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

地震是世界上许多居住地区的重大自然灾害。然而，尽管进行了大量研究，但控制断层滑动稳定性和性质的条件仍然知之甚少。对断层稳定性的一个重要控制源于这样一个事实：断层带通常充满粒状岩石，并且这些颗粒的间隙充满地下水。据观察，这种流体压力的变化会引发断层的运动。拟议的项目将有助于更好地理解断层滑动背后的基本物理原理，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相关的地震灾害，以及当地地下水位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，例如山体滑坡和近海浊流（两者都对生命和结构构成重大自然危害）。饱和断层断块系统包括几种耦合机制，这些机制可以控制断层稳定性并确定断层蠕动、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将导致压力降低，从而强化断层。流入和流出断层的流体会减弱压力波动，但高度依赖于粒状断层泥和围岩的渗透性，而渗透性又会随着断层扩张和应力引起的损伤而发生很大变化。我们将使用粒度数值模型研究该系统。该模型将颗粒动力学的离散元法 (DEM) 与流经渗透介质的流体的连续有限差分解结合在一起。这种颗粒尺度的方法考虑了断层泥内的异质性和局部化，并解释了压力对断层扩张的影响。我们将模拟由易碎围岩限制的充满断层带的部分（用粘合的粘性颗粒材料模拟）。建模的断层将承受各种类型的载荷，包括构造剪切应力的缓慢增加以及地震波和流体压力波的瞬态通过。目标是确定断层稳定性的条件，并量化断层强度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.