Abstract:

On March 19, 2021, an earthquake of M_W5.8 occurred in Ruju County, Tibet. The epicenter of the earthquake is located to the west of Xiaqu town in the Qiangtang Basin in the north of the Qinghai Tibet Plateau. The regional structure around the earthquake is complex and there are many faults developed there. To the south of the epicenter is Peak Shaqiongya and to its west is Peak Ceduo, both are high-altitude peaks. Since 1970(up to July 22, 2021), 27 earthquakes with M_W≥5 have occurred within 200km of the area, most of which are of normal faulting, indicating that tensile stress plays a dominant role in the area. The largest earthquake is the Naqu earthquake with M_W5.9 that occurred near the Bengco Fault in 1972. Therefore, this earthquake is a rare medium strong earthquake in this area. Studying the seismogenic structure of this earthquake has reference significance for understanding the geological structure, fault movement characteristics and seismic rupture attributes of normal faulting earthquakes in this area. In this paper, the coseismic deformation field of this earthquake is obtained by using D-InSAR technology. It shows that the maximum uplift and subsidence are 5cm and 6cm respectively, the long axis direction is NE, and the LOS deformation signs of ascending and descending orbit are the same. It is preliminarily determined that the seismogenic fault is a normal fault, striking NE. On this basis, based on the SDM software and Okada method, the initial seismogenic fault is constructed with a length of 27.56km(92.78°~93.00°E), a width of 20km(set in the SDM program), a dip angle of 50°~63°, and a rake angle of -150°~0°; the smoothing factor is set to 0.08, and the maximum number of iterations is 10000. Secondly, the fault dip angle of the selected range is tested with a step of 1°, and the optimal fault dip angle is determined to be 55°. On this basis, a single fault model is used to divide the fault plane into 1km×1km fine slip distribution model of 1km sub fault structure. Finally, the distribution characteristics of coseismic slip on fault plane are inversed by SDM. For T143 of ascending orbit, the residual RMS is 1.0cm, and for T77 of descending orbit, the residual RMS is 0.4cm. The combined ascending/descending residual range is -0.016~0.015m, and the residual RMS is 0.3cm. Through comprehensive analysis, the model fitting of ascending orbit data is good, which indicates that the inversion result of slip distribution is reliable. According to the determined optimal parameters of the fault, the fine slip distribution model of the fault is drawn. The sliding surface presents an elliptical centralized distribution as a whole, and the maximum sliding amount reaches 0.2m; The average rake obtained by inversion is about -55.56°, the moment magnitude is M_W5.8, the macro epicenter is located at(31.94°N, 92.85°E)and the seismogenic fault is a normal fault with left-lateral strike-slip component.
By setting the friction coefficient to 0.4, taking the Young’s modulus as 3×10¹⁰ and Poisson’s coefficient of 0.25, the coseismic Coulomb stress changes triggered at different depths are calculated with column algorithm at intervals of 5km. The results show that the coseismic Coulomb stress with the seismogenic fault as the receiving fault has the coseismic effect mainly in two directions: One is the NE-SW direction, which is mainly manifested as the Coulomb stress increase in the North Nierong Fault at the southern edge of the Qiangtang block and some sections of the Bangonghu-Nujiang fault zone, and the influence range decreases with the increase of the depth. The other direction is NW-SE, and the Coulomb stress decreases and changes with the increase of depth. The Coulomb stress image of 5km underground shows the alternating distribution of positive and negative stress changes in strike-slip earthquakes. The area with the largest increase in stress is located in the northeast of the Biru earthquake and the stress is partially released. The stress increase at 5km northeast of the epicenter of Biru earthquake turns to stress decrease at 10km northeast of the earthquake, the stress increase area decreases in the northeast and expands in the southwest of the main shock. At the depth of 15km, the stress in the north of the epicenter further decreases, and the stress in the southwest of the epicenter further increases. At the same time, the stress decrease area is relatively large, and the stress decreases in most areas near the epicenter of the earthquake. The slip amount of the fault in the deep is very small, so the magnitude of stress increase at the depth of 20km is relatively small, and the corresponding stress decrease area is relatively large. On the whole, most of the subsequent earthquake events occur at the depth of 5~15km, which is consistent with the stress increase area at the corresponding depth. At the same time, the stress increase at the depth of 5~20km is located at the south and north end of the rupture zone, and its ΔCFS≥0.01MPa, so the seismic risk is worthy of attention. Based on the comprehensive analysis, this paper calculates and analyzes the variation and distribution characteristics of Coulomb stress by taking the North Nierong Fault and the Bangonghu Nujiang fault zone as the receiving faults. The Coulomb stress generated by the earthquake in the depth of 5~20km is calculated by using the above two receiving faults, which is mainly negative. However, the earthquake has obvious Coulomb stress loading effect on the west section of Bangonghu-Nujiang Fault and the east section of North Nierong fault zone. Through comprehensive analysis, the Biru earthquake has a certain static stress loading effect on the above two receiving faults, and there has been no strong earthquake on the two faults for a long time, which needs attention. The Qinghai-Tibet Plateau is composed of several sub blocks. The plate movement is dominated by horizontal and vertical uplift in the east-west direction, and many large active fault zones are developed along the block boundaries. This earthquake is located in the Qiangtang block in the central part of the Qinghai-Tibet Plateau.
There are many conjugate shear faults developed in the block. Among them, the Bangonghu-Nujiang fault zone is a large-scale near EW-strike fault zone at the southern boundary of Qiangtang Basin. According to the interpretation of remote sensing images and historical geological data, medium strong earthquakes of M_S5~6 occurring in this area in the history are all located at the intersection of the NE trending fault. The epicenter of the M_W5.8 Biru earthquake is located at the intersection of the NE-trending Bangong Lake-Nujiang fault zone and the NW-trending North Nierong fault zone. This paper considers that the occurrence of this earthquake is related to the tectonic environment where the Qiangtang block locates, and the slip type of the seismogenic fault has the typical characteristic of seismic rupture inside the Qiangtang block. Judging from the continuous interference fringes of the deformation field of the ascending track, the coseismic rupture did not reach the surface. At the same time, the coseismic surface deformation of this earthquake is concentrated around the North Nierong Fault and Bangong Lake-Nujiang fault zone, which are 9km and 10km away from the earthquake respectively, and the seismogenic fault dips to the northwest. According to the historical research data of the study area, this paper preliminarily believes that the seismogenic fault of this earthquake is a NE-direction hidden secondary fault located in the north of the west section of the Bangong Lake-Nujiang fault zone. The fault activity is mainly of normal faulting with a small amount of strike slip. The relationship between this fault and the main fault needs to be determined comprehensively based on the on-site seismo-geological survey.

