The Recent Activity Features Of Faults And Risk Trend Of Strong Earthquake In Liupanshan Region

The Liupanshan region is an important part which belongs to the northern sectionof the north-south seismic structural zone, and according to the geological structure, itis located in the transitional region between the Ordos block of the western NorthChina Plate and the Qilianshan orogenic belt of the northeastern Qinghai-Tibet block.There is a great theoretical and practical significance to intensively research theLiupanshan region because of its special tectonic location and background of strongearthquakes. In this thesis, we took the Liupanshan western piedmont fault, theLiupanshan eastern piedmont fault and the Longxian-Baoji fault zone as the target forthe study. The recent active characteristics of these fault zones mentioned above sincethe late Pleistocene is studied and the relevant quantitative parameters of the recentactivity is obtained. In addition，historical data about the AD1219Guyuan earthquake,AD1306Guyuan earthquake and AD600Qin-long earthquake are analyzed and thefield investigation is carried out. Finally we comprehensively estimated the future riskarea of the strong earthquake in the Liupanshan region.(1) The total length of the Liupanshan western piedmont fault is~53km. Thefault extends southward from the Sanlidian reservoir to the Taoshan reservoir, then itstrend changes from SN to NNW-SSE. The fault topography is very clear along thefault. Analyzing the geomorphological features and typical fault section of the faultcomprehensively, we believed that the Liupanshan western piedmont fault is ahigh-angle reverse one, and it once moved in the early late Pleistocene, but hasn’tnewly dislocated since the middle late Pleistocene.(2) The Liupanshan eastern piedmont fault can be generally divided into theSunjiazhuang-Kaicheng segment, the Kaicheng-Xiangshuidian segment and theXiangshuidian-Majiaxinzhuang segment. The fault geomorphological characteristicsare distinct along the Sunjiazhuang-Kaicheng segment. The left-lateral slip rate of thisfault segment is3.2±0.5mm/a. Through studying the section at the Haizixia reservoir,we confirmed that the fault had evidently moved since4ka BP. The faultgeomorphological features are clear along the Kaicheng-Xiangshuidian segment too, and the fault is clearly reverse one. Compared with the northern section, theleft-lateral features are gradually weak in this fault segment. TheKaicheng-Xiangshuidian segment is a reverse fault with strike-slip component whosehanginwall relatively eastward moves. The left-lateral slip rate of theKaicheng-Xiangshuidian segment is averagely3.2±0.4mm/a. The faultgeomorphological characteristics are distinct along the Xiangshuidian-Majiaxinzhuang segment, and it performs as thrust nappe with an appreciably sinistralstrike slip component. The recent active characteristics of this fault segment areobvious, and three paleoseismic events occurred in500a BP later,6ka BP ago and15ka BP ago respectively on this segment.(3) Longxian-Baoji faults zone is located in southern Liupanshan area, andcomposed of four secondary faults. The north end of Longxian-Baoji faults zoneconnects with the Liupanshan eastern piedmont fault at the vicinity ofMajiaxinzhuang, and it disappears southeastward in the Weihe River valley near Baoji.Its general trend is325°—335°and length is~130km. The Guguan-Xiangong fault(GXF) of Longxian-Baoji fault zone is a steep (dip up to60-70°) normal fault. Thereare some fresh bedrock fault scarps along Badu-Xiangong segment of GXF; however,the recent active evidences are insufficient on some sections. The Qishan-Mazhaofault (QMF) is another main fault of The Longxian-Baoji faults zone. Preliminarystudy results suggest that QMF is active in late Quaternary, and still partly active inHolocene.(4) It were detailedly researched that the historical data and relics on the AD1219Guyuan earthquake, AD1306Guyuan earthquake and AD600Qin-long earthquakewhich occurred in Liupanshan region. Combining the researching results on activefaults, we checked the parameters of above earthquakes, and discussed their possibleseismogenic structures.(5) Analyzing comprehensively the active fault segment and the rupturecharacteristics of the historical earthquakes in Liupanshan region, we preliminarilysupposed that the Majiaxinzhuang-Guguan segment had the tectonic setting of strongearthquake. In the future, the possibility of strong earthquake to occur in the Majiaxinzhuang-Guguan segment will is relatively large, thus its strong earthquakerisk should be concerned.