At 1:45 on January 8, 2022, a MS6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province. The epicenter(37.77°N, 101.26°E)is located in the western segment of the Lenglongling Fault of the Qilian-Haiyuan fault zone, with a focal depth of 10km. The earthquake is located in the northwest of the MS6.4 Menyuan earthquake on January 21, 2016. According to the survey results of China Earthquake Administration, the highest intensity of this earthquake is IX degree, and the long axis of the isoseismic line is NWW-striking. The earthquake caused serious damage to the Daliang Tunnel between Haomen Station and Junmachang Station, and the Lanxin high-speed railway was interrupted. After the earthquake, the distribution of the earthquake surface rupture zone was quickly determined by interpreting the GF-7 satellite post-earthquake images, and the field surface rupture investigation was carried out at the epicenter site in the first time. The field investigation mainly includes the identification of surface rupture zones, the investigation of rupture characteristics, the survey of fault geomorphology, the high-precision aerial photogrammetry of typical rupture points, the identification and measurement of coseismic dislocation, and the investigation of earthquake disasters. Aerial photogrammetry realizes real-time difference through UAV linked network RTK, and takes high-definition photos from multiple angles. Pix4D software is used to complete calculation and point cloud encryption, etc. DSM (Digital Surface Model) and DOM (Digital Orthophoto Map) are generated for surface rupture space reproduction and feature measurement and analysis. According to the interpretation of high-resolution remote sensing images by GF-7 satellite and field investigation, the surface rupture of MS6.9 Menyuan earthquake can be divided into NW-striking western segment of Lenglongling Fault and EW-striking eastern segment of Tuolaishan Fault. The two surface ruptures are 291° and 86.9°, respectively, and their lengths are not less than 26km and 3.5km respectively. We made detailed observation and measurement on the Jingyangling site, Daogou site, east Daogou site, Shixiamen site, the seven sites along the Liuhuanggou on the Lenglongling Fault, and the Yangchangzigou site on the Tuolaishan Fault. The surface rupture zone is mainly a complex coseismic surface deformation zone formed by the combination of multiple types of fractures, such as tensional fracture, tensional shear fracture, compression bulge and seismic depression, and characterized by sinistral strike-slip motion and partly by thrusting. Generally, the NW-striking ruptures exhibit left-lateral strike-slip characteristics, while NW-striking branch ruptures exhibit a small amount of right-lateral strike-slip characteristics. At Shixiamen site, four pasture fences were continuously offset left-laterally by 2.0~2.15m. At the Daliang Tunel site, the rut was offset left-laterally by 2.77m measured by UAV, which is the largest co-seismic left-lateral displacement of this earthquake. Based on high-resolution remote sensing image interpretation, field investigation, InSAR inversion of focal mechanism, fault rupture model and small earthquake precision location, it is determined that the earthquake occurred at the deep intersection of the Tuolaishan Fault and Lenglongling Fault, and the main seismogenic structure is the western segment of Lenglongling Fault(strike 112°, dip 88°). The Tuolaishan Fault on its west side ruptured simultaneously at the east end. According to the distribution characteristics of the surface ruptures and the field investigation of this earthquake, we believe that the Lenglongling Fault continues to extend westward after passing through the Liuhuanggou No. 1 site until the Jingyangling site, and the NWW-striking Lenglongling Fault has a “Y”-shaped contact relationship with the EW-striking Tuolaishan Fault. The 1986 MS6.4 earthquake occurred at the northwestern end of the Lenglongling North Fault, which protrudes in an arc toward NE, and the 2016 MS6.4 earthquake occurred at the southeastern end of the fault. Affected by the left-lateral strike-slip movement of the Lenglongling Fault, the small block bounded by the Lenglongling Fault and the Lenglongling North Fault also moves in the direction of SEE relative to the northern block. Therefore, the 1986 MS6.4 earthquake showed tensile properties, and the 2016 MS6.4 earthquake showed compression properties. The seismogenic structure of the Menyuan MS6.9 earthquake is the Lenglongling Fault, so the earthquake is mainly characterized by left-lateral strike-slip. The MS6.4 earthquake in 1986, MS6.4 earthquake in 2016 and MS6.9 earthquake in 2022all occurred in the western section of Lenglongling Fault. Three strong earthquakes of M>6 occurred in a short period of time, indicating that this area is still an accumulation area of stress and deformation, and has the potential risk of large earthquakes.
Due to the limitation of the data range of the Gaofen-7 satellite image and the inconvenience of traffic caused by the icing of the river, the location of the easternmost end point of the rupture and the exact length of the rupture have not been determined in this field investigation. We hope that follow-up studies will be carried out to confirm the rupture length when weather conditions are appropriate.