Late Quaternary Activity Research Of The Middle Section Of Huashan Front Fault

This paper conducted the work of “Late Quaternary Activity Research of the Middle Sectionof Huashan Front Fault” based on the Huashan Front Fault1:50000geological andgeomorphological mapping project. The study includes two aspects of tectonic landforms andpaleoseismology. Tectonic landforms work mainly include： dividing the types of tectoniclandforms, studying the characteristics and genesis of various landforms, measuring the typicalfault landform to get displacement data, taking samples in major stratum and landscape unit toconstrain it’s time, calculating the slip rate of fault, building the evolution pattern of majorlandforms. Paleoseismology work is mainly excavated the paleoseismology trench, analysed thepaleoseismology events, got the Holocene paleoseismology events sequence of fault.Huashan Front Fault is one of the main boundary faults of the south edge of Weihe Basin, asa typical normal dip-slip fault, the middle section of which has a strong activity during theHolocene. The middle section had occurred the AD1556M814Huaxian Earthquake, generallyconsidered that it is seismogenic structure or part of seismogenic structure. On this section, anumber of work had undertaken by predecessors and got some consensus. In this paper, on thebasis of previous studies, through theoretical analysis and systematic geological survey, some ofthe results obtained are as follows.(1) Identified the stratum distribution of Late Quaternary near the fault, given the distributionmap of the fault though a lot of fault profiles controlled and geomorphic compared, and discussedthe profile characteristics of fault.(2) Identified the fault was a dip-slip normal fault, left-lateral-slip is not obvious. Tectoniclandforms include: fault facets, diluvia terraces, buried pluvial fans, fault scarps, nick-points andwater fall.(3) Diluvia terraces up developed5levels. T1is accumulation terrace,2000a BP formedaround; T2is the rock-seated terrace, formed around6000a BP; T3is the rock-seated terraceformed in the late stage of Late Pleistocene to early Holocene; T4is the rock-seated terraceformed in the mid-to late stage of Late Pleistocene; T5is eroded terraces formed in the early-tomid stage of Late Pleistocene. According to the displacements of T1, T2level diluvia terracescalculated the average vertical slip rate of fault is1.485mm/a since6000a-2000a BP, and 3.73mm/a since2000a BP.(4) Buried pluvial fans are divided into three phases. The first phase is the mid stage of LatePleistocene to early Holocene pluvial fan, and it’s leading edge from the piedmont3-5km; Thesecond phase is middle and late Holocene pluvial fan, overlaying the first phase, and it’s leadingedge from the piedmont1-3km; The third phase is late Holocene pluvial fan and modern pluvialfan, overlaying the second phase, generally a few hundred meters from the piedmont. Relationshipbetween diluvia terraces and buried pluvial fans is: the third phase pluvial fan corresponds to T1,the second corresponds to T2, and the first corresponds to T3or the level above T3.(5) Fault scarps are divided into four types: diluvia terraces front scarps, flood-plain terracesscarps, slope-deposit scarps, and modern pluvial fans scarps. Most valleys developed2-4levelsnick-points on the upside of the fault, only two levels close to fault intact, Where P1nick-pointand T1diluvia terrace can cope on, P2nick-point corresponding relationship with alluvial terracesdifficult to determine.(6) Through East Shuiyukou trench, Shicai trench of Shangan village, and Lee Bo faultprofile analysis of paleoseismology events are:（8590to7570）a BP, twice and （7570to6180）a BP, once. Paleoseismology events approximately since6000a BP references previous research,finally given the Holocene paleoseismology events sequence of Huashan Front Fault:（8590to7570）a BP, twice;（7570to6180）a BP；5610a BP；2715a BP；AD1556a.