Collaborative Research: Developing a Link between Dynamic Friction and Fracture Mechanics Models of Earthquake Rupture using a New Dynamic Double-direct Shear Apparatus

地球物理学界的研究人员普遍认为，类似的岩石在不同的正应力和/或滑动速度状态下可能会经历非常不同的弱化过程。因此，从与地震物理学相关的界面条件下的实验室实验中推断断层岩的原位弱化行为不能简单地通过缩放练习来完成，法向应力和/或滑动速度的相对较小的变化可以导致滑动弱化距离的数量级变化。 基于这些观测结果，研究者们提出了两种主要的方法来推进我们对地震断层摩擦本构行为的理解：（1）将分离式Hopkinson压杆和双直剪摩擦装置相结合，建立一种新的动剪摩擦试验装置，用于完整和粒状岩土材料的动摩擦研究;以及（2）开发一种方法来测试从动态破裂模型中的动态摩擦实验提取的参数的有效性。 这一建议的智力价值是加强的事实，它解决了一些突出的问题，在地震物理学，包括在一个典型的断层破裂事件的断层强度的滑动和滑动速度的影响。迄今为止，没有任何联合收割机实验将大位移、高滑移率和正应力结合起来，而这些被理解为天然断层界面处动态地震滑移的特征。这些缺陷意味着地震中动态滑动可能发生的过程还没有被实验探索过。本文提出了一种新的实验装置，它是对工程上常用的研究工程材料高应变率行为的实验方法的改进（分离式霍普金森压杆）和岩土材料的准静态摩擦研究（双直接剪切装置），具有提供与地震物理学直接相关的滑动速度和正应力范围内的摩擦数据的潜力。此外，我们所提出的工作的双管齐下的方法旨在从根本上改变我们研究断层摩擦阻力的方式。第一个任务保证显着的结果，将推进状态的理解在地震破裂过程中的相关条件下的动态摩擦，而第二种方法将进一步限制推断的摩擦本构模型，通过比较预测的动态破裂模型，将实验室衍生的摩擦滑移本构行为与实验室破裂实验。拟议的研究将从多个方面促进我们对地震的了解。为了建立地震过程的理论模型，我们必须了解断层上的摩擦阻力在地震中是如何变化的。特别是，我们建议研究的弱化机制对地震期间应力下降的幅度以及因此对强烈地面震动的幅度具有深远的影响。断层强度随位移和破裂速度而变化的方式，以及随着破裂尖端后面的滑移速度减小而发生愈合的速率，可以控制破裂传播的模式，即作为裂纹或脉冲。因此，了解动摩擦力不仅对预测强地面运动和由此造成的破坏的实际问题很重要，而且对回答受到相当关注的重大科学问题也很重要，例如圣安德烈亚斯断层的强度/热流悖论，这个问题最终导致了圣安德烈亚斯断层深度观测（SAFOD）项目。 拟议的计划还提供了跨学科研究和教育互动的令人兴奋的机会，涉及教师和研究生从两个相邻的机构。这两所大学都强烈鼓励本科生参与尖端的教师研究，这也将发生在拟议的工作中。还将特别注意为该项目征聘代表性不足的少数民族学生。计划通过会议介绍和在相关同行评审期刊上发表文章来传播研究成果。调查人员还将利用互联网和大众媒体传播信息，以提高人们对拟议研究在减轻地震灾害方面的潜在影响的认识。

There is broad agreement amongst researchers in the geophysics community that similar rocks may undergo very different weakening processes in different normal stress and/or slip velocity regimes. Consequently, inference of weakening behavior of fault rocks in situ from laboratory experiments at interfacial conditions of relevance to earthquake physics cannot be done simply by scaling exercises, and relatively small changes in normal stress and/or the slip speed can result in changes in the slip weakening distance of an order of magnitude. Motivated by these observations, the investigators propose to advance the current state of our understanding regarding the frictional constitutive behavior of earthquake faults using two principal approaches: (1) implementing a new dynamic shear friction testing apparatus by synergistically combining the split-Hopkinson pressure bar and the double-direct shear friction apparatus to the study of dynamic friction in both intact and granular geo-materials; and (2) developing a methodology for testing the efficacy of parameters extracted from dynamic friction experiments in dynamic rupture models. The intellectual merit of this proposal is strengthened by the fact that it addresses some of the outstanding problems in earthquake-physics, including the influence of slip and slip-velocity on fault strength during a typical fault rupture event. No laboratory experiments to-date combine the large displacement, high slip rates, and normal stresses that are understood to characterize dynamic earthquake slip at natural fault interfaces. These failings mean that processes that may occur during dynamic slip in earthquakes have not been explored experimentally. The new experimental configuration proposed in here, which is a modification of the well-established experimental procedures employed routinely in engineering for investigating high-strain-rate behavior of engineering materials (split Hopkinson pressure bar) and quasi-static friction studies in geo-materials (double-direct shear apparatus), has the potential to provide friction data in the slip-speed and normal stress range of direct relevance to earthquake physics. Furthermore, the two-pronged methodology of our proposed work aims to fundamentally change the way we approach studying the frictional resistance of faults. The first task guarantees significant results that will advance the state of understanding of dynamic friction during earthquake rupture under relevant conditions, whereas the second approach will further constrain the inferred frictional constitutive models by comparing predictions of dynamic rupture models that incorporate lab-derived frictional slip constitutive behavior with laboratory rupture experiments. The proposed research will contribute toward our understanding of earthquakes in several ways. To construct theoretical models of the earthquake process, we must understand how frictional resistance on faults changes during an earthquake. In particular, the weakening mechanism that we propose to study have profound implications for the magnitude of stress-drops during earthquakes and consequently for the magnitude of strong ground shaking. The manner in which fault strength varies with displacement and rupture velocity, as well as the rate at which healing occurs as the slip velocity decreases behind the rupture tip, can control the mode of rupture propagation, i.e. as a crack or a pulse. Thus, understanding dynamic friction is important not only for practical matters related to predicting strong ground motions and resulting damage, but also for answering major scientific questions receiving considerable attention, e.g. the strength of the San Andreas fault/the heat-flow paradox, the question that ultimately is responsible for the San Andreas Fault Observatory at Depth (SAFOD) project. The proposed program also provides exciting opportunities for interdisciplinary research and educational interactions by involving faculty and graduate students from two neighboring institutions.. Both universities are strongly encouraging the involvement of undergraduate students in cutting edge faculty research, and this would occur for the proposed work as well. Special attention will also be given to recruitment of underrepresented minority students for the project. Dissemination of research results is planned by conference presentations and publications in relevant peer-reviewed journals. The investigators will also employ internet and mass-media-based information dissemination to increase awareness of the potential impact of the proposed research in earthquake hazard mitigation.