Analysis on the characteristics of surface rupture and regional seismicity trend of Mengyuan M6.9 earthquake in Qinghai Province

The National Seismological Network officially determined that an earthquake of magnitude 6.9 occurred in Menyuan County, Haibei Prefecture, Qinghai, at 01:45 on January 8, 2022, with a focal depth of 10 km. This earthquake is another strong earthquake activity in the Lenglongling area after the 2016 Menyuan M6.4 earthquake. The macro-epicenter of the earthquake was located in the Liuhuanggou area of ​​Lenglongling, more than 50 kilometers northwest of Chenghaomen Town, Menyuan County, and a large-scale and well-continued surface rupture was formed in the area of Liuhuanggou−Daxigou. Surface surveys show that the total length of coseismic surface ruptures is about 23 km, and the overall trend is N40°~85°W. Surface rupture is mainly composed of echelon earthquake bulge, tensile crack and shear crack, and there are many secondary geological disasters such as landslides and collapses on the surface. According to the scale, strike and fracture characteristics of the surface rupture, it can be divided into three sections: the eastern section (Liuhuanggou section), about 10 km long, strikes N40°~60°W, the scale of the rupture is small, and it is accompanied by gravity. The middle section (Daogou section), about 9 km long, strikes N70°W, with a large scale of rupture, mainly developed large-scale seismic bulges and shear fractures, and large left-handed displacement; The western section (Daxigou section) is about 4 km long and strikes N85°W. This section is the smallest in scale and is dominated by extensional fractures in the geese. The middle and eastern sections together form the eastern branch of the fracture zone, while the western section forms the western branch, both of which have obvious left lateral strike slip characteristics, and it ruptures from east to west, showing a left step distribution as a whole, forming a left step area with tensile characteristics east of G227 National Highway. To the east, a left-order region with extensional features is formed. Comprehensive analysis shows that the earthquake occurred in the Qilian−Haiyuan active tectonic belt of the Qilian Mountains block, and the seismogenic fault should be the Lenglongling−Tuolaishan fault section of the Haiyuan left-lateral strike-slip fault belt. Combined with the preliminary research on the strong earthquake sequence of the Qilian−Haiyuan tectonic belt since 1900 and the Tuolaishan fault, it is believed that the historical seismicity of the tectonic belt as a whole has a trend of continuous westward development, but there is an existing trend between the Hala Lake and the Tuolaishan Mountain. There is a more obvious seismic gap, so it is inferred that the future strong earthquake risk of the Tuoleishan Mountain fault is likely to increase.