Postdoctoral Fellowship: EAR-PF: To roll, flow, or fracture - that is the question: Investigating the mechanisms behind friction and the stability of faults

Kristina Okamoto博士获得了美国国家科学基金会EAR博士后奖学金，在明尼苏达大学进行研究，研究断层泥摩擦行为的物理规律。断层破坏可能发生在从缓慢蠕变（厘米/年）到快速地震（米/秒）不等的滑动速度范围内，这种破坏速率取决于断层的摩擦（抗滑动）。因此，了解地震发生的地点和时间需要一个摩擦模型。主要的模型是一组适合实验室数据的方程，称为速率和状态摩擦。虽然这些方程通常是有用的，但它们不包括系统的任何潜在物理。正因为如此，科学家们无法将结果外推到实验室未直接探索的压力和温度条件。由于实验的限制，许多与地球有关的压力和温度条件是无法达到的。最近，一种新的摩擦模型被定义，其中摩擦状态由滑动过程中晶粒的永久变形控制。这种永久变形被称为塑性变形，并增加了系统的剪切强度，称为背应力。虽然该模型可以拟合类似速率和状态摩擦的实验，但尚未在实验室中系统地研究背应力对摩擦的影响。本项目将改变初始颗粒的背应力量，然后对该材料进行摩擦实验。在550℃和100 MPa法向应力条件下的初步实验表明，起始晶粒中的背应力量会导致材料在稳态滑动所需的剪切应力量发生较大变化。测试和增强这个新模型可以更好地预测地震和缓慢滑动的情况。在这项研究之外，冈本博士将通过UMN岩石变形研究机会（RORD） REU指导学生，并共同监督一个本科生研究项目。冈本博士还将参与并协助UMN正在进行的旨在促进多样性和支持来自代表性不足群体的地球科学家的倡议。这项工作将通过确定材料在一系列压力和温度条件下的速度依赖关系来进一步研究这个新模型，这些条件可能跨越变形机制，如颗粒接触处的膨胀、断裂和塑性变形。在与塑性相关的条件下，稳态摩擦系数和摩擦率依赖于背应力，但当温度和压力较低时，塑性的影响较小，摩擦应是膨胀的函数，而不是背应力的函数。当压力高而温度低时，摩擦主要取决于晶粒的断裂能力。然而，在背应力和断裂之间有一个反馈，目前还没有被绘制出来。背应力基本上是由晶格中被称为位错的小分离的添加引起的。通过新的压痕技术，将探索晶粒断裂能力与位错密度的关系。这将使我们更好地理解摩擦系统在低温高压下的表现。总的来说，探索摩擦是否在一个广泛的参数范围内取决于背应力，将是将实验室摩擦推断到实验室未探索的压力/温度条件以及更大的空间和时间尺度的基础。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。

Dr. Kristina Okamoto has been awarded an NSF EAR Postdoctoral Fellowship to conduct research at the University of Minnesota investigating the physics governing the frictional behavior of fault gouge. Fault failure can occur at a range of slip speeds varying from slow creep (cm/year) to fast earthquakes (m/s) and this rate of failure depends on the friction (resistance to sliding) of the fault. Therefore, understanding where and when earthquakes happen requires a frictional model. The predominant model is a set of equations that fit laboratory data called rate and state friction. While these equations have been generally useful, they do not include any underlying physics of the system. Because of this, scientists are unable to extrapolate results to pressure and temperature conditions not directly explored in the lab. Due to experimental constraints, many pressure and temperature conditions relevant to the earth are not attainable. Recently, a new frictional model has been defined, where the frictional state is governed by the permanent deformation of grains during sliding. This permanent deformation is called plastic deformation, and adds shear strength to the system, called backstress. While this model can fit experiments similar to rate and state friction, the effect of backstress on friction has not been investigated systematically in the laboratory. This project will vary the amount of backstress in the starting grains and then perform friction experiments on this material. Preliminary experiments at 550°C and 100 MPa normal stress show that the amount of backstress in the starting grains causes a large change in the amount of shear stress required to slide the material at a steady state. Testing and enhancing this new model will allow for better predictions of the conditions that allow for earthquakes versus slower slip. Outside of this research, Dr. Okamoto will mentor students through the Research Opportunities in Rock Deformation (RORD) REU at UMN and co-supervise an undergraduate research project. Dr. Okamoto will also engage with and aid in ongoing initiatives at UMN that aim to promote diversity and support geoscientists from under-represented groups.This work will further investigate this new model by determining the velocity dependence of materials over a range of pressure and temperature conditions that may span deformation mechanisms such as dilation, fracture, and plastic deformation at grain contacts. At conditions relevant to plasticity, the steady-state friction coefficient as well as the frictional rate-dependence will depend on backstress, but when temperatures and pressures are low, the effect of plasticity will be low, and friction should be a function of dilation rather than backstress. When pressures are high and temperature is low, friction should mostly depend on the ability of the grains to fracture. However, there is a feedback between backstress and fracture that is currently unmapped. Backstress is fundamentally caused by additions of small separations in the crystal lattice called dislocations. The dependence of the ability for grains to fracture on dislocation density will be explored through novel indentation techniques. This will enable a better understanding of how the frictional system at low temperatures and high pressures will behave. Overall, exploring whether friction depends on backstress over a wide parameter range will be fundamental to extrapolating laboratory friction to pressure/temperature conditions not explored in the lab as well as to larger spatial and temporal scales.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.