Abstract: According to the results of 1/10,000 stripped geologic mapping of the northwest segment of the Maxianshan north marginal fault and historical accounts of past events,we discuss in this paper the range of the magistoseismic area,seismogenic fault,pattern and combination feature of the surface ruptures of the 1125 A.D.Lanzhou M7 earthquake.The results show that the magistoseismic area of this earthquake is located in Lanzhou City and its southwest,and the epicenter can be located at the Xianshuigou area.The seismogenic fault of this earthquake is the Xianshuigou-Maquangou sub-segment on the northwest segment of the Maxianshan north marginal fault.This earthquake has produced a surface rupture zone of about 7km long and 300～1000m wide,extending along the seismogenic fault.The surface ruptures consist of earthquake fractures,fault scarps,seismic fissures,seismic landslides,and seismic pits.The surface rupture zone can be sub-divided into 2 sub-segments:the Maidiwan-Xianshuigou sub-segment in the southwest and the Damajiatan-Maquangou sub-segment in the northwest.Among them,the Maidiwan-Xianshuigou sub-segment consists of two parallel surface ruptures,while the Damajiatan-Maquangou sub-segment comprises a single surface rupture.Basing on large scale mapping,it is determined that the left-lateral displacement produced by this event is 2.4～2.5m,and the vertical offset is 0.45～0.92m.Regionally,the Maxianshan north marginal fault is located at the junction of the northern margin of the Qinghai-Xizang plateau and the northern segment of the North-south tectonic belt,which have been strongly active since neotectonic period.A rhombic block confined by major faults of different strikes is developed in this region,and we call it the Gansu-Ningxia rhombic block.The 1125 A.D.Lanzhou M7 earthquake just occurred on the western edge of the rhombic block,i.e.the Wuwei-Zhuanlanghe-Maxianshan Fault zone.The strong uplift and northeastward pushing of the active Qinghai-Xizang block may cause the stress relief on the boundary faults of the Gansu-Ningxia rhombic block,and hence the occurrence of several strong earthquakes.

Key words: Lanzhou earthquake, epicentral location, surface ruptures, seismic displacement

摘要： 根据马衔山北缘断裂西北段1/10000条带状地质填图和史料考证资料,对兰州1125年7级地震的极震区范围、发震断层、地表破裂类型及分布特征进行了讨论。结果表明,该次地震的极震区范围位于兰州市及其西南,震中在咸水沟一带,发震断层为马衔山北缘断裂西北段咸水沟—马泉沟小段。该次地震形成了长约7km,宽300～1000m的地表破裂,其破裂类型有地震断层、地震陡坎、地震裂缝、地震滑坡、地震陷坑等。其中可细分为2小段,东南小段为麦地湾—咸水沟段,由两条平行的地表破裂组成;西北小段为大马家滩—马泉沟段,由单条地表破裂组成。根据大比例尺平、剖面图实测,该次地震的左旋位移量2.4～2.5m,垂直位移量0.45～0.92m。文章最后,对地震的构造背景进行了讨论。

关键词: 兰州地震, 震中位置, 地表破裂, 地震位移