Paleo-earthquake Of Zemuhe Fault System

Paleo-earthquake research based on the records of strong earthquake on the fault to discover problems such as: the evolvement of the fault, recurrence of strong earthquake using geology survey. As a proverb of Confucius states"Study the past if you would divine the future",paleo-earthquake research is one of the most important methods on active structures and provides clues on earthquake prediction.Sichuan-Yunnan block lies south-east to Tibet Plateau, is under the compression between India block and Eruasia block and restricted by Ganzi-Yushu Fault zone, Xianshuihe Fault zone, Anninghe Fault zone, Zemuhe Fault zone, Xiaojiang Fault zone, Honghe Fault zone and Jinshajiang Fault zone. (Li Ping et.al.,1975;Kan Rongju et.al.,1977;Xu Xiwei et.al. 2003;LǚJiangning et.al.,2003).Zemuhe Fault is sinistral strike slip fault developed between Anninghe Fault zone and Xiaojiang Fault zone and now is the north-east boundary of Sichuan-Yunnan Block.We carry out our work along Zemuhe fault zone, and try to get further on the fault kinematics and the recurrence of this fault based on the precise geomorphology and combining with RS interpretation and trench array.1,Offset of geological block according to the RS interpretationWe can try to find the relationship between geomorphology and fault through research on Earthquake-Geomorphology. We need new methods to survey related earthquake geomorphology. We can get the relative distance between the base station and moving station by Difference GPS(DGPS) (David L.Higgitt et al,1999), we needn't worry about relief reflection on our survey like total station.Zhang Xianfeng(Zhang Xianfeng,2000) and Wang Chunrui(Wang Chunrui,2002) have made some DGPS survey on field survey and beach survey. In this dissertation we try to do some survey on earthquake geomorphology to get some data on active faults research.Using DEM to get the 3D map, analysis the topography and form of the basin, and finally we get the interpretation map of Wudaoqing, Sishiluomo,and Songxin. We get the offset data from 1.9m to 1800m along Zemuhe fault, among them the data of several meters such as 1.9m is due to one earthquake, the data of several ten meters is due to several earthquakes, and the offset of several hundreds even up to thousands meters is corresponding to the terraces of the main river of this region for long time activity of this fault.2,Quantitative measurement by virtue DGPSRelationship of topography and fault evolvement is the foundation of quantitative geomorphology. In this thesis we measured several typical places along the fault by using virtue GPS. Finally we get big scale geomorphology map between DaqingLiangzi and Wudaoqing, data of mole track at Daqing village, terrace data and offset map of gullies at Sixiabushi village,and build the 3D model of the fault. We find that offset of 1.9m was due to earthquake of 1850, tens of meters offset were due to several times of earthquakes, and the offset of hundreds of meters were corresponding to main rivers and terrace in this region.3,Paleo-earthquakeAt Daqing village the biggest mole track was offset to two mole tracks, between them a little sag pond about 1m wide was formed. And there is another big sag pond developed south of it at the turn point of the gully. This big sag pond was formed due to the mole track, the mole track block the gully and make the gully turn at this point, and the sag pond deposition start at the northern bank of the gully. Along with the movement of the fault, sediment area of the sag pond move southern.We can get several layers material contains carbon of the sag ponds of different epoch. This time we make the trench array of three trenches. TC1 cover the little sag pond from east to west,TC2 and TC3 are parallel to TC1 and south of it about 2m and 4m. We discover 17 different layers in the three trenches and get 11 14C samples and 1 OSL(Optical stimulated luminescence, OSL) sample. Finally we get three events by analysis the geology evidence from the trench, according to the age data the three events are E1: Xichang earthquake of 1850,with the evidence of wedge⑿; E2: late than 3160±60a ago, the wedge②being the evidence; E3: early than 5470±70a,with the evidence of sequence from③-⑥. The interval are about 3000a and 2300a,so we get the recurrence interval of M7.5 at the same place are about 2300～3000a.