The Role of Fluids in Earthquake Initiation and Suppression: Models of Fluid-Grain Interaction in Fault Zones

地震是世界上许多有人居住地区的重大自然灾害。然而，尽管进行了大量的研究，控制断层滑动的稳定性和性质的条件仍然知之甚少。断层稳定性的一个重要控制因素是断层带常被粒状岩石充填，而这些粒状岩石的空隙中又充满了地下水。已经观察到这种流体的压力变化会触发断层的运动。拟议的项目将导致更好地了解断层滑动背后的基本物理，特别是断层带中固体应力和流体压力之间的耦合。 从这项工作中获得的见解将有助于评估与个别断层有关的地震危险性，以及当地地下水位的自然或人为变化引发地震的风险。这项工作还应该增加对饱和颗粒流的理解，如滑坡和近海浊流（两者都对生命和结构构成重大自然危害）。饱和断层泥-断块系统包括几种耦合机制，它们可以控制断层的稳定性，并决定断层蠕动、缓慢滑动（慢地震）或加速成地震的条件。流体压力的增加会降低断层上的有效应力，从而削弱断层并促进滑动，而伴随滑动开始的膨胀会导致压力降低，从而加强断层。流入和流出断层的流体将抑制压力波动，但高度依赖于粒状断层泥和封闭围岩中的渗透率，而渗透率又会随着断层泥膨胀和应力引起的损伤而发生很大变化。我们将使用颗粒尺度的数值模型来研究这个系统。该模型将颗粒动力学的离散单元法（DEM）与可渗透介质中流体流动的连续有限差分解耦合在一起。这种颗粒尺度的方法考虑到断层泥的非均质性和局部化，并考虑到压力对断层扩张的影响。我们将模拟断层泥填充断层带的部分，断层带被易碎的围岩限制（用粘结的粘性颗粒材料模拟）。模拟的断层将受到各种类型的载荷，包括构造剪应力的缓慢增加以及地震波和流体压力波的瞬态通过。其目标是绘制出断层稳定性的条件，并量化断层应力与断层泥和受损围岩中不断变化的渗透率和流体压力之间的关系。

Earthquakes represent a significant natural hazard in many inhabited areas of the world. Yet, despite much study, the conditions that control the stability and nature of the slip on faults are still poorly understood. An important control on fault-stability arises from the fact that faults zones are often filled with granulated rock and the interstices of these grains are filled with groundwater. Variations in the pressure of this fluid have been observed to trigger movement on faults. The proposed project will result in a better understanding of the basic physics behind slip on faults, in particular the coupling between solid stresses and fluid pressures in the fault zone. The insight gained from this work will contribute to the efforts to assess the seismic hazard associated with individual faults, and the risk of triggering earthquakes by natural or man-made changes in local groundwater levels. This work should also add to the understanding of saturated granular flows, such as landslides and offshore turbidity flows (both of which constitute significant natural hazards to lives and structures). The saturated gouge-fault block system includes several coupled mechanisms that may control fault stability and determine the conditions under which faults creep, slip slowly (slow earthquakes), or accelerate into earthquakes. Increases in fluid pressure reduce the effective stress across a fault, which weakens the fault and promotes sliding, while the dilation that accompanies the onset of slip will lead to pressure reductions that can strengthen the fault. Fluid flow into and out of the fault will mute pressure fluctuations, but is highly dependent on permeability in both the granular fault gouge and in the confining wall rock, which in turn can vary greatly with gouge dilation and stress-induced damage. We will study this system using a grain-scale numerical model. This model couples together the discrete element method (DEM) for granular dynamics with a continuum finite-difference solution for fluid flow through permeable media. This grain-scale approach allows for heterogeneity and localization within the fault gouge, and accounts for effects of the pressure on the dilation of the fault. We will simulate sections of a gouge-filled fault zone confined by breakable wall rock (simulated with bonded cohesive granular material). The modeled fault will be subjected to various types of loading, including slow increases in tectonic shear stress and the transient passage of seismic waves and fluid pressure waves. The goal will be to map out the conditions for fault stability, and quantify the relationships between fault stresngth and the evolving permeability and fluid pressure in both the fault gouge and damaged wall rock.