应用非均匀性对地震周期和地震破裂速度影响的数值模拟

发布时间：2018-05-05 08:37

本文选题：有限元数值模拟 + 特征地震　；参考：《中国科学技术大学》2015年博士论文

【摘要】：弹性回跳理论目前被认为是构造地震发生的主要机制。所谓的弹性回跳理论是：断层两边岩石在构造应力的作用下发生形变,应力和应变能逐渐累积,当应力达到一定程度就会造成断层的突然活动,导致地震发生,应变能也得到突然释放。野外地质调查和岩石物理实验都显示,地震的这种过程可以简化为断层的粘滑行为。而且断层的这种粘滑可以用滑移弱化摩擦准则或一个与速率和状态相关的摩擦准则来表示。研究也发现,地震发生时,断层面的破裂速度可能会出现超剪切现象。而超剪切破裂可能会在水平方向上产生扩张的平面S波以及马赫波,这些波会显著地加强断层破裂方向上的地面震动以及破坏力,即有着更强的能量辐射。地震断层的粘滑行为以及断层面的破裂过程与周围地质环境密切相关,如断层的应力扰动或孔隙压变化可能会影响地震的周期以及地震的大小(长时间尺度,年)、也有可能阻碍断层的破裂或触发超剪切破裂(短时间尺度,秒)。深刻认识周围环境变化对断层滑动和破裂过程的影响,对进一步理解地震的物理过程以及防震减灾工作都具有重要的理论和现实意义。本文意欲通过建立物理模型的方法,研究周围环境应力变化对断层粘滑行为以及断层面的破裂过程的影响。具体来说,就是利用有限元方法以及滑移弱化摩擦准则,通过模拟断层应力加载的过程来认识断层的滑动规律、并探索断层的动态破裂过程,以期能加深对地震物理过程的理解,并为防震减灾提供一些有价值的科学依据。首先,建立一个二维走滑断层数值模型,并结合断层滑移弱化摩擦准则对断层长期滑动规律以及应力扰动对其影响进行了研究；然后,以日本2011年Tohoku Mw9.0地震为具体实例,建立一个二维曲线断层数值模型,并且利用高精度的形变观测来约束模型；最后,在二维平面走滑断层中,研究局部有效正应力增加的障碍体(barrier)对超剪切破裂的触发作用。主要结果如下：(1)数值计算结果表明,在均匀应力分布情况下,平面断层滑动显示出典型的特征地震规律,断层面上的应力扰动对断层滑动规律产生影响,压应力增加明显延迟地震的发生时间,并增加地震释放的能量。应力扰动发生在地震破裂临界区时的影响比在震前滑移区时的影响显著。当发生在地震滑移区时,若应力扰动足够大,则压应力增大会造成地震发生时部分动力断层被暂时锁住,使得地震释放的能量变小,但可增加后续地震的能量；而压应力减小则可导致地震规律产生更加复杂的变化,会即时触发地震。如果应力扰动发生在一个地震周期的早期,则触发的地震较小,但可导致随后的地震提前发生；如果应力扰动发生在一个地震周期的后期,则会触发大地震。当应力扰动位于震前滑移区或破裂临界区时,小的扰动也可能产生类似的效果。应力扰动产生越晚,这种影响也越明显。应力扰动发生在破裂临界区的影响最明显。应力扰动的影响一般主要集中在应力发生扰动后的1-2个地震周期内。后续地震基本恢复无应力扰动时的特征地震规律。(2)数值模拟计算结果显示,模型在1000年间的6次大地震表现出特征重复地震的规律。模型的数值结果与地表同震GPS位移、震间GPS速度分布都具有较好的一致性。此外,模型结果也显示了在两次大地震中间会发生一次小地震。其中大地震的重复周期约为161年、单位破裂长度地震大小约为1.13 x 1020N m/km、小地震的地震矩约为5.62×1018Nm/km.进一步的计算结果表明,模型地幔楔的岩石圈和软流圈的粘性大小对震间GPS速度场产生显著影响。地幔楔的岩石圈和软流圈的粘性存在参数折中。如果软流圈粘性从1020 Pa s减小为2.5×1019 Pa s,为得到与观测基本相符的震间GPS速度分布,模型地幔楔岩石圈的粘性需从1020 Pa s增加到2.5×1020 Pa s。数值计算也显示,在一个地震周期内,模型空间重力变化值和地面点重力变化值基本上随时间均匀变化,在大陆一侧距海沟100km处分别可达-366 μgal和667μgal,但在陆地上变化较小,最大变化处约距海沟200 km,变化数值约为159 μgal和-77μgal;速度场的变化主要发生在震后约5年的时间内,此后基本保持稳定增加。(3)模拟结果证实,障碍体不仅会减缓或者阻止地震破裂的传播,但是同样也可能触发超剪切破裂。研究结果表明,超剪切破裂出现在一个参数区域内,这个区域介于两条直线边界之间。而如果障碍体的宽度位于下边界的下方,那么地震破裂能够克服这个障碍体,并且保持亚剪切破裂速度传播。如果障碍体的宽度位于上边界的上方,则障碍体始终能够阻止地震破裂的传播。此外,本文结果还表明,这种由障碍体触发产生的超剪切破裂传播可能最终会减速至亚剪切,这个过程依赖于障碍体的宽度以及位置。随着障碍体的宽度从下边界增加至上边界,障碍体导致的主破裂速度减量逐渐增加,并且超剪切破裂传播持续的距离也随之增加。这些结果表明断层上的障碍体可能并没有阻止地震破裂传播,反而可能触发超剪切破裂转变。而这种超剪切破裂转变对于近场强地面运动以及地震破坏力具有非常重要的影响。
[Abstract]:Elastic rebound theory is considered to be the main mechanism of tectonic earthquake. The so-called elastic rebound theory is that the rock on both sides of the fault is deformed under the action of tectonic stress, and the stress and strain can accumulate gradually. When the stress reaches a certain extent, the sudden movement of the fault leads to the occurrence of the earthquake, and the strain energy can also be suddenly released. Field geological survey and rock physics experiments have shown that this process can be simplified as a slip of the fault. And this kind of slip of the fault can be expressed by sliding weakening friction criterion or a friction criterion related to rate and state. The super shear rupture may occur in the horizontal direction of the plane S wave and Maher wave in the horizontal direction. These waves will significantly enhance the ground motion and destructive force in the direction of fault rupture. That is, it has stronger energy radiation. The slip behavior of the seismic fault and the fracture process of the fault layer are closely related to the surrounding geological environment. Correlation, such as the stress disturbance or pore pressure change of the fault may affect the period of the earthquake and the magnitude of the earthquake (long time scale, year). It may also impede the rupture of the fault or trigger the super shear rupture (short time scale, second). The physical process and the earthquake prevention and disaster reduction work have important theoretical and practical significance. This paper intends to study the influence of the ambient stress changes on the slip behavior of the fault and the rupture process of the fault surface by establishing the physical model. In particular, the finite element method and the sliding weakening friction criterion are used to simulate the fracture. The process of layer stress loading to recognize the sliding law of the fault and explore the dynamic fracture process of the fault in order to deepen the understanding of the physical process of the earthquake, and provide some valuable scientific basis for the earthquake prevention and reduction. The sliding law and the effect of the stress disturbance are studied. Then, taking the 2011 Tohoku Mw9.0 earthquake in Japan as a concrete example, a two-dimensional curve fault numerical model is set up, and the high precision deformation observation is used to restrain the model. Finally, in the two-dimensional plane strike slip layer, the obstacle (B) with the increase of local effective positive stress is studied. The main results are as follows: (1) the main results are as follows: (1) the numerical results show that, under the uniform stress distribution, the plane fault slide