CHARACTERISTICS OF THE HOLOCENE ACTIVITY OF THE GUANGUANLING FAULT ZONE

The newest active time and segmentation of the fault are of special significance in the safety evaluation of major engineering projects.This paper discusses the active times and segmentation characteristics of the Guanguanling Fault zone through interpreting aerial photos,field investigation,topographic and geomorphic surveys and analysis of trench logs on paleoearthquake in connection with the study of the geological and seismologic problems in Heishanxia project of the Huanghe River.The Guanguanling active fault zone lies on the northeastern margin of the Qinghai-Tibet block.It is part of the Zhongwei-Tongxin arc active fault zone,striking near EW generally,with a total length of about 60km.It consists of 5 discontinuous secondary faults arranged in left step en echelon,namely,Jingtaixiaohongshan(F1-1),Guanguanling(F1-2),Shajing(F1-3),Zhongweixiaohongshan(F1-5)and Qingshan-Gushanzi(F1-4),respectively.Since Late Quaternary,the fault is characterized with intense sinistral strike-slip and compressional thrust and has offset a series of ridges and small gullies and terraces.At the same time,fault scarps were developed along the fault zone.The study reveals that the latest earthquake occurred 700～1200a BP,the largest displacement took place in Guanguanling,and the maximum horizontal sinistral displacement reaches 6m since Holocene.