汶川地震形成位移场的空间分布

采用Yoshimitsu Okada及Steketee的断裂位错模型和汶川地震现有成果,通过坐标转换和合成矢量的方法将断裂位错模型用于映秀—北川断裂、灌县—安县断裂和北川—青川断裂组成的断裂系统的错动研究中,理论上计算龙门山近断裂地区的水平位移场(包括沿走向和垂直走向方向)和垂直位移场从震源到地表的分布.模型参数源于现有的研究成果和野外实地考察,计算得到的地表位移场与先前研究成果,包括GPS实测数据,具有的近似性表明了模型的正确性.但GPS只能测定地表水平和垂直变形,对于地下变形情况及其分布却无法描述;目前对于地下变形的研究主要基于对大量仪器记录的地震资料进行反演或通过野外观测进行推测;但是反演多集中于空间较大范围,这样虽可阐述断裂运动引起的大范围位移趋势,而对于震源附近空间介质位移的描述却略显粗糙;同时,野外观测误差较大.本文通过理论模型计算汶川地震中近断裂区域(距断裂50km)内的位移及分布.由计算发现在近断裂区域内垂直位移场和垂直走向方向位移场变化趋势一样,幅值都是从震源到地表逐渐减小;沿断裂走向方向的水平位移场从震源到地表逐渐变大,同时计算还表明位移场的变化在断裂上盘比下盘剧烈,余震分布主要集中于断裂上盘,这说明余震分布和位移剧烈程度存在某种相关性.

The spatial distribution of displacement fields in 2008 Wenchuan earthquake

Basing on the Yoshimitsu Okada or Steketee’s dislocation model and achievements of present researches on Wenchuan earthquake,through the coordination transform and vector composition,we apply dislocation model to fault system and make the dislocation model of Yingxiu-Beichuan fault Guanxian-Anxian fault and Beichuan-Qingchuan fault.We calculate theoretically the evolvement of region displacement fields including horizontal and vertical displacement from the source to the ground surface in the region near the faults.In the computation,the model parameters are from the present research achievement and field scientific research.Compared the computational result with GPS data achievements,we prove the validity of model.In fact,GPS can only measure the horizontal and vertical displacement along the ground surface,but it can not depict the deformation fields of underground.Through the much data from the seismic instrument or field survey,people mainly compute the large area underground deformation now.Although this computation can give the trend of displacement due to the motion of fault in large zone,it can not depict the displacement near the source nicely.In addition,field survey has relatively large error.This paper,we will apply the dislocation model to compute the displacement fields near the fault.The computation results show that vertical displacement field and horizontal displacement field vertical to strike direction have the same variation trend which change gradually small from the underground to ground surface.At the same time,horizontal displacement field along the strike direction varies gradually larges from the underground to ground surface.Furthermore,our computational results also show displacement fields vary stronger in hanging wall of fault than that of foot wall.The distribution of aftershock mainly focusing on the hanging wall of fault shows that the distribution of aftershock has the some correlation to the strong deformation displacement of strata.