| 2017年西藏米林MS6.9地震发震构造初探 |
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| 引用本文： | 王林,田勤俭,李文巧,赵妍,徐岳仁.2017年西藏米林MS6.9地震发震构造初探[J].地球物理学报,2019,62(7):2549-2566. |
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| 作者姓名： | 王林 田勤俭 李文巧 赵妍 徐岳仁 |
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| 作者单位： | 中国地震局地震预测研究所, 地震预测重点实验室, 北京 100036 |
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| 基金项目： | 中国地震局地震预测研究所基本科研业务费专项（2015IES010202、2017IES010105）和雅鲁藏布江下游地区区域构造稳定性研究（A404HX-2015-003）共同资助. |
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| 摘 要： | 2017年11月18日在西藏米林发生了MS6.9地震，目前尚未发现地表破裂带，发震构造尚不明确.震源机制解表明该次地震为逆冲型地震.精定位结果显示余震集中在加拉白垒东北坡上一个NW走向的长约36 km、宽约8 km的狭长条带之内.余震条带的走向及长度严格受到派乡构造岩片NE边界走向及长度的控制，垂直于该条带的地震剖面清晰地揭示出一条倾向NE的低倾角逆冲断层面，结合震源机制解及GPS同震位移场的已有结果，初步推断它可能就是发震断层面.雅鲁藏布江大拐弯上游加拉-米林河段两岸的湖相基座阶地面和山脊线在南迦巴瓦、加拉白垒脚下都发生了倾向SW的翘起变形，发震断层面构成了其上盘加拉白垒、南迦巴瓦强烈隆升区与其下盘地貌发生翘起变形的弱隆升区的分界面，推断加拉白垒峰沿着这一断层面不断地逆冲、隆升，以此来调节其两侧的不均匀挤出，而下盘近断层处的褶皱、拖曳等作用逐渐造成了阶地面、山脊线的翘起、弯曲变形.基于夷平面的区域变形分析，认为雅江缝合带作为主干断裂带从整体上控制着印度板块与欧亚板块在东构造结地区的碰撞-挤压格局.印度板块东北犄角的强烈顶撞引起了东构造结附近强烈的断块运动，嘉黎断裂带北侧的地壳显著增厚，主夷平面随之发生裂解.与此同时，由于碰撞带来的强烈挤压，派乡构造岩片、多雄拉变质穹隆沿着缝合带大拐弯内侧不均匀地挤出，南迦巴瓦、加拉白垒随之隆升.此次的米林地震仅仅是该不均匀挤出过程所引发的一次具体的事件，是派乡构造岩片内部的一条次级断层发生的一次逆冲运动造成的.此外，紧邻此次余震条带的南迦巴瓦NEE边界以及SE边界是一个潜在的地震空区，其未来地震危险性值得关注.
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| 关 键 词： | 米林地震 逆冲断层 河流阶地 夷平面 构造变形 发震构造 |
| 收稿时间： | 2018-01-09 |
Preliminary investigation of the seismogenic structure of the 2017 MS6.9 Milin earthquake in Tibet |
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| WANG Lin,TIAN QinJian,LI WenQiao,ZHAO Yan,XU YueRen.Preliminary investigation of the seismogenic structure of the 2017 MS6.9 Milin earthquake in Tibet[J].Chinese Journal of Geophysics,2019,62(7):2549-2566. |
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| Authors: | WANG Lin TIAN QinJian LI WenQiao ZHAO Yan XU YueRen |
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| Institution: | Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting, China Earthquake Administration, Beijing 100036 |
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| Abstract: | The MS6.9 Milin earthquake occurred at November 18,2017 in Tibet. The seismic surface rupture has not been found until now, and the seismogenic structure is also uncertain. The focal mechanism shows that the Milin earthquake is mainly caused by thrust structures. The relocated result shows that the aftershocks concentrate in a narrow stripe along the northeast slope of the Gyala Peri with 36 km length and 8 km width. The trend and length of the aftershock stripe is restrictly controlled by the NE boundary of the Paixiang tectonic slice. The seismic profile perpendicular to the aftershock stripe clearly reveals that a low angle NE-trend thrust plane existing below the aftershock distribution zone. Combined with the known results of the focal mechanism and the GPS co-seismic displacement field, we preliminarily infer that this low angle thrust is the probable seismogenic fault plane. Along both sides of the Jiala-Milin river segment in the upstream of the Yarlung Zangbo gorge, two levels of rock-base terrace with lacustrine deposit and the ridges continually tilt and turn up towards SW on the foot of Namcha Barwa and Gyala Peri. The seismogenic fault plane can be considered as the boundary between the Namcha Barwa-Gyala Peri strongly uplifting zone on its hanging wall and the weekly uplifting zone on its footwall where the terraces and the ridges have tilted and deformed. We infer that the Gyala Peri has been thrusting and uplifting continually along this low angle thrust plane to accommodate the unevenly squeezing out on both sides of it, and the folding and dragging near the fault on the footwall has caused the tilting and bending of the terrace and the ridges. Based on the regional deformation of the planation surface, we think that the Indus-Yalung Zangbo suture zone (IYS) is an main fault belt which totally controls the whole collision-compression framework of the India Plate and the Eurasian Plate at the eastern Himalayan syntaxis. The strong collision at the NE corner of the India Plate has caused intensive faulted-block movement around the syntaxis. The crust on the northern side of the Jiali fault has obviously thickened, and the main planation plane has also been disassembled accordingly. At the same time, due to the strong compression resulting from the collision, the Paixiang tectonic slice and the Duoxiong-La metaphorical dome have unevenly extruded up from the inside of the IYS gorge, while Namcha Barwa and Gyala Peri have also uplifted accordingly. The Milin earthquake is only one events during this unevenly squeezing-out process of the Paixiang tectonic slice, which is caused by a thrusting movement of a secondary fault in the Paixiang tectonic slice. Besides, the NEE and SE boundary of the Namcha Barwa close to the Milin aftershock stripe is a potential seismic gap, and its seismic risk deserves our attention. |
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| Keywords: | Milin earthquake Thrust fault River terrace Planation surface Tectonic deformation Seismogenic structure |
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