基于流固耦合的静态库仑应力变化的数值计算及其在地震触发研究中的应用

发布时间：2018-05-11 07:05

本文选题：地震应力触发 + 静态库仑应力变化　；参考：《中国地震局地球物理研究所》2017年博士论文

【摘要】：地震“应力触发”是近年来地震科学研究中的热点之一,由地震引起应力转移从而对后续事件产生触发作用的应力触发模型已被大量震例的研究成果所支持。应力触发理论中的最关键问题是如何计算库仑应力变化,但传统方法的计算一般都是基于Okada的解析解,其中不考虑流体对固体骨架力学行为的影响。同时,模型中通常仅仅计算同震位错所产生的库仑应力变化,而不考虑震后余滑、孔隙流体迁移等其他因素造成的影响。但实际上,流体广泛存在于地下岩土介质中,对固体变形有着非常重要的影响。当主震发生后,随着时间的推移,震后余滑、介质孔隙压变化等震后效应的作用逐渐突出,由此引起的静态库仑应力变化也逐渐发挥作用。为此,本文基于孔隙弹性理论,考虑流-固之间的完全耦合作用,针对三种不同类型的断层错动模型(走滑型、逆冲型以及正断型),利用有限元数值模拟方法,分别计算同震静态库仑应力变化的空间分布,然后将其结果与传统算法的进行比较,考察流-固耦合的效果。在此之上,进一步计算震后余滑、孔隙流体迁移等引起的库仑应力变化的时空分布,定量分析这些因素对地震触发的影响。研究中,主要获得的成果如下:1.三种不同类型地震模型得到的介质孔隙压变化在空间的分布格局完全不同:走滑型地震产生的同震孔隙压变化图案在空间中呈正负相间的四象限分布,近场的静态库仑应力明显下降,流-固耦合作用对静态库仑应力变化的影响较大;逆冲和正断层地震产生的介质孔隙压变化在空间的分布图案类似,但正负区域正好相反;孔隙压在逆冲地震的震源附近上升,而在正断层地震的震源附近下降。同传统方法计算的库仑应力相比,逆冲地震产生的介质孔隙压变化使得震源附近的应力影区面积减小,这将会触发更多的余震;而正断层地震产生的孔隙压变化则正好相反,增大了震源附近的应力影区范围,这样可能会减小该区域余震发生的机会。2.三种不同类型地震的震后流体扩散效果也很不相同:在渗透率相同的介质中,走滑型地震的孔隙压力变化衰减速度相对较慢,在20天内逐渐衰减;逆冲和正断层地震产生的孔隙压变化则会在10天内急剧降低;但三种不同类型地震的孔隙压变化最终都会在60天后衰减完毕。虽然孔隙压力变化会在震后不断地演化,但由于孔隙压力变化对库仑应力变化的贡献所占比例较小,因而对总体静态库仑应力变化的影响非常有限。由此可见,单纯震后流体孔隙压力变化对地震触发的能力较弱,震后流体运移触发地震的物理机制还需深入研究。3.三种不同类型地震的震后余滑效果也很不一样:走滑型地震的震后余滑能整体上提升库仑应力变化的幅度,触发更多的地震;逆冲地震的库仑应力上升区域会在空间中整体扩展,触发范围更大的余震;而正断层地震的震后余滑对库仑应力变化的影响较小,仅沿断层两端有较小的扩展。4.此外,研究中还专门利用传统方法计算了四川芦山地震(MS7.0)对其余震的触发情况。当选择最优破裂面投影、或选择余震节面投影或使用不同的震源位错模型以及不同的有效摩擦系数时,大量的数值计算均发现,芦山地震对其余震没有明显的触发效果。总之,计算地震引起的库仑应力变化不是一个简单的问题,它涉及地震发生、震后响应、构造应力场和介质环境变化等。只有充分利用数值模拟手段,考虑诸如介质的流-固耦合等多种物理因素,才有可能获得比较接近真实的库仑应力变化,从而使得地震触发理论更为完善,更加科学合理地分析地震趋势及评估地震灾害。
[Abstract]:Earthquake "stress triggering" is one of the hotspots in recent seismological research. The stress triggering model, which is caused by the earthquake induced stress transfer and triggering the subsequent events, has been supported by the research results of a large number of seismic examples. The key problem in the theory of stress triggering is how to calculate Coulomb stress change, but the traditional method is calculated. It is generally based on the analytic solution of Okada, which does not consider the effect of fluid on the mechanical behavior of the solid skeleton. At the same time, the model usually only calculates the Coulomb stress changes caused by the dislocation of the same earthquake, but does not consider the influence of other factors, such as the residual slip after the earthquake, the migration of pore fluid and other factors. In the mass, it has a very important influence on the deformation of the solid. After the occurrence of the main shock, the aftershock effect of the aftershock and the change of the pore pressure of the medium gradually protruded with the passage of time, and the static Coulomb stress changes caused by it gradually play a role. This paper, based on the theory of pore elasticity, takes into account the complete coupling between the flow and solid. Using three different types of fault dislocation models (strike slip type, thrust type and positive fault type), the spatial distribution of static Coulomb stress changes of the same earthquake is calculated by finite element numerical simulation method, and then the results are compared with the traditional algorithm, and the effect of the fluid solid coupling is investigated. On this, the residual slip of the earthquake is further calculated. The temporal and spatial distribution of Coulomb stress changes caused by pore fluid migration, and quantitative analysis of the effects of these factors on earthquake triggering. The main achievements in the study are as follows: 1. the distribution pattern of pore pressure changes in the medium of three different types of seismic models is completely different: the variation of pore pressure caused by a strike