Abstract: Based on the survey and study of active faults at three sites,i.e.Yaodian,Shiheyang and Dujiapu on the north bank of Weihe River in Xianyang,Shaanxi,this paper probes into the methodology of survey of the loess-covered active faults coincident with terrace scarps,and presents the displacement amount of the Weihe Fault zone at Shiheyang in late Pleistocene.At Shiheyang,exploration of the Weihe Fault zone was carried out by means of shallow seismic prospecting,drilling,topographic analysis and age dating.The initial survey result showed a displacement of 17.94m of the stratum S1 on the Weihe Fault zone.The causes leading to this false result were mainly due to incorrect judgment on geomorphic unit,and followed by the so big spacing of drill holes that the subtle change of strata tilting due to erosion couldn't be seen.The drop of the same stratum at the profile detected at two drill holes far away from each other was mistaken for fault displacement.With the scarp caused by erosion added to the fault displacement,the fault throw was magnified.By densifying the drill holes to a spacing of 1.9m between holes,we get the displacement of the top of S₁ to be only about 1.2m.At Yaodian,data are available,including the 200m deep drilling section data,the densified mid-deep drilling data and shallow seismic prospecting data.Drilling data with borehole spacing of 30m revealed an offset of 4.8m on the top of S1 by the Weihe Fault.Since the two holes were located at scarp change zone,the 4.8m height difference of the top of S1 might be the elevation difference of tilted terrain superimposed possibly with certain amount of faulting.The 30m hole spacing is too large to affirm that S1 has been faulted.The drilling section at Dujiapu was implemented at last,in which deficiencies in dealing with the first two ones were avoided.At this site,the shallow seismic methods couldn't be performed,therefore the fault was located by combining the deep drilling with shallow drilling at a hole spacing as small as possible(2～3m).In spite of the small borehole spacing,it was difficult to identify the displacement amount of the fault according to the paleosol layer S1,which is probably due to too small fault throw.All the explorations of fault at the above three sites have a certain deficiency in methodology,mainly in the depth and spacing of drill holes.The common shortcoming is that no deep trenches were excavated.If allowable,it would be better to verify the fault location and activity by trenching.The above results show that the exploration of loess-covered active faults coincident with terrace scarps shall be carried out with comprehensive method combining topographic analysis,shallow seismic survey,drilling and trenching.Particularly for drilling exploration,deep,medium and shallow holes shall be combined in use with the medium and deep holes drilled to determine the location of faults at depth,and the shallow holes used to identify the location and activity of faults near surface.Due to river erosion,the fluvial deposition layer in terrace scarp zone is tilted.Aeolian paleosol layer draping over the tilted layer is tilted too.As a result,the spacing between holes must be small(2～3m preferable)when such strata are used to identify the location and movement of faults.Excessive spacing may lead to the addition of the height of erosion-formed scarp to the fault offset,thus greatly overstating the later.It is highly recommended to make verification by trenching in the end.The above exploration results show that the Weihe Fault zone coincides with the scarps of the third terrace at Yaodian,Shiheyang and Dujiapu.The displacements associated with faulting only have a small proportion of the terrace scarp and the 1～2m offset of the first late Pleistocene paleosol layer by faulting is much less than the difference in elevation of terrace surface.The previously thought 4.8m and 17.94m displacements are incorrect.

Key words: Weihe Fault zone, terrace scarp, loess cover, drilling section, shallow seismic method

摘要： 陕西咸阳渭河北岸窑店、石何杨、杜家堡渭河断裂活断层探测结果表明,对于有黄土覆盖的、与阶地陡坎重合的活断层探测,要综合采用地形地貌分析、浅层人工地震、钻探和探槽等方法进行。特别是对于钻孔探测,要深、中、浅孔结合。首先用中、深孔确定断层在深部的位置,再用浅孔确定断层在近地表的位置和活动性。由于河流侵蚀,阶地陡坎区的河流相沉积地层是倾斜的,风成的古土壤层披盖在已有的倾斜地层上亦呈倾斜状态,因此,用以上地层判断断层的位置和活动量时,钻孔孔距一定要小,以2～3m为宜,孔距太大,会把侵蚀形成的已有陡坎高度加入断层的错距中,严重放大断层的错动量。探测结果表明,渭河断裂在窑店、石何杨、杜家堡等处与Ⅲ级阶地陡坎重合。该断裂在阶地陡坎上的活动量很小,错断晚更新世第1古土壤层1～2m,远小于2个阶地面的高差。可见,以前认为S1错距4.8m、17.94m是不准确的。

关键词: 渭河断裂带, 阶地陡坎, 黄土覆盖区, 钻孔剖面, 人工浅层地震