马衔山北缘断裂为兰州市区南侧一条规模较大的区域性活动断裂带,总体走向N60°W,长约115 km.该断裂大致可划分为4条次级断裂段,自东向西分别为内官营段、马衔山段、七道梁段和雾宿山段.其中,内官营段为晚更新世活动段,雾宿山段为全新世活动段并为1125年兰州7级地震的发震断裂,而中间的马衔山段和七道梁段以往研究所获得的新活动证据不多,认识不统一.近期经过详细的追踪考察,获得了马衔山段和七道梁段晚第四纪活动的新证据,结合该区多次历史地震的活动特征,表明其为晚第四纪活动断裂,具左旋走滑兼倾滑运动特征.%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 faults eastern end begins in Neiguanying, moves a-long Miaowan, Yangzhai, and Yinshan, then meets with the southern margin fault of Xinglong Shan in Moyunguan of Qidaoliang and continues along Tianjiagou, Hutan, Guanshan, Xianshui-gou, 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.rnThe 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 Hol-ocene and produced the Lanzhou Ml 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.rnAccording 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 Ml 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.rnThe 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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