兰州马衔山北缘断裂带古地震初步研究

A PRELIMINARY STUDY ON PALAEOEARTHQUAKE EVENTS OF THE NORTHERN MARGIN ACTIVE FAULT ZONE OF MAXIAN MT IN LANZHOU AREA

兰州马衔山北缘断裂带为一条全新世活动断裂 ,大致由 4条次级断裂段组成 .沿断裂带发现了多期古地震事件 ,其活动具有时空不均匀性 .其中东段的马衔山段可以确定 2次古地震事件 ,距今 5850± 50 0aB .P .,2 0 60± 42 0aB .P .,复发间隔约 380 0a ;震级 7～ 7.5级左右 .中段的七道梁段发现 2次古地震事件 ,距今 1 682 0± 80aB .P .,1 0 80 0± 1 40aB .P ..西段的雾宿山咸水沟段可以确定一次古地震事件 ,其年代为 1 2 45± 560aB .P .,结合史料考证结果 ,认为就是 1 1 2 5年兰州 7级地震 .从古地震活动年代及复发间隔分析 ,马衔山北缘断裂带未来的强震危险段应为东段的马衔山段和西段的雾宿山咸水沟段 .

The northern margin fault zone of Maxian Mt. in Lanzhou area is a Holocene active fault which includes about 4 secondary fault segments. Along the active fault zone, several palaeo earthquake events are found in trench profiles. The activities of palaeo earthquakes exist inhomogeneity in time and space. In the east, on Maxian Mt.segment, there are two events occurred in 5 850±500 a B.P.,2 060±420 a B.P.,respectively. Their magnitudes are about 7～7.5, the recurrence interval is about 3 800 a. In middle segment, Qidaoliang segment,two events are ditermined,in 16 820±80 a B.P.,10 800±10 a B.P., both are in Late Pleistocene. In west segment, Xianshui valley segment in northern margin of Wusu Mt.,one palaeo earthquake with age of 1 245±560 a B.P.is found.Considering the distribution of loess landslides and ancient town sites, this event is mostly the Lanzhou M7 in 1125. According to the palaeo earthquake ages and recurrence interval features,the strong earthquake hazard segments of the northern margin active fault zone of Maxian Mt.will be the east segment Maxian Mt.fault segment and the west segment Xianshui valley fault segment of Wusu Mt..