The Holocene Activity Evidence of the Yema River-Daxue Mountain Fault in Western Qilian Mountain

Altyn Tagh fault controls the deformation characteristics of the northern margin of the Qinghai‐Tibet Plateau. The sinistral slip rate of the eastern segment of the fault reduces gradually where the reduction transforms into the deformation within Qilian Mountain, forming a series of thrust faults and strike‐slip faults. Among them, the Yema River‐Daxue Mountain fault is one of the important structural transform faults in the study area. Based on the differences of the geometrical characteristics and activities, the fault is divided into four segments, the Yema River segment, the Shibandun segment, the Liushapo segment and the Baishitougou segment, among which the former three are Holocene active faults, and the Baishitougou segment belongs to late Pleistocene fault. The excavated trenches imply a total of 6 paleoearthquake events, and at least 4 events have occurred during Holocene, whose occurrence times are 8300±700 yr BP, 6605±140 yr BP, 4540±350 yr BP, 2098±47 yr BP, respectively. The recurrence interval is 2600±600 yr BP that is close to the lapsed time of the last one, 2098±47 yr BP, which suggests that the Yema River‐Daxue Mountain fault is in a high risk of major earthquakes in the future. The vertical coseismic displacements of the four Holocene paleoearthquake events are 100 cm, 42 cm, 40 cm and 50 cm, respectively, the horizontal coseismic displacements are 5 m, 4.5–5.5 m, 5–8 m and 4–5.5 m, separately, and then the reference magnitude of the paleoearthquake events is conjectured to be M7.6±0.1.