COSEISMIC SURFACE DEFORMATION AND SLIP MODELS OF THE 2021 MS6.4 YANGBI(YUNNAN, CHINA)EARTHQUAKE

doi:10.3969/j.issn.0253-4967.2021.03.014

Abstract

Abstract: Due to the ongoing collision between Indian and Eurasian plates, the internal blocks of the Tibet plateau are experiencing eastward extrusion. Resulting from the blocking of the Sichuan Basin along the eastern boundary of the Bayanhar block, the plateau begins to rotate clockwise around the eastern syntaxis, and continues to move toward the IndoChina Peninsula. Such process forms the Hengduan Mountains with thousands of gullies in the Sichuan-Yunnan region, and generates major earthquakes across the entire Red River Fault, where infrastructures and residents are seriously threatened by the frequent earthquakes. InSAR observations feature a high spatial resolution and short intervals, ranging from several days to over a month, depending on the satellite revisit period.
On May 21, 2021, an earthquake struck the Yangbi city. This event provides a rare opportunity to look at the local tectonic and seismic risk in the north of the Red River Fault. We processed the Sentinel-1 SAR data with D-InSAR technology and generated the surface deformation caused by the Yangbi M_S6.4 earthquake occurring on May 21, 2021. Due to the abundant vegetation and moisture in Yunnan, significant atmospheric noise needs to be corrected for the derived InSAR displacement field. The results show a maximum deformation of~0.07m in line-of-sight for ascending track and~0.08m for descending track. The quality of interferogram on the ascending track is low, and only one of the quadrans can be distinguished, the rest of the interferogram is regarded as phase noise. However, the descending interferogram contains two deformation regions, with its long axis roughly along the NW-SE direction. The northeast part of interferogram moves towards the satellite, while the southwest part moves away from the satellite. The InSAR interferograms pattern shows a right-lateral strike-slip movement. Then, we combined coseismic displacement data obtained from the Global Navigation Satellite System(GNSS)and InSAR(both the ascending and descending)to invert the coseismic slip model of the Yangbi earthquake. The inversion test shows that our data cannot give strong constraints for the dip orientations, and the two slip models with opposite dip orientation can explain the observations within the noise level. No matter what the dip orientation is, the slip models show that the coseismic slip concentrated at depth of 2~10km, with a maximum slip of~0.8m, which corresponds to a moment magnitude of M_S6.4, and is consistent with body-wave-based focal mechanism. But the relocated aftershocks in 3 hours immediately after the mainshock reveal a SW-dipping fault plane 10km away to the west of Weixi-Qiaohou-Weishan Fault, we therefore conclude that the Yangbi earthquake ruptured a SW-dipping dextral fault, which is previously unknown. To analyze the effects of the Yangbi earthquake on the seismic risk of the regional dextral faults, we estimated the Coulomb stress change caused by our preferred slip model. The Coulomb stress at 7.5km depth is negative, indicating stress unloading, while the Coulomb stress at 15km depth is positive, indicating slightly loading, but still less than the empirical triggering threshold. The results indicate that Yangbi earthquake partially relieved the strain accumulated on the nearby faults, thus restraining the seismic risk of these faults.

Key words: Yangbi M_S6.4 earthquake, coseismic deformation field, fault slip distribution, coseismic Coulomb stress, seismic hazard

摘要： 2021年5月21日云南漾濞县城西侧发生M_S6.4地震。文中以欧洲航空局升、降轨Sentinel-1 SAR为数据源, 基于D-InSAR技术获取了此次地震的InSAR同震形变场: 升轨LOS(Line of Sight)向最大形变量约为0.07m, 降轨最大LOS向形变量约为0.08m。以升、降轨同震形变场数据和GNSS数据为约束, 反演了断层滑动分布。反演结果表明, InSAR/GNSS数据对断层倾向的约束能力较弱, NE倾向和SW倾向2种断层模型都能够在误差范围内拟合观测数据, 其中: SW倾向断层模型的最大滑动量约为0.8m, NE倾向断层模型的最大滑动量约为0.6m; 其余参数, 如滑动角为-170°、倾角为80°、矩震级为M_W6.07均一致, 破裂也都集中在地下2~10km深度范围内。结合主震发生后3h内的余震精定位结果分析, 我们认为倾向SW的发震断层模型可能更符合实际情况。根据断层滑动模型、同震滑动分布并结合破裂运动学特征, 初步判定发震断层为维西-乔后-巍山断裂以西的一条次级断裂。基于反演得到的断层滑动模型计算了漾濞地震破裂导致周边断裂的库仑应力变化情况, 结果表明漾濞地震对维西-乔后-巍山断裂、红河中段断裂和红河北段断裂的南段具有显著的应力卸载作用, 初步认为漾濞地震之后周围断裂的地震危险性有所缓解。

关键词: 漾濞地震, 同震形变场, 断层滑动分布, 同震库仑应力, 地震危险性

CLC Number:

P315.2

WANG Shao-jun, LIU Yun-hua, SHAN Xin-jian, QU Chun-yan, ZHANG Guo-hong, XIE Zhao-di, ZHAO De-zheng, FAN Xiao-ran, HUA Jun, LIANG Shi-ming, ZHANG Ke-liang, DAI Cheng-long. COSEISMIC SURFACE DEFORMATION AND SLIP MODELS OF THE 2021 M_S6.4 YANGBI(YUNNAN, CHINA)EARTHQUAKE[J]. SEISMOLOGY AND GEOLOGY, 2021, 43(3): 692-705.

王绍俊, 刘云华, 单新建, 屈春燕, 张国宏, 解朝娣, 赵德政, 范晓冉, 华俊, 梁诗明, 张克亮, 代成龙. 2021年云南漾濞M_S6.4地震同震地表形变与断层滑动分布[J]. 地震地质, 2021, 43(3): 692-705.

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