The Role of Fault Strands and Roughness in Fault and Earthquake Mechanics

断层是非平面的、粗糙的，而不是平坦的表面，这一观察结果给试图理解它们是如何运动的提出了许多挑战。首先，对这样的曲面进行建模在数值上是困难的。其次，当滑动在单一粗糙表面上累积时，法向应力就会累积，断层很快就会锁定，无法克服摩擦。一种标准的方法是通过“向后滑动”的方法来绕过这种锁定，借此规定滑移，然后施加允许这种规定滑移发生的所产生的非均匀加载应力。不幸的是，由此产生的行为在许多方面看起来更接近于平面断层的行为，因此，几何图形的一些内在影响似乎被这种有意构建的非均匀载荷抵消了。这个项目试图开发一种新的方法来解决这个问题，基于这样的观察，即断层不是作为一个单独的断层表面发生的，而是作为断层带中的一个断层表面系统发生的。特别是，这项工作将检查断层链，将断层建模为一组粗糙表面，而不是一个粗糙表面。通过这种方式，该建议旨在找到动态一致的断裂带，这些断裂带都是由粗糙的断层组成的，并且在均匀加载时不会锁定。为了实现这一目标，利用其预期的成功，并测试其影响，将进行三个广泛的研究元素。第一，测试假设，一个粗糙的断层系统可以在均匀载荷下滑动而不锁定。一种新的数值方法的持续发展和应用，这种方法不需要断层与底层网格对齐，将开辟潜在几何研究的新领域。然后，使用这种数值方法来构建和测试相关的粗糙故障集。展示成功将是理解和模拟断层机制的重要一步。第二，将模型断裂带应用于与断层和地震力学有关的问题。SLIP在系统中是如何分区的？是否存在最小数量的有效链？分配会随着静摩擦系数的改变而改变吗？研究动态破裂的序列，可以提出对地震力学重要的问题。粗糙断层上的震级分布比平面断层上的更像古登堡-里希特吗？个别事件是倾向于向下传播一条链，还是从一条链跳到另一条链？断裂沿走向同时断裂多股的频率是多少？沿走向滑动与断层粗糙度有何关系？第三，将模型系统与地质、地震观测结果进行对比。将进行两项新的测量，量化地质观测数据。其中一项测量涉及量化弯曲和相邻线束之间的曲率之间的相关性。第二项测量涉及测量大地震中地表侧向滑动应变的分布。这方面的初步工作表明，地表滑动应变的典型值与其他人以前测量的断层粗糙度之间存在有趣的潜在关系。这种测量方法可以通过直接的地质观测为地震的物理学提供基本的限制。地震对公共安全和财产的危害给社会带来了许多挑战。了解地震行为的起源将有助于我们更好地了解这些灾害。这个项目旨在解决地震动力学中的一个基本问题，以及地震发生在其上的断层系统的动力学，即复杂的几何学在这个问题中扮演什么角色。从单个断层或断层集合的角度来看待问题的传统方法，已经遇到了如何适应非平坦不规则几何图形上累积滑移的问题。PI将研究断层的一种观察到的性质，即它们以断层链的形式出现--断层中的多个表面--在允许滑动累积到非平面结构上所起的作用。

The observation that faults are nonplanar, rough rather than flat surfaces,poses a number of challenges in trying to understand how they move. First, modeling such surfaces is difficult numerically. Second, as slip accumulates on a single rough surface, normal stress builds up and the fault soon locks, unable to overcome friction. A standard approach has been to circumvent this lock-up by the method of ``backslipping'', whereby slip is prescribed, and the resulting heterogeneous loading stresses which would allow such prescribed slip to occur are then applied. Unfortunately, the resulting behaviors appear in many ways much closer to how planar faults behave, and thus some of the intrinsic impacts of the geometry appear to be canceled out by this purposefully constructed heterogeneous loading. This project seeks to develop a new approach to the problem, based on the observation that faults occur not as an individual fault surface, but as a system of fault surfaces in a fault zone. In particular, this work will examine fault strands, modeling faults not as one rough surface, but a set of rough surfaces. In this way, the proposal aims to find dynamically consistent fault zones which are both made of rough faults and when loaded uniformly do not lock up. To achieve this goal, take advantage of its anticipated success, and test its implications, three broad research elements would be carried out.First, testing the hypothesis that a system of rough fault strandscan slip under uniform loading without locking up. Continued development and application of a new numerical approach which does not require faults to align with an underlying mesh will open up new realms of study of potential geometry. Constructing and testing correlated sets of rough faults with this numerical approach would then occur. Demonstrating success would be a significant step in understanding and modeling the mechanics of faults. Second, applying the model fault zone to questions relevant to fault and earthquake mechanics. How is slip partitioned in the system? Are there a minimum number of active strands? Does partitioning change as the static coefficient of friction changes? Examining sequences of dynamic ruptures, questions of importance to earthquake mechanics could be asked. Is the distribution of sizes of events on rough faults more Gutenberg-Richter like than on planar faults? Do individual events tend to propagate down one strand, or jump from strand to strand? How often do ruptures break simultaneously multiple strands along-strike? How is slip along-strike related to fault roughness? Third, comparing the model system with geological and seismological observations. Two new measurements quantifying geological observables would be carried out. One measurement concerns quantifying correlations in bends and curvature between neighboring strands. A second measurement concerns measuring the distribution of lateral surface slip strains in large earthquakes. Preliminary work on this showsinteresting potential relationships of typical values of surface slip strain with previous measurements by others of fault roughness. This measurement could provide fundamental constraints on the physics of earthquakes from direct geological observations.Earthquakes pose a number of challenges to society in terms of hazards to public safety and property. Understanding the origins of earthquake behaviors will help us better understand these hazards. This project aims to tackle one of the fundamental questions in earthquake dynamics, and the dynamics of the fault systems on which earthquakes occur, namely, what is the role of the complicated geometry in the problem. Traditional ways of looking at the problem in terms of individual faults, or collections of faults, have run into problems dealing with how to accommodate accumulating slip on nonflat irregular geometries. The PI will examine the role that an observed property of faults, that they occur as fault strands-- multiple surfaces within faults--plays in allowing for slip to accumulate on the nonplanar structures.