part)at the southwest side of West Qinling orogenic belt(west segment)under the depth of 20km in the lower crust, which is shallow in the northeast and deep in the southwest, and the Longxi Basin at its northeast has a stable layered structure, suggesting that West Qinling orogenic belt(west segment)is being subject to the northward extrusion of the Songpan-Ganzi block and southward resistance of the Longxi Basin. The East Kunlun Fault(Tazang segment)faulted the low-resistivity layer in the lower crust of Songpan-Ganzi block. The Diebu-Bailongjiang Fault and Guangaishan-Dieshan Fault zone extend to a shallow depth and merge into the East Kunlun Fault(Tazang segment)in the deep part. The characteristic of low-resistivity of the media in the deep-seated structures in the East Kunlun Fault(Tazang segment)is the underlying cause for the gradual decrease of horizontal slip rate and gradual increase of vertical movement of the Tazang segment. The West Qinling Fault is a main geoelectric boundary zone, which extends through the Moho; Lintan-Tanchang Fault zone behaves as a low-resistivity layer with a certain width, which extends into the low-resistivity layer in the mid to lower crust. The source region of Minxian-Zhangxian M_S6.6 earthquake locates in the core of inverted "trapezoid" of the low-resistivity layer in the West Qinling orogenic belt(west segment), that is, in the contact area between the high to low resistivity layers, and also in the low-resistivity fractured zone near the Lintan-Tanchang Fault. The interaction of southwest-northeast pushing from Songpan-Ganzi block and resistance of Longxi Basin block at its northeast is external dynamics of the Minxian-Zhangxian M_S6.6 earthquake, and the high- and low-resistivity medium property and their contact relation in the seismic source region of the earthquake are the internal factor to generate this earthquake.

Key words: Minxian-Zhangxian M_S6.6 earthquake, magnetotellurics, low-resistivity layer, West Qinling orogenic belt

摘要：

2013年7月22日甘肃岷县漳县交界处发生M_S6.6地震后,横跨西秦岭造山带和地震区沿NE方向的剖面进行了45个大地电磁测点的观测。使用远参考和 "Robust"技术以及相位张量分解技术处理数据, 采用NLCG 2维反演方法, 获得的深部电性结构图像揭示: 西秦岭造山带自地表至深度约20km存在东北和西南浅、中部深的倒 "梯形"高电阻体, 在高阻体之下为低电阻层, 高、低电阻层相互契合; 西秦岭造山带西南侧的松潘-甘孜地块(北部)在深度约20km存在西南深、东北浅的中下地壳低阻层, 其东北侧的陇西盆地具有稳定的成层性结构, 显示出西秦岭造山带正处于松潘-甘孜地块向北挤压和陇西盆地向南的阻挡挤压作用中。东昆仑断裂带(塔藏段)错断了松潘-甘孜地块中下地壳低阻层, 迭部-白龙江断裂和光盖山-迭山断裂带延伸深度不大, 在深部归并于东昆仑断裂带(塔藏段), 东昆仑断裂带(塔藏段)内部结构和介质的低阻特性是东昆仑断裂带在塔藏段水平滑动速率逐渐减小、垂向运动逐渐增强的深层原因。西秦岭北缘断裂为陡立的大型电性边界带, 延伸深度穿过莫霍面; 临潭-宕昌断裂带表现为具有一定宽度的低阻带, 延伸深度归并到中下地壳低阻层中。2013年甘肃岷县漳县6.6级地震震源区处于倒 "梯形"高阻体的西秦岭造山带的核部, 即位于高低电阻体接触区, 同时发生在低阻破碎带的临潭-宕昌断裂带附近。松潘-甘孜地块从SW向NE推挤、东北侧陇西盆地阻挡的相互作用是2013年岷县漳县M_S6.6地震发生的动力学原因, 岷县漳县地震震源区特殊的高低阻介质属性和接触关系是该次地震发生的内部因素。

关键词: 岷县漳县M_S6.6地震, 大地电磁, 低阻层, 西秦岭造山带

CLC Number:

P315.72⁺2

ZHAO Ling-qiang, ZHAN Yan, ZHAO Guo-ze, CHEN Xiao-bin, YANG Hao, JIANG Feng. THE SEISMOGENIC ENVIRONMENT OF THE 2013 MINXIAN-ZHANGXIAN M_S6.6 EARTHQUAKE BASED ON THE DEEP ELECTRICAL STRUCTURE[J]. SEISMOLOGY AND GEOLOGY, 2015, 37(2): 541-554.

赵凌强, 詹艳, 赵国泽, 陈小斌, 杨皓, 姜峰. 基于深部电性结构特征的2013年甘肃岷县漳县M_S6.6地震孕震环境探讨[J]. 地震地质, 2015, 37(2): 541-554.

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