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

地震包括沿断裂延伸穿过地球的固体地壳，称为“断层”的摩擦滑动。地质断层可以被像水这样的流体饱和，这些流体会影响滑动的摩擦阻力，从而影响地震的大小和时间。断裂带内流体压力是一个重要的参数，它是由流体的生产和运移控制的。量化流体对断层滑动的影响是更好地评估地震危险性的关键。在这里，研究人员使用并开发了最先进的计算机模拟来模拟断层上的地震，这些断层被认为有活跃的流体流动，比如圣安德烈亚斯断层。该项目侧重于流体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地震活动有关；后一种地震是由人类活动引发的，这些活动涉及从地球上注入或提取流体。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荷载、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-状态摩擦系数与有效法向应力（总法向应力减去孔隙压力）的乘积。在本项目中，孔隙压力的确定采用了断裂带流体输运模型，而不是像通常那样采用先验方法。流体在渗透率随时间变化的断裂带中通过地壳上升；地震间由于愈合和封闭作用而减小，而在断层滑动过程中由于裂缝和膨胀作用而增大。这种行为被称为“故障阀”。初步模拟结果揭示了超压循环积累和释放对地震复现的影响。流体模型在这里被合并到一个热力学地震序列框架中。它使用温度相关的幂律来解释从局部摩擦滑动到深度分布粘性流动的转变。温度服从沿断层及其粘性根部产生热异常的摩擦和粘性剪切加热的热方程。该模型被推广为考虑孔隙蠕变闭合、流体（和孔隙）热膨胀和流体加压的孔粘弹性框架。系统行为是根据相关时间尺度的无因次比率进行量化的，例如地震复发间隔、愈合/密封时间尺度以及整个发震深度的孔隙压力扩散时间。模拟结果预测了地壳变形、地震和地震滑动模式、热流、应力剖面和其他可与地球物理数据和地质观测相比较的特征。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。

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.