The Study Of New Activities On Western Segment Of Northern Margin Of Western Qinling Fault And Laji Shan Fault

Northeastern margin of Qinghai-Tibet Plateau(NE Tibet) is a sensitive zone ofnew tectonic deformation, which experienced northeastward contraction andwest-eastward extension, and it’s still in the process of strong extension[1-4]. The studyarea is located in the eastern end of NE Tibet, in which new activities has been verystrong, the topography there has been complex, with the highly Qinling Mountain,Jishi Shan and Laji Shan, and adjacent Linxia Basin, Xunhua-Guide Basin andXining-Minhe Basin[5,6]. Active tectonics developed intensively in study area,Northern Margin of Western Qinling Fault Zone (WQLF) is one of the mainleft-lateral active fault in NE Tibet, and regional tectonics and topography wereaffected greatly by its activity; the contrasting topography between high Laji Shan andits adjacent much lower and more smooth basin encourage people to pursue itscausative mechanism; there is also some questions need to be answered, such as: whatabout the faults activity in study area? How faults in study area relate to each otherand how these faults control the topography in study area?Based on those question, we did a detailed research on late Quaternary activity ofwestern segment of WQLF and surface ruptures in extreme seismic area of Kangleearthquake happened in1936(M=6, unique historic event happened in westernsegment of WQLF), and we surveyed late Quaternary activity of Laji Shanpreliminary, combined with previous research, we got the following conclusions:(1) Through satellite image interpretation and field geological investigations,we found west of Tufang village, Northern Margin of WQLF, remain strong activity, itextends westward and stretches across Daxia River, and presents significant Holoceneactivity characteristics in Qielonggou，the fault trace vanish in Ganjia Basin. So LateQuaternary activity of Northern Margin of WQLF terminates at Ganjia Basin. Thefault moves mainly left-lateral slip and with little dip-slip component. Finally, we gottwo paleoseismic events since Middle Holocene:（1932±54） a B.P. and（3602±89~5880±115）a B.P., respectively, which may correspond to later two stageof three active stage in Holocene on Northern Margin of WQLF. As a result of elapsedtime of the most recent paleoseismic event is about2000years, we can deemquantitatively that future earthquake risk of western segment of Northern Margin ofWQLF is relatively high[7-10].(2) The rupture phenomenon in extreme seismic zone are fault scarp, earthquakefissure, rock falls, earthquake landslides and so on. Rock falls distribute all along theWZSF, the scale of rock falls in eastern segment is greater than western segment, andwe can identify at least two stages of earthquake rock falls. Surface deformationmainly distribute in the eastern segment, left-lateral silp and vertical slip caused byKangle earthquake was2.5m and0.6~0.7m, respectively. It represent that the faultactive mainly left-lateral, with a little thrust component. Earthquake fissure mainlydistribute both side of WZSF, they develop in the diluvia deposits composed ofgravels and muck. We found several different scale of left-lateral slip in differentgullies in field investigations and probably WZSF has experienced several stages of seismic activity[7].(3) Laji Shan Fault (LJSF) is composed of four secondary faults: Easternmargin of Jishi Shan Fault, western margin of Jishi Shan Fault, Northern Margin ofLaji Shan Fault and Southern Margin of Laji Shan Fault, respectively, and constituteboundary fault between Minhe-Xining Basin to the north, Xunhua-Guide Basin to thesouth and Linxia Basin to the east[11-14]. Through satellite image interpretation andfield geological and geomorphology investigations, we found LJSF active mainlythrust-slip, western segment of LJSF also had significant left-lateral component. LJSFmainly active in late Pleistocene and offset some stream channel terraces, littleevidence was found to support fault activity in early Holocene. Holocene activity ofLJSF might extend into basins on both side of Laji Shan, but it need more evidence tosupport this assumption[11-14].(4) Late Quaternary activity of Northern Margin of WQLF terminate at GanjiaBasin, which is adjacent to Jishi Shan, explain that strike-slip component of westernsegment, Northern Margin of WQLF, may be absorbed by Jishi Shan or Ganjia Basin,thus prompt uplifting of Jishi Shan. Uplifting of Jishi Shan in Holocene also promptthe faults both side of the range extending into interior basin in the form of buriedfault and active fold, resulting in Jincheng-Longxi M6earthquake (138AD) andmore than20moderate earthquakes both side of Laji Shan[11-15].(5) Combination of quantitative parameters on fault activity, historic earthquakesand weak seismicity, we think future seismic hazards of moderate earthquake onWQLF and LJSF worthy of more attention, and should focus on seismic hazards oflarge earthquakes on near Lianlu Basin, western end of WQLF (whose elapsed timeget into the range of recurrence interval) and western end of LJSF (Riyue Shan Faultdextrally extrusion) and Laji Shan arc-vertex and the basin northeast ahead.