Key words: Biru earthquake, InSAR, coseismic deformation, fault slip, static Coulomb stress

摘要：

文中运用D-InSAR技术获取的2021年3月19日西藏比如县M_S6.1 地震的同震形变场显示: 升、 降轨LOS向同震位移场的长轴为NE向, 其中最大抬升量和沉降量分别约为5cm和6cm; 在此基础上, 基于Okada模型反演断层面的精细滑动分布特征。反演结果表明近场残差得到有效控制, 其中发震断层参数为: 走向228°, 为SE倾向断层, 宏观震中位于(31.94°N, 92.87°E), 矩震级为 M_W5.8, 平均滑动角为-56.42°, 最大滑动量达0.2m, 倾角为55°。最后, 运用Column33软件计算以反演断层为接收断层的不同深度的同震库仑应力变化, 结果显示比如地震震中附近产生了明显的应力降, 深部断层滑动量很小。后续地震事件通常发生在5~15km深度, 这与相应深度的应力增加区域一致; 以班公湖-怒江断裂和聂荣北断裂为接收断层的应力变化显示, 比如地震在2条断裂的部分区域产生了应力加载, 库仑破裂应力值ΔCFS>0.01MPa, 需要引起关注。结合地表形变观测资料和前人的研究成果初步认为, 比如 M_W5.8 地震的发震断层为NE向隐伏次级断裂, 位于班公湖-怒江断裂带西段北侧, 断裂活动方式以正断为主, 兼具少量走滑分量, 发震断层与主断层的关系需要通过野外地质调查资料综合确定。

关键词: 比如地震, InSAR, 同震形变场, 断层滑动, 静态库仑应力