青藏高原东缘岷江断裂北段全新世活动特征

HOLOCENE ACTIVE CHARACTERISTICS OF THE NORTHERN SEGMENT OF THE MINJIANG FAULT IN THE EASTERN MARGIN OF THE TIBETAN PLATEAU

有史料记载以来,沿岷山断块边界断裂曾发生多次6.0~7.2级强震,是中国南北地震带的重要组成部分。文中针对岷江断裂北段,采用高分辨率遥感影像解译、地质地貌调查、微地貌测量及探槽开挖相结合的方法,准确厘定了该断裂段的空间几何展布形态,揭示了断裂的逆冲兼左旋走滑性质,获取了断裂的全新世最新活动参数。川盘村附近的Ⅱ级阶地断错地貌线性较好,实测断层陡坎垂直高度3.1m,冲沟累积走滑量约3.1m,由断层作用造成的水平累积缩短量约3.0m;这与探槽揭示的结果相吻合。按照位移相依的特征地震计算,同震垂直位移量和水平走滑量均约为1.0m,同震水平缩短量为1.0m。结合地层年代测试结果,计算获得该断裂段的垂直位移速率和水平走滑速率为0.7~0.9mm/a,水平缩短速率为1.0~1.1mm/a。垂直断裂布设的探槽不仅揭示了断裂为低角度逆冲性质,产状为倾向260°、倾角29°;同时,根据断层、崩积楔与地层的切盖关系,探槽还揭示了3次古地震事件,最新1次事件发生在距今0~295a,第2次事件发生在距今1 405~1 565a,第1次事件发生在距今2 750~2 875a,复发周期约1 110~1 565a,离逝时间约0~295a。根据相关关系式计算结果,认为该断裂段具备发生7.0级以上地震的潜在能力,目前处于应力积累阶段,具有一定的地震危险性。

As a part of the north-south seismic zone in China,a lot of M6.0-7.2 earthquakes have occurred in the margin faults of the Minshan block in history.This work attempted to characterize the geometry and activity of the north section of the Minjiang fault in this region based on high-resolution satellite images,geologic and geomorphic investigations,micro-geomorphic surveys,and trench excavation.The results show left-lateral-slip and Holocene activity of this structure.Along it,the offset landform has a continuous linearity onⅡterraces near the Chuanpan village.The vertical height of the fault scarp measures 3.1 meters,which is almost the same as the accumulative horizontal displacement of the gully.The accumulative horizontal shortening due to faulting is 3.0 meters.Calculation using the model of displacement-dependent characteristic earthquakes shows both the vertical and horizontal co-seismic displacements and the horizontal shortening amount are about 1.0 meter.While strata dating suggests that the vertical and horizontal slip rates are all about 0.7-0.9mm/a,and the horizontal shortening rate is approximately 1.0-1.1mm/a.The excavated trench,perpendicular to the fault trace,reveals low-angle thrust dipping in 260°at 29°.From the relationship of the fault,colluvial wedge and stratigraphy ages,three palaeoseismic events are identified from youngest to oldest at 0-295a BP,1 405-1 565a BP,and 2 750-2 875a BP,respectively,with recurrence intervals 1 110-1 565 years and elapsed time about 0-295 years.According to the relationship between magnitude and active parameters,it is considered that the northern segment of the Minjiang fault is capable of generating M7 or greater earthquakes.Now it is in the process of stress accumulation,having a certain seismic risk.