shows typical characteristic seismic law, the stress disturbance on the fault surface affects the fault slip law, and the pressure stress increases obviously and increases the time of the earthquake. The influence of the stress disturbance on the critical area of the earthquake rupture is more significant than that of the slip zone before the earthquake. When the stress disturbance occurs in the earthquake zone, if the stress disturbance is large enough, the increase of the pressure stress will result in the temporary locking of some dynamic faults when the earthquake occurs, so that the energy released by the earthquake will be smaller, but the follow-up can increase the follow-up. The energy of an earthquake, and the decrease of the pressure stress can cause more complex changes in the law of the earthquake, which triggers the earthquake immediately. If the stress disturbance occurs at the early stage of an earthquake cycle, the triggered earthquake is smaller, but it can lead to a subsequent earthquake to occur in advance; if the stress disturbance occurs in the later period of an earthquake period, it will trigger. A large earthquake. When the stress disturbance is located in the slip zone or the critical zone before the earthquake, small disturbances may also have similar effects. The later the stress disturbance occurs, the more obvious the effect is. The effect of the stress disturbance on the critical zone is most obvious. The effect of the stress disturbance is mainly concentrated on the 1-2 earthquake weeks after the stress is disturbed. During the period of the earthquake, the following earthquake basically recovered the characteristic seismic law without stress disturbance. (2) the numerical simulation results show that the model shows the characteristics of repeated earthquakes with 6 large earthquakes in 1000 years. The numerical results of the model are consistent with the GPS displacement of the ground surface and the GPS velocity distribution between the earthquakes. In addition, the model results are also obvious. A small earthquake occurs in the middle of the two large earthquakes. The repetition period of the large earthquake is about 161 years, the size of the unit rupture length is about 1.13 x 1020N m/km, and the seismic moment of the small earthquake is about 5.62 x 1018Nm/km.. The calculation results show that the viscosity of the rock rock ring and the soft flow circle of the model mantle wedge has the GPS velocity between the earthquakes. There is a significant impact on the field. The viscosity of the lithosphere and the asthenosphere of the mantle wedge exists in the parameter medium. If the viscosity of the asthenosphere decreases from 1020 Pa s to 2.5 x 1019 Pa s, the viscosity of the model mantle wedge lithosphere needs to be increased from 1020 Pa s to 2.5 * 1020 Pa S. numerical calculation. During the earthquake period, the spatial gravity change values and the ground point gravity change values vary with time, which can reach -366 gal and 667 gal respectively in the continental margin of the trench, but on the land, the change is smaller and the maximum change is about 200 km from the trench, and the change value is about 159 Mu gal and -77 Mu gal, and the change of the velocity field is mainly occurring. After about 5 years after the earthquake, it has been basically stable after the earthquake. (3) the simulation results show that the obstacle not only slows down or prevents the propagation of earthquake rupture, but also may trigger the super shear rupture. The results show that the super shear rupture occurs within a parameter region, and this region is between two linear boundaries. The width of the obstacle body is below the lower boundary, then the earthquake rupture can overcome this obstacle and keep the velocity of the subshear rupture. If the width of the obstacle is above the upper boundary, the obstacle can always prevent the propagation of the earthquake rupture. In addition, this paper also shows that this kind of overshoot is triggered by the obstacle body. The propagation of shear rupture may eventually decelerate to subshear, which depends on the width and position of the barrier body. As the width of the barrier increases from the lower boundary to the upper boundary, the reduction of the main rupture velocity caused by the obstacle increases gradually, and the continuous distance of the propagation of the super shear rupture increases. These results indicate that the fault is on the fault. The barrier body may not prevent the propagation of earthquake rupture, but may trigger the transition of super shear rupture. This super shear fracture transition has a very important influence on the near field strong ground motion and the earthquake damage force.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：P315

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