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

博士后奖学金：EAR-PF：滚动、流动或断裂 - 这就是问题：研究摩擦和断层稳定性背后的机制

• 批准号: 2305630
• 负责人: Kristina Okamoto
• 金额: $ 18万
• 依托单位: Okamoto, Kristina
• 依托单位国家: 美国
• 项目类别: Fellowship Award
• 财政年份: 2024
• 资助国家: 美国
• 起止时间: 2024-04-01 至 2026-03-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2305630&HistoricalAwards=false
• 关键词: Postdoctoral; Fellowship; EAR; PF; roll

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.

Kristina Okamoto博士已获得NSF博士后奖学金，在明尼苏达大学进行研究，调查断层泥摩擦行为的物理学。断层破坏可以发生在从缓慢蠕变（cm/年）到快速地震（m/s）的滑动速度范围内，并且这种破坏率取决于断层的摩擦力（滑动阻力）。因此，了解地震发生的地点和时间需要一个摩擦模型。主要的模型是一组方程，适合称为速率和状态摩擦的实验室数据。虽然这些方程通常是有用的，但它们不包括系统的任何基础物理。正因为如此，科学家们无法将结果外推到实验室中没有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，一种新的摩擦模型已被定义，其中的摩擦状态是由在滑动过程中的颗粒的永久变形。这种永久变形称为塑性变形，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于速率和状态摩擦的实验，但背应力对摩擦的影响尚未在实验室中系统地研究。这个项目将改变初始颗粒中的背应力的大小，然后在这种材料上进行摩擦实验。在550°C和100 MPa法向应力下的初步实验表明，起始晶粒中的背应力的量导致材料在稳定状态下滑动所需的剪切应力的量发生很大变化。测试和增强这个新模型将允许更好地预测允许地震与较慢滑动的条件。在这项研究之外，冈本博士将通过UMN的岩石变形研究机会（RORD）REU指导学生，并共同监督本科生研究项目。Okamoto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计划。这项工作将通过确定材料在压力和温度条件下的速度依赖性来进一步研究这一新模型，这些条件可能跨越变形机制，如膨胀，断裂和颗粒接触处的塑性变形。在与塑性相关的条件下，稳态摩擦系数以及摩擦率依赖性将取决于背应力，但是当温度和压力较低时，塑性的影响将较低，并且摩擦应该是膨胀而不是背应力的函数。当压力高而温度低时，摩擦力主要取决于颗粒的断裂能力。但是，在背应力和断裂之间存在当前未映射的反馈。背应力基本上是由晶格中称为位错的小分离的增加引起的。将通过新颖的压痕技术探索晶粒断裂能力对位错密度的依赖性。这将有助于更好地了解摩擦系统在低温和高压下的行为。总的来说，探索摩擦是否取决于背应力在一个广泛的参数范围内将是基本的推断实验室摩擦的压力/温度条件下，没有在实验室探索，以及更大的空间和时间scales.This奖项反映了NSF的法定使命，并已被认为是值得通过评估使用基金会的智力价值和更广泛的影响审查标准的支持。