Characteristics of co-seismic surface rupture zone of Menyuan M(S)6. 9 earthquake in Qinghai Province on January 8, 2022 and seismogenic mechanism

At local time 01 :45 on January 8, 2022, an earthquake of M(S)6. 9 occurred in Menyuan County (N37. 77 degrees, E101. 26 degrees), Haibei Prefecture, Qinghai Province, with a focal depth of 10 km. Based on the detail post-earthquake field investigation, it was confirmed that the epicenter of Menyuan M(S)6. 9 earthquake was located at the regional structural transformation area of Lenglongling fault and Tuoleshan fault,which were the part of middle-to-west segment of Qilian-Haiyuan fault, both faults were left-lateral strike-slip active faults in Holocene. The earthquake formed two obvious surface rupture zones with a total length of about 31 km. The northern main surface rupture mainly distributed along the western section of Lenglongling fault zone, starting from head of the Liuhuanggou in the east, passing through Daogou westward further ending at the lower Daquangou, with a length of about 22 km. Combining field measurement and verification by high-resolution UAV image, the maximum horizontal dislocation confirmed was 2.6 +/- 0.3 m in the middle segment of northern rupture, which decreased gradually to the two ends. From the field observation, it is concluded that the macro epicenter is located in the area between great bend of Liuhuanggou and the east of Daogou. The southwestern secondary rupture zone distributed on the partial eastern segment of Tuoleshan fault, which started from Daquanwo in the east to Yangchangzigou in the west and ends up at Daxigou, with a length of about 9 km, the maximum horizontal dislocation about 1.0 +/- 0.1 m. Two surface ruptures displayed left-stepped with a minimum 1.0 km wide distance. Overall, the slip behavior of the surface ruptures in the M(S)6. 9 earthquake was mainly left-lateral strike-slip with a slight thrust component. The patterns of surface rupture were typical and abundant, each secondary fracture was consisted of combination of echelon left-lateral tension or right-lateral compression, showing the typical strike-slip dislocation landforms, such as left-lateral dislocated small gullies, riverbed, pastoral barbed wire fence, road subgrade, ruts, sidewalks and animal footprints, etc. Besides, the surface rupture was also characterized by a series of typical deformation phenomena such as compressive ridges or mole tracks, tensile cracks, fault scarps, and so on. Furthermore, by comprehensive analysis of the co-seismic surface ruptures and the deep structure inferred from aftershock activity of Menyuan earthquake, the result shows that the seismogenic structure should be mainly the western segment of Lenglongling fault and the partial eastern segment of Tuoleshan fault. The Y-shaped bifurcated co-seismic surface rupture style between the regional structure transformation area was the joint action of these two faults. The 2022 Menyuan earthquake is a strong earthquake occurred on the Lenglongling main active fault after the Menyuan M(S)6. 4 earthquakes in 1986 and in 2016, both occurred on the northern sub-fault of Longlongling. Therefore, more attention should be paid to the potential larger earthquake activity in the Qilian-Haiyuan fault zone, especially in the west segment in the future.