Studies On The Geometry,Kinematics,and Paleoseismology Of The Yushu Fault

Ganzi-Yushu fault belt is the northwestern continuation of the Xianshuihe fault system,Southeastern Tibetan Plateau.Previous studies are mainly focused on the eastern segment of the Ganzi-Yushu fault belt,which is also maned as Ganzi fault.By contrast,studies on the Western of the Ganzi-Yushu fault belt,which is also maned as Yushu fault,are scarce due to that the Yushu fault is thought to be inactive in early investigation.After the Yushu earthquake occurred in 2010,many research groups carried out lots of studies on this earthquake including field investigation of the earthquake surface rupture zone,fast inversion of rupture process of the earthquake,co-seismic deformation field and parameters inversion of the Yushu earthquake from In SAR,and survey of secular slip rate across the fault obtained from GPS data.However,studies on the geometry,kinematics and recurrence of strong earthquakes along Yushu fault are weak.Firstly,based on interpretation of satellite imagery,field geological and geomorphological investigation as well as trace of co-seismic surface rupture of Yushu earthquake,we obtain the distribution of geometrical structure of Yushu fault.Secondly,we apply unmanned aerial vehicle(UAV)photography and radiocarbon dating(14C)and optically stimulated luminescence dating(OSL)to study several presentative faulting sites to get the quaternary slip rate along Yushu fault.Thirdly,by excavating trenches across the fault and combining with previous similar studies,we determined several pale-earthquake events,discussed the recurrence of strong earthquakes and compared the pale-earthquake events along differnet segment of the Yushu fault.At last,our dissertation comes to the following conclusions:(1)Yushu fault emerge with Ganzi fault at Shango village to the east.It pass through Jiegu town and Ganda village with a strike of NWW,then terminating at the western of Jielong basin.The total length of the Yushu fault is 140 km.We divide it into three segments which distribute as left en-echelon distribution according to its geometrical features.They are Jielong segment to the west,Jiegu segment in the middle,and Changu segment to the east.Jielong basin is cira 30 km in length and 5-6km in width.It can be severed as a geometrical segmentation zone because of its tectonic barrier.But the co-seismic surface rupture of Yushu earthquake in2010 distributed along Jielong segment and Jiegu segment are not all produced by the Ms7.1 earthquake event which is the mainshock of the Yushu earthquake.We argue that the rupture along Jielong segment is related to Ms6.3 earthquake which is the largest aftershock of the earthquake in 2010.The geometrical characteristic between Jiegu segment and Changu segment is Jiegu basin which is 5km in length and 1.5km In windth.The Jiegu basin is passed through by surface rupture of the Ms7.1 earthquake event.The geometrical characteristics of the ends of the Yushu fault are a compressional stepover which is about 25 km in width.to west and a fault branch to east.The stepover is defined by the Yushu fault and the Dangjiang fault.The fault branch ismade of the Yushu fault and the southern boundary fault of Batang basin.Combined with tectonic analog,deformation partitioning determination and historical surface rupture analysis,we suggest that both of the irregular geometrical structure located at the ends of the eastern and western Yushu fault are highr level geometrical boundary area than Jielong basin.Because they have a larger scale and ability to serve as a strong tectonic barriers.Therefore,we infer that Yushu fault is an individual rupural segment.(2)We chose two representative faulting sites to study the slip rate along the Yushu fault.The results show that the slip rate of the Yushu fault is 6.6±0.1 ~7.4±1.2mm/a.Compared with that of the Ganzi fault,we find there is a discrepancy between them.The reason for this discrepancy is that there is a slip partitioning along the Ganzi fault during its westward transfer process of deformation,which transfer partial deformation to Southern boundary fault of Batang basin.The existence of Southern boundary fault of Batang basin explain why the slip rate along Yushu fault is less than Ganzi Fault.Considering the regional deformation,however,the tectonic deformation from eastern to western segment of the Ganzi-Yushu fault belt is consistent since late Quaternary.And if we concede that the slip rate of Xianshuihe fault belt is~10mm/a,Ganzi-Yushu fault belt is consistent with Xianshuihe fault belt in horizontal tectonic deformation.They constitute the lateral extrusion and clockwise boundary of Chuandian block and modify the crustal deformation of southeastern Tibet Plateau.(3)We excavated two trenches along Jiegu segment of Yushu fault at Guoqing village and western Luorongda village.We interpreted the occurrence of three paleo-seismic events through trench analysis and dating.Combined with previous paleo-seismic investigations within a few kilometers,we obtained five paleo-seismic events since 9ka in all,their occurrence are,from oldest to youngest,E1:~7.5ka BP,E2:5300~5420a BP,E3:4120~3990a BP,E4:~2210a BP,E5:in2010s.Based on the occurrence of theses earthquake events,we calculated the recurrence of them.The results are E1-E2:2080~2200a,E2-E3:1180~1430a,E3-E4:1780 ~ 1910 a respectively,suggesting recurrence interval of strong earthquake events is relatively long and random.In addition,we compared the comparable paleo-seismic events along Jiegu segmeng and Changu segment of the Yushu fault.Compared with the analysis of the geometrical characteristics along the Yushu fault,and investigation of the co-seismic displament of the surface rupture zone,we conclude that the earthquake occued in 2010 is not a characteristic slip earthquake.