Surface rupture investigation of the 2022 Menyuan M_S 6.9 Earthquake, Qinghai, China: Implications for the fault behavior of the Lenglongling fault and regional intense earthquake risk

The M_S 6.9 Menyuan earthquake on January 8, 2022 occurred in the Qilian Mountain fault block in the northeastern margin of the Tibetan Plateau. The instrumental epicenter is located in the Lenglongling fault zone in the west section of the Haiyuan active fault system. It is another strong earthquake with M>6.5 that occurred in the fault system after the Haiyuan M 8.5 earthquake in 1920. The preliminary conclusion of the investigation results shows that the Menyuan earthquake generated two main fracture zones in the south and north separately, which are distributed in left-step oblique arrangement with a total length of nearly 23 km, and there is a surface rupture cavity with a length of about 3.2 km and a width of nearly 2 km between them. The south branch rupture (F₁) appears in the east section of the Tuolaishan fault, striking 91°, with a length of about 2.4 km. It is mainly characterized by southward thrust and sinistral strike slip, with a maximum horizontal displacement of nearly 0.4 m. The north branch main rupture (F₂) appears in the west section of the Lenglongling fault, with a total length of nearly 20 km. It is mainly sinistral strike-slip deformation, and presents an overall slightly convex arc distribution to the northeast, including the west, middle and east sections of 102°, 109° and 118°, respectively. The maximum strike-slip displacement occurs in the middle section, which is 3.0±0.2 m. In addition, a new north branch secondary fault (F₃) with a cumulative length of about 7.6 km, dominated by normal-dextral fault, is discovered on the north side of the middle to east section of the main fault of the north branch, with a cumulative maximum horizontal displacement of about 0.8 m and a maximum normal vertical displacement of about 1.5 m. Comprehensively, the whole coseismic rupture is mainly sinistral strike-slip and has the characteristics of bilateral rupture, whereas the macroscopic epicenter is located in the north branch of midway through the main fracture zone, thus the strike-slip displacement of the surface may be associated with the shallow rupture of the source. The normal-dextral rupture is likely to be caused by the differential movement of the north block dragged by the active south plate. The south Qilian block was extruded eastward along the Haiyuan sinistral strike-slip fault system due to the strong collision and compression between the Indian and Eurasian plates, which led to the simultaneous rupture of the Tuolaishan fault and the Lenglongling fault. This is the main dynamic mechanism leading to the strong earthquake. Under the background of continental dynamics, further attention should be paid to the future strong earthquake risk of the Qilian Mountain fault block and its surrounding area with the Haiyuan sinistral strike-slip fault system as the main boundary.