A Paleoeathquake Study On The Moxi Fault And Tarwan-Chijiaciwo Fault At The Margin Of Tibetan Plateau

In order to recognize the the differences of geomorphic characteristics, paleoseismic phenomena and the research methods in different environments of active faulst, in this work, I choose two different research areas. The first is the Moxi fault and Kangding fault, the south east segment of the Xianshuihe fault on the south east edge of the Qinghai-Tibat plateau; the second is the Tarwan-Dengdegnshan-Chijiaciwo fault nearby Yumen town at the noth-east edge of the Qinghai-Tibat plateau and the most west part of the Hexi corridor. The Xianshuihe fault is located at the west plateau of Sichuan province. Because of intense incision, the mountains are high and vegetation cover is dense, but the Qaternary is aplastic, so it is difficult to study the active fault here. The Tarwan-Chijiaciwo fault is situated at the north east boundary of the Qinghai-Tibet Pateau and the most west part of Hexi corridor nearby Yumen town. Since the beginning of the Quaternary time, the Hexi coerridor took place strong sedimentation and deep Quaternary deposited. On the earth surface, the low hill and complanate desert are distributed abroudly, so the active tectonic phenomena can be sonserved perfectly. It is helpful to choose the segment to excavate trenches and observe carefully. In this work, through the observation of the two different types of active faults, I select the feasible place to excavate trenches to make research of paleoseimology and the active characteristics of those faults.The study contents and the used techniques of this work are summarized below.1) Data collection and interpretation for the faults and field geological reconnaissance along the faults in order to select suitable locations for research on active faults or paleoearthquake.2) Survey of displacement on the faults. On the Moxi, Kangdong fuaults, 1 selected the places where the active physiognomy maintained well, such as Ertaizi, Yuejinping and Xinyulin to measure the terrain and excavate the trenches. I have selected the suitable locations of the Tarwan-Dengdengshan-Chijiaciwo fault, and survey the fault scarp, in order to memerize the displacement physiognomy characters.3) Analysis of active phenomena and paleoseimic events on the profile of the trenches, collections of the age samples to test in the chronology laboratory, to establish list of the paleoearthquakes, analyze active characters of the faults.The main results of the work are as follows:1) Acorrding to the observation of the fault landforms along the Moxi and Kangding fault, the appearance of the fault belt is clear, and the structure is simple relatively. There are only several embranchments at Xinxing-Caiyangping-Ertaizi and Xinmin. At Puziba, the scarp between small trenches PT1 and PT2 is the fault scarp. The measured height of the scarp is about 3m, and the caculating averge vertical slip rate with the known age of the stratum is about 3.0mm/a.2) In this work, there are 3 paleoseismic and historic earthquake events have been revealed on the Moxi fault. The first event occurred in 502±183a B.P., the second occurred in about 332±35 a B.P., The third one is the A.D.1786 M73/4 earthquake event.3) At Xinyulin of Kangding, I have determined 3 paleoseismic events,and their occurrence time respectively are 595±35a B.P.,3050±40 a B.P. and 2628±40 a B.P., and the average reccurrence interval is about 486a. This trench is the only one excaved on the Kangding fault in this work. According to previous research, when the Kangding-LudingMoxi M73/ 4 earthquake occurred, unfortunately the trace on the trench profile might be destroyed by the stream, but it is possible to recognize the event from the micro-physiognomy around the trench.4) According to the landform characteristic it can be judged it is a Holocene active fault. Because of the uplift of the Tibetan plateau, there took place many times of sintense and important thrusts on its north edge, and the last thrust made the Yumen conglomerate fold intensely, even standing erect. The Tarwan fault lies just at this place. The Yumen conglomerate's waterproof performance is so excellent that it can prevent the draining of the water on the scarp, then the vegetation can grow massively. According to the sedimentology, it is known that the sand flowing with the wind will form scarp once it is prevented by vegetation. According to that, it can be inferred that the Tarwan fault scarp is not formed by tectonic movement directly.5) Along the Dengdengshan-Chijiaciwo fault segment, there are 6 paleoseismic events since middle Pleistocene, and their occurrence time are 354.18ka B.P.,152.3ka B.P., 77.56ka B.P., 64.27 ka B.P., 19.79 ka B.P. and 10ka, respectively. The reccurrence intervals range from 13.29ka to201.88ka.6) By analysis of the active ciecs of the fault exposed in the trenches, it can be concluded that on the Tarwan-Chijiaciwo fault, the activities of the Tarwan segment, Dengdengshan segment and Chijiaciwo segment became stronger from west to east gradually. Judging from the physiognomy of scarp, the Tarwan fault segment should be a Hocene active fault, but there is no evidence to prove that from the excaved trenches, so it is preliminarily concluded that it is early-middle Pleistocene fault. The Dengdengshan fault segment may be a forepart active fault of the late Pleistocene; the Chijiaciwo fault segment is a Hocene active fault.