slip earthquake The four quadrant distribution in the space is positive and negative in the space, the static Coulomb stress of the near field decreases obviously, the flow to solid coupling has great influence on the static Coulomb stress change, and the pore pressure variation produced by the thrusting and normal fault earthquakes is similar in the spatial distribution pattern, but the positive and negative regions are just the opposite; the pore pressure is the source of the thrusting earthquake. Near the source of the normal fault earthquake, it rises near the source of the normal fault earthquake. Compared with the Coulomb stress of the traditional method, the change of pore pressure caused by the medium pore pressure caused by the thrusting earthquake reduces the area of the stress area near the source, which will trigger more aftershocks, and the pore pressure variation produced by the normal fault earthquake is just the opposite and increases near the seismic source. The scope of the stress area, this may reduce the chance of aftershock in the region.2. three different types of earthquakes after earthquake fluid diffusion effect is very different: in the medium with the same permeability, the attenuation velocity of the pore pressure change of the strike slip earthquake is relatively slow, gradually attenuates in 20 days, and the pore of the thrusting and normal fault earthquakes The pressure change will decrease sharply in 10 days, but the pore pressure changes in the three different types of earthquakes will eventually decay after 60 days. Although the pore pressure changes will continue to evolve after the earthquake, the contribution of the pore pressure changes to the Coulomb stress change is smaller, so the influence on the overall static Coulomb stress change is not. It can be seen from this, it can be seen that the fluid pore pressure change after the simple earthquake is weak to the earthquake triggering, and the physical mechanism of the fluid migration triggering the earthquake after the earthquake is also necessary to study the residual slip effect of the.3. three different types of earthquakes after the earthquake. More earthquakes, the Coulomb stress rising region of the thrusting earthquake will expand in the whole space and trigger a larger aftershock, while the residual slip of the positive fault earthquake has little influence on the Coulomb stress change, and only along the faults at both ends of the fault has a small expansion.4.. In the study, the Sichuan Lushan earthquake (MS7.0) is also calculated by the traditional method. A large number of numerical calculations show that the Lushan earthquake has no obvious triggering effect on its aftershocks when selecting the optimal projection surface, or selecting the aftershock surface projection or using different seismic source dislocation models and different effective friction coefficients. In a word, the Coulomb stress changes caused by the calculation of the earthquake are not one. A simple problem involves the occurrence of earthquakes, the response of the earthquake, the tectonic stress field and the change of the medium environment. Only by making full use of the numerical simulation methods and considering various physical factors such as the fluid solid coupling of the medium, it is possible to get more close to the real Coulomb stress change, which makes the theory of earthquake triggering more perfect and more scientific. Learn to analyze earthquake trend reasonably and evaluate earthquake disaster.

【学位授予单位】：中国地震局地球物理研究所
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P315

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