龙门山断裂带西南端地壳电性结构

doi:10.3969/j.issn.0253-4967.2014.02.002

摘要/Abstract

摘要： 在龙门山断裂带中段于2008年5月12日发生了汶川M_S 8.0地震，5a之后于2013年4月20日在其西南侧即龙门山断裂带SW段发生了芦山M_S 7.0地震。而在汶川地震前，沿龙门山断裂带主体部分存在7a间未发生4.0级以上地震的相对平静期。因此，汶川地震后人们研究了龙门山断裂带的地壳结构及其与汶川地震的成因关系，仅仅相隔5a时间，就在龙门山断裂带的SW段发生了芦山地震，其深部结构和孕震环境以及与汶川地震的关系又成为人们关注的热点科学问题。为了研究龙门山断裂带西南端附近的地壳结构，布置了一条穿越龙门山断裂带西南端附近的大地电磁探测剖面LS6，该剖面位于芦山地震破裂带的西南端。通过采用先进技术对大地电磁数据的分析和二维反演，发现LS6剖面与其东北侧的穿过芦山地震区汶川地震后完成的LMS4剖面的地壳电性结构既有相似性，但也存在明显的差别，其电性结构更复杂。研究表明，尽管2008年发生了汶川地震，但是龙门山断裂带受到的西北侧松潘-甘孜地块向SE的运动和对龙门山断裂带的推挤作用，以及东南侧四川盆地的阻挡作用仍然存在，同时龙门山断裂带西南端及其附近地区的地壳结构更复杂，而且还受到其西南侧川滇地块和鲜水河断裂等变形作用的影响，因此推测芦山地震与汶川地震既是相互独立的2次地震，但也有一定关联。

关键词: 龙门山断裂带西南端, 大地电磁, 地壳电性结构, 芦山地震, 地震危险性

Abstract: On 20 April 2013, the Lushan M_S7.0 earthquake happened in the southwestern segment of the Longmen Shan Fault zone, which is a shock only five years after the devastating 2008 Wenchuan M_S8.0(M_W7.9) earthquake that ruptured the northeastern section of the same fault zone. At least seven years before the Wenchuan event, the most parts of this tectonic zone looked seismically quiet, without M>4.0 quakes, apparently consistent with the low slip rates along the fault evidenced in geological research and GPS measurements. The occurrence of these two major shocks has stimulated researchers to explore what is the cause, particularly the deep conditions, that made the Longmen Shan Fault zone wreak subsequent havoc within five years.To probe the crustal structure beneath Longmen Shan, a magnetotelluric (MT) profile LS6 was deployed for data collection, which runs through the southwestern fault trace and facilitates comparison with another MT profile LS04 that crosses through the Lushan seismic area to the northeast. Using the advanced methods, MT data processing and 2D inversion have been carried out on both the two profiles. The comparison shows that the profile LS6 seems to reveal a more complex electric structure of crust with respect to that from the profile LS04, though both have similarities to some extent. The formation of such a deep structure is associated with the southeastward motion of the Songpan-Ganzi block in the northwest, which is hampered by the Longmen Shan Fault zone and by the rigid Sichuan Basin farther to the southeast. It is also influenced by the motion of the Chuandian block as well as active faults, such as the Xianshuihe Fault in the southwest. The occurrence of the 2013 Lushan earthquake implies a subsequent release of the elastic strain accumulated within the Longmen Shan Fault zone, which is an independent event to the 2008 Wenchuan earthquake, though both have some relationship between each other.

Key words: the southwest end of the Longmenshan Fault zone, magnetotellurics, electric structure of the crust, Lushan earthquake, seismic risk

中图分类号:

P542⁺.5

王立凤, 赵国泽, 詹艳, 陈小斌, 肖骑彬, 赵凌强, 王继军, 乔亮, 韩冰. 龙门山断裂带西南端地壳电性结构[J]. 地震地质, 2014, 36(2): 302-311.

WANG Li-feng, ZHAO Guo-ze, ZHAN Yan, CHEN Xiao-bin, XIAO Qi-bin, ZHAO Ling-qiang, WANG Ji-jun, QIAO Liang, HAN Bing. ELECTRIC STRUCTURE OF CRUST BENEATH THE SOUTH- WESTERN SEGMENT OF THE LONGMENSHAN FAULT ZONE[J]. SEISMOLOGY AND GEOLOGY, 2014, 36(2): 302-311.

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