汶川(Ms8.0)地震的河流地貌响应

摘要

2008年5月12日龙门山汶川(Ms8.0)地震导致了北川断裂、彭灌断裂和小鱼洞断裂发生了地表破裂,显示为两条近于平行的北东向逆冲-走滑型断层.同震变形在瞬间就改变了地形坡度,并产生了巨量的滑坡和泥石流,导致河流地貌产生相应的变化和响应.本文在整合汶川地震所导致的地表破裂、地形和水系变化相关数据的基础上,标定了汶川地震驱动的逆冲-走滑型构造作用对河流坡折点、河流转折点、河道走向的控制作用,刻画了平行的走滑-逆冲断层对河流地貌与不规则水系样式的控制作用,讨论了汶川地震驱动的隆升作用对河床梯度剖面的影响,探讨了汶川地震和暴雨驱动滑坡、泥石流和洪水及其剥蚀卸载作用对河道地貌和龙门山地形演化的影响.研究结果表明:1)汶川地震驱动的右行走滑作用导致水平位错和偏转,使水系产生新的河流流向的转折点；2)汶川地震驱动的逆冲作用导致的垂向位错使水系产生新的河流坡折点；3)汶川地震驱动的活动断层走向对河道走向具有控制作用；4)汶川地震驱动抬升作用导致河床梯度平衡剖面和剥蚀基准面的变化；5)汶川地震驱动的滑坡使得该地区剥蚀作用加强,同震滑坡量远大于同震岩石隆升增加的山脉体积,导致了龙门山造山带的物质亏损；6)龙门山地形雨的“雨影区”产生的强降雨带与汶川地震驱动的地表破裂带和滑坡带的空间分布位置一致,暴雨季节来临时容易出现滑坡、泥石流和洪水,这是未来几十年所面临的地质灾害.因此认为,在龙门山地震活动频繁的地区,河流地貌和水系样式主要受地震构造活动控制.%The(Ms8.0)Wenchuan Earthquake which occurred on May 12,2008 in the Longmenshan Mountain range area led to surface ruptures of the Beichuan,Pengguan and the Xiaoyudong faults.These are two roughly parallel NE trending thrust and strike-slip ruptures.Coseismic deformation changed the topographical gradient and produced massive landslides and debris flows,causing a corresponding response of the fluvial landforms.In this paper,based on the data of surface ruptures and changes in the topography and river channel drainage,the influence of the thrusting and strike-slipping driven by the Earthquake on fluvial landforms and the drainage are integrated and analyzed.The results are mainly shown in the following six aspects:(1) The strike-slipping driven by the Earthquake caused the formation of new tectonic diversion points of river channels; (2) the thrusting driven by the Earthquake caused the formation of new tectonic slope-break points of the river channel drainage ; (3) strikes of the active faults driven by the Earthquake control on the channel strike ; (4) the uplifting driven by the Earthquake leads to changes of riverbed gradient profiles and their base levels ; (5) coseismic landsliding produced erodible material greater than the net volume added to the orogen by coseismic rock uplift,the Wenchuan earthquake will lead to a net material deficit in the Longmenshan Mountain; and(6)extreme rainfall after the earthquake easily induces landslides,debris flows and floods,geo-hazards which will exist for several decades.