关键词: 活动断层, 滑动速率, 古地震, 祁连山, 酒西盆地

Abstract: The Western Jiuquan (Jiuxi) Basin is located in the westernmost part of the Hexi Corridor. The basin is bounded by the Qilian Mountain fault on the south, by Alytn Taugh fault on the north, and by Jiayuguan fault on the east, respectively. The Hexi Corridor is one of the seismically active regions in western China. According to historical records, a large number of strong earthquakes had occurred in this area. Recently, we have discovered three Holocene active faults through the interpretation of aerial photos and field investigation in the Jiuxi basin. These three faults are called Xinminpu, Yinwashan and Yumen faults, respectively. The Xinminpu fault is a Holocene thrust fault, which is 17km in length, striking 315°and dipping southwest, located in the northern part of the basin. A fault scarp of 14m height was developed on the hanging wall of the fault, and it is superposed by the newly formed fault scarp with free surface. The rate of vertical motion along the fault is determined to be 0.24mm/yr. The Yinwashan fault is a Holocene thrust fault located on the alluvial fan at the eastern piedmont of the Yinwashan Mountain, striking 315°with a length of 17km and dipping southwest. The rate of vertical motion along the fault is determined to be 0.18mm/yr. Two Holocene paleoearthquake events have been identified through trenching on the fault. The first event occurred 10.64±0.83ka B.P., while the second event occurred between 4.09±0.31ka B.P. and 8.22±0.63ka B.P. The Yumen fault is also a Holocene thrust fault, which is nearly EW-striking and south dipping, located on the alluvial fan at the northern piedmont of the Qilianshan Mountain. A fault scarp of less than 2m height was developed along the fault. The fault scarp was perhaps produced by a historical earthquake. The rate of vertical motion along the fault is determined to be 0.25mm/yr. Two Holocene paleoearthquakes were revealed by trenching on the fault. The first earthquake occurred at 3.05±0.24 ～3.20±0.25ka B.P. The second occurred after 3.05±0.24ka B.P. As mentioned above, all the three Holocene faults belong to thrust fault, and thus no obvious horizontal displacement can be observed along the fault. This may indicate that this area is dominated by compressional deformation. According to historical records, the Jiuquan earthquake of 756 A.D. is the latest historical earthquake occurred in this area. It is postulated that the Xinminpu fault or Yumen fault would be ruptured during this earthquake, but currently we are unable to determine which fault was ruptured by this earthquake on the basis of available dating data. The ages of paleoearthquakes and the characters of surface ruptures along the three faults suggested that the three faults were activated independently or sometimes in cluster.

Key words: Active fault, Slip rate, Paleoearthquake, Qilianshan Mountains, Western Jiuquan basin