Late Quaternary Activity Of The Tazang Fault

2001 Kunlun Earthquake,2008 Wenchuan Earthquake, 2010 Yushu Earthquake occurred at the edge fault of Bayan Har Block after 2000 because of the movement of the Bayan Har Block.Tazang fault is one of the gap of the edge fault at the North-eastern edge of the Bayan Har Block.Its activity characteristic is close related with the motion state of the Bayan Har Block. This researth focus on the Tazang fault, through the analysising of the late quaternary active characters and existing research results of edge faults of the Bayan Har Block to analysis the geometry, segmention of the Tazang fault and the block movement background of the Wenchuan earthquake and so on.The paper gets the next results:(1)The research get the geometry and the subsection of the Tazang fault through remote sensing image interpretation, DEM anlysising, field investigation: Tazang fault initiated at north of Luocha village Ruo er gai county, eastward goes through Xia huangzhai village, Zhai jiu village, Dongbei village, Tazang village, Jiu zhaigou, Tangzhou, Miaozhai, Majiamo, Xia wujiao,up to Sha gali.The full length is approximately 150km. The strike trend is NW.(2) There are three Layered geomorphic surfaces about Tazang fault Luocha segment. The first Layered geomorphic surface is first terrace of the gully, the second Layered geomorphic surface is the second terrace of the gully, the first Layered geomorphic surface age is (0.66±0.04)ka BP, the second Layered geomorphic surface age is (4.15±0.12)ka BP, the third Layered geomorphic surface is (22.41±2.63)ka BP through the age samples test collect from The river terraces floodplains phase silty sand layer. Evry Layered geomorphic surfaces and evry level gully have been displaced,it means Tazang fault active many times and active for a very long time. Field investigation find many temporary gullies were displaced for 2-5.7 meters, Pull-apart sag in the surface, Geese column rupture in surface turf can be recognized.It means the form time of the surface ruptures is short and it must in hundreds years. The Left-lateral strike-slip rate of west part of Luocha segment is 6.5-10.3mm/a through the test of the second Layered geomorphic surface displacement and the age sample. Comprehensive the rate and the single displacement, the Recurrence period of the earthquake big than 7 grade on Tazang fault is 553-876a.(3)Tazang fault can be subsection into Luocha, Dongbei, Tazang, Majiamo four segments according to the change of the strike trend and the fault geometry discontinuous. According to the contrast analysis of the west and east part of the Tazang fault Luocha segment, find that the Tazang fault Luocha segment actived major in strike-slip way on western part, but the main active way is changed from left-strike to thrust to eastern part of Luocha segment, the ratio is 5:1.As the north-east edge fault of the Bayan Har Block, the active characteristic change is the geological evidence of the SEE diretion movement in west part of Bayan har block change to large part of vertical movement at the eastern part of the Bayan har block.