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

点击翻译按钮获取中文摘要

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.