Effect of Pore Pressure Rate on Rate and State Frictional Slip In Experiments

向地壳注入液体会引起地震。这种风险存在于地热开发、能源储存和回收以及碳封存的背景下。一个戏剧性的例子是美国中部地震的迅速增加。了解诱发地震活动对于评估和减轻风险至关重要。然而，由于现场复杂的地质条件和材料性质的不确定性，这是一项具有挑战性的工作。实验室实验提供了注入液体如何引发地震的重要信息。本文采用理论分析和数值模拟相结合的方法，研究了流体增压对岩石摩擦滑移的影响。将模拟结果与实验结果进行比较，研究了孔隙压力变化对断层滑动的影响。计算机模拟允许在实验室无法达到的条件下测试参数。在这里，它们被用来将实验结果外推到野外观测的时间和尺度上。该项目还支持西北大学（IL）研究生和本科生的培训。其结果改进了与诱发地震活动有关的危险的评估。更具体地说，该项目考察了施加的孔隙压力变化率对速率和状态摩擦滑移稳定性的影响。这项工作的动机是最近的实验，以不同的速率施加压力。这些结果表明，加速滑移事件的滑移速度和剪应力降与孔隙压力速率有关，而与孔隙压力大小无关。对一个简单的弹簧滑块模型进行数值模拟，假设滑动受速率和状态摩擦控制，结果表明压力速率可以控制快速滑动事件的频率。模型的改进表明，在实验室中观察到的滑移事件的特征如何取决于摩擦参数、加载速率、孔隙压力增加的速率和幅度以及扩散系数。这项工作为理解在实验室试验和现场注射流体时孔隙压力率的影响提供了基础。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Injection of fluid into the Earth’s crust can cause earthquakes. Such risks exist in the context of geothermal stimulation, energy storage and recovery, and carbon sequestration. A dramatic example is the rapid increase of earthquakes in the central US. Understanding induced seismicity is critical to assess and mitigate the risks. Yet, it is challenging because of the complex geology of field sites and uncertainty on material properties. Laboratory experiments provide essential information about how injecting fluids may trigger earthquakes. Here, the researchers use theoretical analysis and numerical simulations to study the effects of fluid pressurization on rock frictional slip. Comparing simulations with experimental observations, they examine how pore pressure changes affect fault sliding. Computer simulations allow testing parameters at conditions unattainable in the laboratory. Here, they are used to extrapolate experimental results to the time and scales of field observations. The project also supports the training of graduate and undergraduate students at Northwestern University (IL). Its outcomes improve the assessment of hazards associated with induced seismicity.More specifically, the project examines the effects of imposed rate of pore pressure change on the stability of rate and state frictional slip. The work is motivated by recent experiments that impose pressure at different rates. These show that the slip velocity and shear stress drop of accelerated slip events correlate with pore pressure rate rather than the magnitude of the pore pressure. Numerical simulations for a simple spring – slider model assuming sliding is governed by rate and state friction - show that the pressure rate can control the frequency of rapid slip events. Refinement of the model indicates how the features of slip events observed in the laboratory depend on frictional parameters, rate of loading, rate and magnitude of pore pressure increase, and diffusivity. This work provides a fundamental basis for understanding the effects of pore pressure rate accompanying injection of fluids in laboratory tests and at field sites.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.