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，在不同的正应力和/或滑动速度下，相似岩石可能经历非常不同的软化过程。因此，在与地震物理相关的界面条件下，从实验室实验中现场推断断层岩的弱化行为不能简单地通过刻度练习来完成，而相对较小的正应力和/或滑动速度的变化可以导致滑动弱化距离的变化一个数量级。基于这些观察结果，研究人员建议采用两种主要方法来推进我们对地震断层摩擦本构行为的理解：(1)通过协同结合分离式Hopkinson压杆和双向剪切摩擦仪来实现一种新的动态剪切摩擦测试仪，以研究完整和散体岩土材料中的动态摩擦；以及(2)开发一种方法来检验从动态摩擦实验中提取的参数在动态破裂模型中的有效性。由于它解决了地震物理学中的一些突出问题，包括在典型的断层破裂事件中滑动和滑动速度对断层强度的影响，这一建议的学术价值得到了加强。到目前为止，还没有实验室实验将大位移、高滑移率和正常应力结合在一起，这些被认为是天然断层界面上的动态地震滑动的特征。这些失败意味着，在地震动态滑动过程中可能发生的过程还没有经过实验研究。本文提出的新的实验结构是对工程中常用的研究工程材料(分离式Hopkinson压杆)高应变率行为的实验方法和岩土材料准静态摩擦研究(双向剪切仪)的改进，它有可能提供与地震物理直接相关的滑移速度和法向应力范围内的摩擦数据。此外，我们提出的双管齐下的工作方法旨在从根本上改变我们研究断层摩擦阻力的方式。第一个任务保证了重要的结果，这些结果将在相关条件下提高对地震破裂过程中动态摩擦的理解，而第二个方法将通过比较包含实验室得出的摩擦滑移本构行为的动态破裂模型的预测与实验室破裂实验来进一步约束所推断的摩擦本构模型。拟议中的研究将在几个方面有助于我们对地震的理解。为了建立地震过程的理论模型，我们必须了解地震过程中断层上的摩擦力是如何变化的。特别是，我们建议研究的减弱机制对地震期间应力降的大小具有深远的影响，从而对强地面震动的大小具有深远的影响。断层强度随位移和破裂速度变化的方式，以及破裂尖端后滑动速度减小时的愈合速度，可以控制破裂的扩展模式，即裂缝或脉冲。因此，了解动态摩擦不仅对于预测强地面运动及其造成的损害的实际问题很重要，而且对于回答备受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，这个问题最终是圣安德烈亚斯深部断层观测站(SAFOD)项目的责任。拟议的计划还提供了令人兴奋的跨学科研究和教育互动的机会，让来自两个相邻机构的教师和研究生参与进来。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招收代表人数不足的少数族裔学生。通过在相关同行评议期刊上发表会议报告和出版物，计划传播研究成果。调查人员还将利用互联网和基于大众媒体的信息传播，以提高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.