Study On New Features Of Tectonic Activity And Seismic Hazard At The Middle-eastern Segment Of Western Qinling Fault Zone

West Qinling strike-slip fault zone is a seismic sensitive fault which located in the northeastern margin of Tibetan Plateau [1-4]. The topography of this area is complex and active faults are densely distributed, which imply that Cenozoic tectonic activity is ongoing. Many destructive earthquakes have been recorded in historical materials on this fault, such as Gangu West M7 earthquake in AD 143, Tianshui M7 earthquake in AD 734[5-6] and Kangle M6? earthquake in AD 1936, et al[7], so its strike-slip behavior and future large earthquake risk are highly concerned. Previous studies on this fault mainly concentrated on quantitative parameters such as fault geometry, slip rate, paleoearthquakes of the western part of West Qinling fault[7-9], however, relatively less works and more disputes exists on the eastern segment because of geological and other kinds of factors [10-12]. So, this paper focus on the most recent tectonic activities of Wushan – Tianshu segment and Tianshui- Baoji segment of the fault zone in eastern part of West Qinling fault, we research geological and geomorphological evidence of most recent activity, and we acquire its quantitative parameters, finally, we discuss its future earthquake risk. The main conclusions are as follows:(1) Submeter satallite image of Wushan- Tianshui segment exposes clear linear expressions. Field investigation reveals abundant geological and geomorphological expressions of most recent activity, including faulting controlled valleys, fault scarps, synchronous sinistral offseted ridges and streams, left laterally dislocated rivers and their terraces. Secondary expressions including densely distributed landslides and pitfalls on both sides of fault. Through trench excavation and sample testing, Buzi village trench in Wushan County shows the latest event occurred after 4.21 ± 0.45 ka B.P., Wujiahe trench in Gangu County shows a 710 ± 30 a B.P. event, and Maoji village trench in Tianshui shows earthquake about 1210 ± 30 a B.P. Comprehensively analysis show that Wushan- Tianshui fault is a late Holocene fault. Through geomorphic mapping by small unmanned aircraft vehicles(sUAV) and differential GPS(DGPS), and sample testing by radiocarbon dating(C-14) and optical stimulated luminescencedating(OSL), strike slip rate of puzi village in Wushan is estimated to be 2.7 ± 0. 5mm / a since Holocene, Wangjiazhuang in Wushan is 2.17 ± 0.16 mm / a since Late Pleistocene, Wujiahe in Gangu is 0.35 ± 0.026 mm / a since the late Pleistocene, Maoji village in eastern end of Wushan-Tianshui fault is 0.35 ± 0.02 mm / a since the late Pleistocene, strike slip rate on Wushan-Tianshui fault decreases eastward.(2) East of Tuoshi town on Tianshui- Baoji fault exposes perfect linear trace, while west of Tuoshi fault trace are not so clear for thicken covered loess along Weihe valley. Fault perfomances include fault controlled valleys, fault scarps, U-shaped valleys, sinistral offseted ridges and gullies. Trench studies reveal Holocene activity of Tianshui-Baoji fault. Yanghe village in Tianshui actived about 6970 ± 370 a.B.P., Shangmatouliang in Baoji show the latest event range from 1780 ± 30 a B. P. to 2440 ± 30 a B.P. SUVA Mapping and smaple testing get a slip rate of ? since Holocene.(3) Paleoearthquakes and historical earthquakes exists on Western Qinling fault. Four events could be roughly determined: event Ⅰis ~ 3500 ± 550 a B.P. occurred on Wushan secondary faults section; events Ⅱis ~ 2110 ± 330 a B.P. occurred on Tianshui- Baoji fault; event Ⅲ is 1210 ± 30 a B.P. occurred on the southern secondary section of the Tianshui fault, which coincides with historical records of AD 734 Tianshui M7-7? earthquake; event Ⅳ is 620 ± 40 a B.P. occurred on Gangu fault segment, roughly coincide with 1765 Gangu 6? earthquake. Paleoearthquakes and historical earthquakes on western Qinling fault varied by segment, and show clustering and periodic perfornance to a certain extent. The latest event on south Tianshui fault occurred in AD 734, it has passed 1282 years, the destructive earthquake risk is relatively high. The greatest earthquake risk place is on Tianshui-Baoji fault, where no destructive earthquake recorded in historical materials, and the latest paleoearthquake event has passed over 2100 years, so more concern needed.