Earthquake Sequence Simulations with Thermomechanical Coupling and Fault-Zone Fluid Transport

地震涉及沿着穿过地球固体地壳的裂缝摩擦滑动，这些裂缝被称为“断层”。地质断层可能被水之类的流体饱和，这会影响滑动的摩擦阻力，从而影响地震的大小和时间。断裂带内的流体压力是一个重要的参数，它受流体的生产和运移控制。量化流体对断层滑动的影响对于更好地评估地震风险至关重要。在这里，研究人员使用并开发最先进的计算机模拟来模拟断层上的地震，这些断层被认为沿途有活跃的流体流动，比如圣安德烈亚斯断层。该项目的重点是流体的运移以及压力和摩擦阻力的变化。其结果与自然发生的地震以及诱发的地震活动有关；后者是由涉及地球流体注入或提取的人类活动触发的。该项目还为一名女研究生提供支持，并为本科生提供培训。开发的建模软件将以开源许可证发布，供科学界使用。地震序列模拟捕捉到缓慢的构造加载、无地震滑动、地震成核和破裂传播。它们正在成为研究地震过程和解释观测结果的主要工具。断层强度是速率和状态摩擦系数与有效正应力(总法向应力减去孔压)的乘积。在这个项目中，孔隙压力是使用断裂带流体传输模型来确定的，而不是像通常所做的那样设定先验。在渗透率随时间变化的断裂带中，流体通过地壳上升；在愈合和封闭过程中，流体在两次地震之间减少，在断层滑动期间，由于破裂和膨胀而增加。这种行为被称为“故障阀”。初步模拟揭示了周期性超压累积和释放影响地震重复性的断层关闭行为。流体模型在这里被合并到热机械地震序列框架中。它解释了从局部摩擦滑动到深度分布粘性流动的转变，并使用了依赖于温度的幂定律。温度服从热方程，具有摩擦和粘性剪切加热，沿断层及其粘性根部产生热异常。该模型被推广到多孔粘弹性框架，该框架考虑了孔隙的蠕变闭合、流体(和孔隙)的热膨胀和流体加压。用相关时间尺度的无量纲化比率来量化系统的行为，如地震复发间隔、愈合/封闭时间尺度和跨孕震深度的孔压扩散时间。模拟的结果预测了地壳形变、地震和无地震滑动模式、热流、应力分布和其他可以与地球物理数据和地质观测相比较的特征。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Earthquakes involve frictional sliding along fractures extending through the Earth’s solid crust, called "faults." Geologic faults can be saturated by fluids like water which affect the frictional resistance to sliding, hence the size and timing of earthquakes. Fluid pressure within the fault zone, an important parameter, is controlled by the production and migration of that fluid. It is critical to quantify the effects of fluids on fault sliding to better assess earthquake hazards. Here, the researchers use and develop state-of-the-art computer simulations to model earthquakes on faults that are thought to have active fluid flow along them, like the San Andreas fault. The project focuses on fluid migration and changes in pressure and frictional resistance. Its outcomes have relevance to naturally occurring earthquakes, as well as to induced seismicity; these latter earthquakes are triggered by human activities that involve injection or extraction of fluids from the Earth. The project also provides support to a female graduate student and training to undergraduate students. The developed modeling software will be released with an open-source license for use by the scientific community.Earthquake sequence simulations capture the slow tectonic loading, aseismic slip, earthquake nucleation, and rupture propagation. They are becoming major tools to study earthquake processes and interpret observations. Fault strength is the product of a rate-and-state friction coefficient and the effective normal stress (total normal stress minus pore pressure). In this project, pore pressure is determined using a fault-zone fluid-transport model, rather that set a priori as is typically done. Fluids ascend through the crust in a fault zone whose permeability varies in time; it decreases interseismically from healing and sealing processes and increases from cracking and dilatancy during fault slip. This behavior is called "fault valving." Preliminary simulations reveal fault valving behavior with earthquake recurrence influenced by cyclic overpressure build-up and release. The fluids model is here merged into a thermomechanical earthquake sequence framework. It accounts for the transition from localized frictional sliding to distributed viscous flow at depth using a temperature-dependent power law. Temperature obeys the heat equation with frictional and viscous shear heating that create a thermal anomaly along the fault and its viscous root. The model is generalized to a poroviscoelastic framework that accounts for creep closure of pores, thermal expansion of fluids (and pores), and fluid pressurization. The system behavior is quantified in terms of dimensionless ratios of relevant time scales, such as earthquake recurrence interval, healing/sealing time scale, and pore pressure diffusion time across the seismogenic depth. Outcomes of the simulations predict crustal deformations, seismic and aseismic slip patterns, heat flow, stress profiles, and other features which can be compared to geophysical data and geological observations.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.