Abstract:

On May 22, 2021, an M_S7.4 earthquake occurred in the Madoi area of Banyan Har block, with a focal depth of about 8km. The seismogenic fault is deduced as the Jiangcuo Fault, a branch of the east Kunlun strike-slip fault. Different with previous strong earthquakes which located at the boundary faults around the Bayan Har block, the Madoi M_S7.4 earthquake occurred inside the block and about 70km away from the boundary fault. Furthermore, there is a contradiction between the small strike-slip component of the seismogenic fault and the large earthquake magnitude. The above phenomena indicate that the Madoi earthquake may have special seismotectonic background and seismogenesis. Strong earthquakes in Tibetan plateau are always closely related to the deep crustal structure and dynamic process. Therefore, it is of great significance to study the crustal structure and the distribution of deep faults in the Madoi area in order to reveal the deep tectonic background and genesis of the Madoi M_S7.4 earthquake. To research the deep seismotectonic environments of the M_S7.4 Madoi earthquake, we reinterpret the deep seismic sounding(DSS)results in Madoi area. The DSS profile reveals fine crustal structure beneath the Madoi area, and divides the crust into 3 crustal layers. From the crustal velocity structure of the Madoi and adjacent area, we found the generation of the Madoi earthquake is closely connected with the deep structure and crustal medium. Through analysis on the velocity structures, we get the following understanding: 1)There is an interface in the upper crust of the Madoi area, which represents the velocity changing from 5.8km/s to 5.6km/s and divides the upper crust into two layers. The upper layer is composed of high velocity structure, indicating a brittle medium environment, while the lower layer consists of low velocity zone and provides the strain accumulation condition for the Madoi earthquake. In addition, the transition between local high velocity zone(HVZ)and the normal crust in the focal area provides an ideal medium environment for earthquake preparation. 2)A wedge-shaped low velocity zone(LVZ)exists in the lower crust south of Madoi, which provides an environment for the movement of weak materials from the SW to NE direction. However, the high-velocity lower crust beneath Madoi area resists the crustal flow and thus transforms the horizontal movement to vertical upwelling, resulting in the stress concentration of the upper crust beneath Madoi area, which may provide dynamic for the preparation of the Madoi M_S7.4 earthquake. 3)The Jiangcuo Fault merges into the East Kunlun Fault in the deep crust, forming a reverse thrust fault structural style dominated by the East Kunlun strike-slip fault. As a branch of the East Kunlun Fault, the strike slip of the Jiangcuo Fault is the adjustment results of strain and movement of the East Kunlun Fault. Moreover, the Jiangcuo Fault and adjacent faults constitute the horsetail-shaped fault zone, combined with the imbricated thrust fault zone profile, reflecting the compressive stress of Modoi area that facilitates the strain concentration. Therefore the occurrence of the Madoi earthquake is related to the left-lateral strike-slip movement of the East Kunlun Fault and the special imbricated thrust fault assemblages. On the other hand, the upwelling of the lower crustal flow and the corresponding sliding of the upper crust may be related with the occurrence of the Madoi earthquake. In conclusion, the Madoi M_S7.4 earthquake is closely related to the ideal medium environment of the upper crust, the lower crustal flow and vertical upwelling beneath Madoi area, as well as the left-lateral strike-slip of the East Kunlun Fault.

Key words: Madoi M_S7.4 earthquake, crustal velocity structure, medium environment of earthquake preparation, lower crustal flow, seismotectonic background

摘要：

2021年5月22日, 巴颜喀拉块体内部的玛多地区发生M_S7.4地震, 震源深度约为8km, 发震断层为江错断裂。 文中利用玛多地区的深地震测深结果, 对玛多M_S7.4地震震源区的地壳速度结构和深部构造背景进行了研究。 结果显示: 1)玛多地震震源在深度上位于上地壳脆性层内, 具体位于上地壳局部高速体的边缘, 上地壳特殊的上下分层结构和局部的高速异常体为玛多地震的孕育提供了介质环境。 2)玛多以南地区的深部地壳存在楔状低速体, 为深部软弱物质向NE运动提供了环境, 而玛多地区高速的下地壳阻挡了软弱物质的运移而使其垂向上涌, 导致了玛多地区上地壳应力的集中, 这可能为此次玛多地震的孕育提供了深部动力。 3)江错断裂在深部归并到东昆仑断裂上, 在剖面上构成以东昆仑走滑断层为主的反向逆冲断裂构造样式, 因此玛多地震的发生与东昆仑断裂的左旋走滑运动存在一定联系; 此外, 下地壳流的上涌也可能在一定程度上促进了玛多地区上地壳的水平滑动以及地震的发生。 综上所述, 此次玛多M_S7.4地震的孕育和发生, 与玛多上地壳特殊的介质条件、 中下地壳物质的挤压流动和垂向上涌以及东昆仑主走滑断裂的水平运动密切相关。

关键词: 玛多M_S7.4地震, 地壳速度结构, 地震孕育介质环境, 下地壳流, 地震构造背景