关键词: 兰州市, 地震活断层探测, 岩性界线, 构造变形, 城市规划和工程建设

Abstract: This paper makes a comprehensive analysis of the recent progress of the seismic active fault prospecting in Lanzhou city. Based on the satellite and aerial photos interpretation,geological and geomorphic investigation,geochemistry prospecting,shallow seismic investigation,resistivity imaging,drilling,especially large-scale trenching along the 7 active fault zones in Lanzhou city,we have achieved very important progress and gained new knowledge about the recent activity of main active faults and deformation features in Lanzhou Basin. The main conclusions are summarized bellow: (1) The Jinchengguan Fault is a thrust fault,constituting the northern boundary of the Lanzhou Tertiary Basin. It is revealed by geophysical prospecting and drilling that the newest strata offset by the Jinchengguan Fault are the early-Pleistocene sandstone and conglomerate,and that the overlying second and third terraces of the Yellow River remain intact. So,it's an early and middle Pleistocene active fault.(2) The Liujiabu Fault and Shengouqiao Fault constitute the northern and western boundaries of the Qilihe Subsidence,respectively. Revealed by geophysical prospecting,drilling and large trenching,they are not faults but lithologic boundaries of different rocks between Pliocene and early Pleistocene.(3) The Leitanhe Fault is the eastern boundary of Qilihe Subsidence,a boundary fault separating the Tertiary Lanzhou Basin into the east and west basins. According to the geophysical prospecting and drilling,the Leitanhe Fault is a thrust fault and its newest activity age is early and middle Pleistocene. It is not active since late Quaternary and does not cut the third terrace of the Yellow River.(4) The Siergou Fault is the southwestern boundary of Lanzhou Basin,a thrust fault too. It's an early and middle Pleistocene active fault and does not offset the forth terrace of Yellow River. While the Xijincun Fault is much nearer to the south margin of Lanzhou Basin and forms the southern boundary of the Tertiary Lanzhou Basin. It's an early Pleistocene fault.(5) The northern margin of Maxianshan Mountains fault is a major seismic fault on the southern margin of Lanzhou Basin,and its movement is characterized by segmentation. The east segment,the Neiguanying sub-fault,is a late Pleistocene fault. The middle segment,the Maxianshan and Qidaoliang faults,are active during late Pleistocene and early Holocene. The west segment,the Wusushan sub fault,is active during late Pleistocene and Holocene,and it's also the seismic fault of the M7 Lanzhou earthquake.On the whole,we correct the previous recognitions about the activity times of 4 faults,i.e. the Jinchengguan Fault,Leitanhe Fault,Siergou Fault and Xijicun Fault. They are all early and middle Pleistocene instead of late Pleistocene active faults. Especially,we find that the Liujiabu Fault and Shengouqiao Fault directly across Lanzhou city are not late Pleistocene or Holocene active faults but lithologic boundaries between Pliocene mudstone and early Pleistocene conglomerate. The results are very important for the urban planning and engineering construction,and will produce obvious economical and social benefits.

Key words: Lanzhou city, active fault prospecting, lithologic boundary, tectonic deformation, urban planning and engineering construction