A microstructural study of deformation in currently-inactive fault rocks from the San Andreas Fault Observatory at Depth pertinent to aseismic creep in central California

根据历史记录、地表地质证据和大量已发表的研究，加利福尼亚中部从霍里斯特到帕克菲尔德的约150公里（90英里）的圣安德烈亚斯断层被认为是一个爬行段（沿着一个不太可能产生破坏性震动的活跃断层的段）。然而，累积的地震证据和监测技术的令人印象深刻的进步使一些科学家质疑，在可预见的未来，这一段是否还会继续爬行。断层这一段破坏性地震的潜在危险将直接影响到350万人。自2006年以来，圣安德烈亚斯断层观测站（SAFOD）提供了大量的物证，包括从爬行断层带内部钻出的大量岩石，垂直深度约为2.7公里（1.6英里）。直接接触正在被积极剪切的岩石，揭示了断层如何在构造力的作用下移动而不会引起大的震动。但不确定的是，同样的机制是否适用于天文台以下的深度和更高的温度。该项目将使用从天文台目前不活跃的岩石中提取的岩心样本，对最终确定断层的过程进行基础科学研究。沿蠕变段产生破坏性地震的趋势。这个期望是现实的，因为天文台的不活动岩石被认为保存着断层在更深和更高温度下运动的信息。该提案采用传统的信息收集工具，如x射线探针和电子显微镜，同时应用新的和创新的信息分析和处理技术（地理信息科学）来完成项目的既定目标。地理信息系统软件的使用也使得通过基于网络的地图应用程序和可视化更容易地向公众在线提供结果成为可能。本研究的主要目的是探讨研究较少的SAFOD损伤带岩石的信息含量，以建立圣安德烈亚斯断裂带（CSAF）中部地震蠕变发展的微观结构-组成模型。该研究将结合传统的数据收集方法，包括扫描电子显微镜、扫描/透射电子显微镜、电子背散射衍射和x射线衍射，以及地理信息科学（GIS）的非传统使用技术，以产生更容易可视化、分析、存储和传播的结果。大多数基于SAFOD试样的研究表明，弱粘土矿物相（摩擦系数1 ~ 0.01）的剪切局部化是导致CSAF上部截面地震蠕变的主要原因。目前的模型还指出，在SAFOD以下3km深处存在蛇纹岩衍生的粘土矿物。然而，考虑到SAFOD场址附近存在深部低速带，CSAF内反复出现的微地震，以及与活动蠕变带相邻的破坏带断层泥的证据，我们认为以下问题值得回答：什么样的累积和重新激活的变形过程和模式定义了SAFOD损伤区，我们可以了解到SAFOD以下深度的变形过程？2. 破坏区与SAF的主动蠕变核心之间可能的相互作用的空间范围是什么？驱动这种相互作用的过程（例如变形机制）是什么？3. 这种相互作用对CSAF抗震性能的潜在影响是什么？该研究为当前关于加利福尼亚中部的圣安德烈亚斯断层爬行段是否会像历史记录显示的那样保持稳定的地震模式，或者将来可能发生强震事件的讨论提供了物质基础。一个直接基于证据的CSAF地震蠕变模型也有助于更好地理解西海岸板块边界系统中类似的蠕变断裂段。该研究结果不仅对构造地质学家有重要意义，而且对构造物理学、地震学和岩土工程的研究提供了物理约束。该项目通过将GIS方法引入固体地球和微观构造地质学，为跨学科科学做出了贡献。研究生和本科生将被训练使用这两个学科的方法和概念。使用分析设备和数据集尤其有助于本科生体验科学研究的实践和科学方法的应用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Based on historic records, surficial geological evidence, and numerous published studies, approximately 150km (90 mile) of the San Andreas Fault in central California, from Hollister to Parkfield, has been recognized as a creeping segment (segment along an active fault that is unlikely to produce damaging tremors). However, cumulative seismological evidence and impressive advances in monitoring technology have lead some scientists to question if the segment will remain creeping for the foreseeable future. Potential hazards from damaging earthquakes in this section of the fault would directly affect 3.5 million people. Since 2006, the San Andreas Fault Observatory at Depth (SAFOD) has provided material evidence including a large suite of rocks drilled from inside the creeping fault zone down to a vertical depth of about 2.7km (1.6 miles). Direct access to rocks that are being actively sheared off has revealed much about how the fault moves by tectonic forces without causing major tremors. But it is uncertain that the same mechanics apply at depths and higher temperatures below the observatory. This project will use drill core samples from the currently inactive rocks in the observatory for conducting basic-science research into the processes that ultimately determine the fault?s tendency to produce damaging earthquakes along the creeping segment. This expectation is realistic because the inactive rocks of the observatory are believed to hold information about the fault movement at greater depths and higher temperatures. The proposal employs traditional tools of information gathering such as x-ray probes and electron microscopes while applying new and innovative techniques of information analysis and processing (the Geographic Information Science) to accomplish the stated objectives of the project. The use of GIS software also makes it possible to provide the results more readily to the public online through web-based mapping applications and visualizations. The primary objective of the proposed research is to probe the information content of the less studied SAFOD damage zone rocks in order to construct a microstructural-compositional model of the development of aseismic creep in central San Andreas Fault Zone (CSAF). The research will be carried out by combining the traditional methods of data collection including a scanning electron microscope, scanning/transmission electron microscope, electron backscatter diffraction, and x-ray diffraction with non-traditional use of techniques of Geographic Information Science (GIS) to produce results that are easier to visualize and analyze, store, and disseminate. A majority of studies based on the SAFOD samples indicate that shear localization on weak clay mineral phases (coefficient of friction 1-0.01) is responsible for the aseismic creep in the upper sections of CSAF. The current models also point to the presence of serpentinite-derived clay minerals at depths 3km below the SAFOD. However, in view of the presence of deep low velocity zones near SAFOD site, the repeating microearthquakes in CSAF, and evidence from the damage zone gouge bordering the active creep zones, we believe the following questions are worth answering: 1. What cumulative and reactivated deformation processes and patterns define the SAF damage zone and what could we learn about the deformation processes at depths below the SAFOD? 2. What is the spatial extent of possible interactions between damage zone and the actively creeping core of the SAF, and what processes (e.g. deformation mechanisms) drive such interactions? 3. What is the potential for such interactions to affect the aseismic behavior of CSAF? The proposed research provides a material basis for current discussion of whether the creeping section of the San Andreas Fault in central California will remain in a stable aseismic mode as the historical records indicate, or strong seismic events are probable in the future. A directly evidence-based model of aseismic creep in CSAF also contributes to better understanding of similar creeping fault segments in the West Coast plate boundary system. The results of this research are of interest not only to structural geologists, but also serve to provide physical constraints for research in tectonophysics, seismology, and geotechnical engineering. The project contributes to interdisciplinary science by introducing methods of GIS into solid earth and microstructural geology. Graduate and undergraduate students will be trained to use methods and concepts of the two disciplines. Working with analytical equipment and datasets help undergraduate students, in particular, to experience the practice of scientific research and applications of the scientific method.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.