The latest surface rupture along the western segment of the Qilian–Haiyuan fault and its implications

The Qilian‒Haiyuan fault is a boundary fault with strike slipping in the Qilian Shan (Shan means mountain or mountain range in Chinese) on the northeastern Tibetan Plateau. Several strong earthquakes (M ≥ 7) have occurred in this fault zone, and the risk of future major earthquakes is highly concerning. We identified a seismic surface rupture zone of unknown age at the Halahu fault, which is located at the western end of the Qilian‒Haiyuan fault. The Halahu fault plays a crucial role in structural transformation at the westernmost point of the Qilian‒Haiyuan fault zone, and historical seismic records are vital for assessing the future seismic risk associated with the fault. We found via field investigation that the existing length of the rupture zone is more than 24 km, which is characterized mainly by cracks, the offset of the trench, and the offset of terrace edges. Twenty-eight left-lateral coseismic offsets were determined using field measurements and UAV image analysis, with a minimum offset of 0.6±0.1 m and a maximum offset of 2.5±0.2 m. Historical earthquake records confirm that the surface rupture zone was caused by the M6½ earthquake that occurred east of Hala Lake on July 14, 1930 east of Hala Lake; however, the magnitude of that earthquake should be revised to be approximately M6¾ on the basis of field survey data. Research has shown that major earthquakes have shifted from the Bayan Har active block to the Qinghai‒Tibet block boundary fault zone. This study identified and dated earthquake surface rupture zones, offering essential guidance for future research and assessments of fault earthquake risk. Our results indicate that the Halahu, Tuolaishan, and Jinqianghe‒Laohushan faults have high potential for strong future earthquakes.