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New evidence for Late Quaternary Activity in the Middle Segment of the Northern Margin Fault of Maxian Shan in Lanzhou

YUAN Dao-yang1,2,LIANG Ming-jian3,LEI Zhong-sheng1,LIU Xing-wang1(1.Lanzhou Institute of Seismology,CEA,Lanzhou,Gansu 730000,China; 2.Lanzhou National Observatory of Geophysics,Lanzhou,Gansu 730000,China; 3.Earthquake Administration of Sichuan Province,Chengdu,Sichuan 610041,China)  
In the southern mountainous region of Lanzhou city,there are four large-scale regional active fault zones of late Quaternary.These are the northern margin fault of Maxian Shan("Shan" means Mountain)(F1),the southern margin fault of Maxian Shan(F2),the southern margin fault of Xinglong Shan(F3),and the northern margin fault of Xinglong Shan(F4).They make up the Maxian Shan-Xinglong Shan active fault system.The northern margin fault of Maxian Shan is the longest among them.The fault's eastern end begins in Neiguanying,moves along Miaowan,Yangzhai,and Yinshan,then meets with the southern margin fault of Xinglong Shan in Moyunguan of Qidaoliang and continues along Tianjiagou,Hutan,Guanshan,Xianshuigou,and the western part ends in Bapanxia,Yellow River.The fault strike direction is about N60 °W and the total fault length is about 115 km. The geometrical features of the northern margin fault of Maxian Shan are simple.The fault is divided into four segments according to the fault displacement,fault bending,fault step,and differences in recent activity.The secondary faults are the Neiguanying segment(F1-1),the Maxian Shan segment(F1-2),the Qidaoliang segment(F1-3),and the Wusu Shan segment(F1-4).Field investigations indicate that the northern margin fault of Maxian Shan is a seismic fault zone characterized by segmentation.Its eastern segment,the Neiguanying secondary fault,is a late Pleistocene fault.The west segment,the Wusushan secondary fault,is active in the Holocene and produced the Lanzhou M7 earthquake.However,there is no geological and dating evidence for the recent activity of the middle segments,that is,the Maxian Shan and Qidaoliang secondary faults.Nonetheless,in recent years,field investigations have indicated that the middle segments were active during the late Pleistocene and early Holocene,on the basis of left-lateral strike-slip and dip-slip features. According to historical data in the Lanzhou area,several moderate to strong earthquakes with magnitude of about 5~6 occurred in the Maxian Shan area in 1524,1629,and 1901 A.D.besides the Lanzhou M7 earthquake in 1125 A.D.and Yongjing M6.5 earthquake in 1590 A.D.These earthquakes may have correlated with the activity of the northern margin fault of Maxian Shan. The northern margin fault of Maxian Shan was active in late Quaternary and produced several moderate to strong or even large historical earthquakes;therefore,this fault is active and has the potential to produce earthquakes in future.
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