Evidence for Dynamic Weakening Mechanisms in the San Andreas Fault: Microgeochemistry and Microthermometry of Fault-related Rocks from SAFOD Core and Exhumed Faults

非技术描述地震对美国和世界的许多地区构成了重大危险，了解地震发生的过程是当今地球科学中的一个关键问题。一种主要的观点是，断层岩在地震的最早阶段必须减弱；否则，断层滑动停止，就不会发生大地震。如果这种“动态减弱”发生，我们应该在经历过地震的断裂带的岩石中看到它的证据。该项目正在研究圣安德烈亚斯断裂带的岩石，以揭示地震造成的动态减弱的痕迹，并了解什么条件可能导致这种减弱。这项工作的结果将使大地震造成的危险模型得到改进。此外，该项目通过实践研究经验对学生进行广泛的地震地质学和断层岩专题教育，并通过社交媒体、视频和面对面演示向更广泛的受众传播基础地震地质学和物理学、科学钻探和岩心。技术摘要PI团队正在利用圣安德烈亚斯深部断层观测站(SAFOD)岩心和Cajon Pass钻探岩心的野外观测和样品，结合圣安德烈亚斯断层新的和以前的野外和钻探岩心样品，阐明大型发震断层中热激活动态弱化机制的证据。该团队在样本中寻找这些机制的证据，这些证据可能会保留深层滑移减弱的结构和组成。该团队正在评估这些机制在一系列空间和时间尺度上的分布，并测试挖掘出的断层在深部代表断层的程度。该团队正在将光学和扫描电子显微镜与详细的X射线衍射矿物学、微尺度全岩地球化学和稳定同位素分析相结合。PI正在使用激光拉曼显微测温仪对流体包裹体进行滑面检查，以确定流体成分和矿脉中的温度，并使用拉曼和X射线近边光谱来研究断层上碳、铁和锰在热驱动下元素变化的潜在新指标。这项工作的更广泛影响包括：1)在一系列地震地质学和断层岩石学专题方面教育学生，最终为STEM队伍增加训练有素的技术研究人员；2)通过开展动手活动、课堂练习、视频和社交媒体平台，提高更广泛的受众对基础地震地质学和物理学、科学钻探和岩心的认识。

Non-technical DescriptionEarthquakes pose a significant hazard in many areas of the United States and the world, and understanding the processes that generate earthquakes is a key problem in Earth science today. A leading idea is that fault rocks have to weaken during the earliest stages of the earthquake; otherwise, fault slip stops and large earthquakes would not occur. If such "dynamic weakening" occurs, we should see evidence for it within rocks from fault zones that have experienced earthquakes. This project is studying rocks from the San Andreas Fault zone to uncover traces of dynamic weakening from earthquakes, and to understand what conditions might lead to such weakening. The results from this work will enable improved models of the hazards posed by large earthquakes. In addition, this project is educating students in a wide range of earthquake geology and fault rock topics via hands-on research experience, and communicating fundamental earthquake geology and physics, scientific drilling, and coring to broader audiences via social media, videos, and in-person presentations. Technical Abstract The PI team is using field observations and samples from the San Andreas Fault Observatory at Depth (SAFOD) core and the Cajon Pass drill core, combined with new and previous field and drill core samples of the San Andreas Fault, to elucidate evidence for thermally activated dynamic weakening mechanisms in large seismogenic faults. The team seeks evidence for these mechanisms in samples that may preserve the texture and composition of slip weakening at depth. The team is evaluating the distribution of these mechanisms across a range of spatial and temporal scales, and testing how well exhumed faults represent faults at depth. The team is integrating optical and scanning electron microscopy with detailed X-Ray diffraction mineralogy, micro-scale whole-rock geochemistry, and stable isotope analyses. The PIs are examining slip surfaces with Laser Raman microthermometric spectroscopy on fluid inclusions to determine fluid compositions and temperatures in veins, and with Raman and X-ray Near Edge Spectroscopy to investigate potentially new indicators of thermally driven elemental changes of carbon, iron, and manganese on the faults. Broader impacts of this work include: 1) Educating students in a range of earthquake geology and fault rock petrological topics, ultimately adding uniquely trained technical researchers to the STEM workforce; and 2) Increasing awareness of fundamental earthquake geology and physics, scientific drilling, and coring to broader audiences through the use of engaging hands-on activities, class exercises, videos, and social media platforms.