【作者】 郑文衡；

【导师】 倪樵；

【作者基本信息】 华中科技大学 ， 固体力学， 2009， 博士

【摘要】 运用三维有限元动力学分析方法,结合地球物理观测数据,研究了地壳介质中应力的传播特性,得出了大陆地壳对海洋潮汐的响应、仿真地震图、全球模型、断裂面对地震波的影响、动应力对断层错动的影响等研究结果。基于后两项内容,探讨了地震触发的动力学机理。在此基础上,进一步研究了由此形成的地震波与断层活动之间的正反馈关系,解释了大地震发生有一个长时间酝酿过程这一现象。通过研究,对地壳中的力学现象从应力的传播和影响角度作出物理解释。同时探讨了计算力学与地球物理观测相结合的可行性。本文的主要内容包括:(1)将三维动态有限元方法应用于地球动力学分析,研究了大尺度的地球物理对象,本文致力于深入地进行三维动力学分析与固体地球的动力学过程的联合研究,在方法上,着眼于地球物理的动力学现象,对地球物理现象作动力学机理的探讨。地球物理的研究对象具有大尺度特征,通常的研究,往往着眼于缩小的模型,再外推到大尺度的实际情况,外推必须非常谨慎。研究小尺度模型而外推到大尺度,涉及到相似性分析,其正确性还需要论证,比较繁琐。本文利用了动态有限元建模,直接研究大尺度的对象。在大陆地壳对海洋潮汐的响应、仿真地震图、全球模型、断裂面对地震波的影响、动应力对断层错动的影响等问题的研究中,均采用了真实尺寸地壳建模,进行仿真计算。得到了直观的结果。(2)利用反演思想实现了仿真地震图研究,基于传统的理论地震图的研究方法,本文提出仿真地震图方法。针对地壳的有限元模型,做了实际的仿真地震图分析,与地震观察结果进行对比研究。理论地震图是一种解析方法,而仿真地震图基于数值方法。前者采用一种解析的格林函数,代表波在地壳中的传播特性,而后者对地壳介质及其结构具有明确的定义,对波的传播过程有具体的描述。仿真地震图属于反演方法,通过假设介质和震源参数,得到理论上的仿真地震图,将实际观察到的地震图与之比较,用试错法调整介质和震源参数,最终使仿真地震图与观察到的地震图符合较好,这时的介质和震源参数就是所求出的未知量。(3)研究了地震触发的动力学机理:地震机理有很多模式,本文提出了一种可能的模式。这个模式指出,库伦应力的变化可能触发地震。动态应力的传播即地震波,波在自由面上反射会发生半波损失,也就是说,压缩波会变成拉伸波,因此,无论是压缩波还是拉伸波,都有可能激发应力降。而应力下降将会减小(内)摩擦,导致库伦-莫尔断裂准则中的库伦等效应力增大,从而触发岩石破裂现象。岩石的破裂会导致地震波的辐射,因此,应力的下降和岩石的破裂会相互激发,形成一个正反馈环路,最终触发大规模的岩石破裂事件,这就是大地震。本项研究从冲击载荷下的断裂现象入手,论述拉伸波的作用,考虑到围压的影响,指出拉伸波虽然可以在爆破中直接引起岩石的断裂,但是在地层深处,不能直接引起断裂事件。因此引出了内摩擦和库伦应力的影响机理。在此基础上,理解到大地震的触发,为一个不断通过内摩擦改变形成的正反馈过程,解释了大地震发生有一个漫长的地应力下降过程和局部变形过程这一实际现象,从而对已有的地震的触发机理给出了合理的补充。更多还原

【Abstract】 The dynamic process of stress transmition in crust is studied by three dimensional dynamic FEM based on geophysical observation. The response of crust to ocean tide, simulation of seismogram, the globe model, the relation between geological structures and its influences to the crust stress are studied. Besed on the last two contents, the mechanism of earthquake triggering is studied. Furthermore, the feedback relation between the structure and the stress’ evolution has been studied. The studies give an interpretation to those dynamic effects in crust. Analysis says that the joint research of computational mechanics and geophysical observation is acceptable.The creations in this dissertation are on four aspects.Firstly, we conduct a joint research combined computational mechanics and geophysical observations. This joint research is realistic because the dynamic FEM meets the request of geophysics research where many equation of movtion can not be expressed briefly. Therefore the three dimensional finite elemental dynamic analysis offers a way for simulating the mechanical waves in crust. In addition, by applying the nonlinear analysis to geophysics research, the physics in geodynamic process can be understood.Second, we simulate large scale geophysical objects with the full size and dynamic analysises applys to geophysical research. Usually, geophysics studies large scale object via small experimental samples. Anyhow, to extrapolate the experimental results is not always safe. In this paper, we directly study full scale model. All the research contents, include the response of crust to ocean tide, simulated seismogram, the globe model, earthquake triggering mechanism, are conducted by full scale model.Third, the concept of "simulating seismogram" is proposed as a successor of theoretical seismogram that is an inversive method. The new concept includes a material definition of media and a transmition process of waves. The S and P waves can not be distinguished from each other in a small experimental sample, save the real rarth’s crust. By simulation of seismogram, we study the S and P wave in a model and comparing to the seismological observation. With the full scale model, the S and P waves are identified and successfully shown some information of the source. This is one of the meanings of simulation seismogram. Generally, any kind of theoretical seismogram, include simulated seismologram, is for getting information from the source or the media of waves by trial-and-error method.Finally, the dynamic mechanism of earthquake triggering is studied. The seismic waves are the propagations of the vibrations in crust, in which include the propagations of dynamic stresses. The compressive wave can be turned into tensile wave when it reflected on a free surface. However, the tensile or compressive wave may reduce or increase the inviromental stress in deep crust. It infuences the inner friction and, in turn, changes the Coulomb’s equvlent stress. By this way, it may trigger an small rupture. As the rupture of rock will emit the seismic wave, the dynamic stress and the events of rupture form a loop of positive feedback. The positive feedback process, namely, will finally build a big event of rock rupture, that is, earthquake. The study begins with the phenomenon of rupture under an impact load. And the effect of tensile wave is researched. Taking the giant ambient stress into consideration, though the tensile stress of impact load can cause a rupture under common condition near the earth’s surface, the tensile in deep crust cannot break the rock. Therefore, the mechanism of inner friction and of the coulombs’ stress is proposed. Based on the train of thoughts above, we interpret the earthquake as a result of a positive feedback process when the inner friction declines and the crannies enrich and big deformation appears. This explanation matches well to the phenomenon that a big shock occurrence often accompanying a long period’s normal stress decline and an obvious located deformation. By this train of thoughts, we have given a full description for earthquake triggering in dynamic mechanism.更多还原