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发表的研究，加利福尼亚州中部从霍利斯特到帕克菲尔德的圣安德烈亚斯断层大约150公里(90英里)已被确认为蠕动段(沿着活动断层的段，不太可能产生破坏性地震)。然而，累积的地震学证据和监测技术令人印象深刻的进步让一些科学家质疑，在可预见的未来，这一部分是否会继续爬行。这段断层破坏性地震的潜在危险将直接影响350万人。自2006年以来，圣安德烈亚斯深度断层观测站(SAFOD)提供了大量证据，包括从爬行断层带内钻取的一大套岩石，垂直深度约为2.7公里(1.6英里)。直接接触到正在被积极剪切的岩石，揭示了断层如何在不引起大地震的情况下通过构造力移动的许多信息。但目前还不确定同样的机制是否适用于天文台以下的深度和更高的温度。该项目将使用来自天文台目前不活跃的岩石的钻芯样本，对最终确定断层的过程进行基础科学研究-S倾向于沿蠕动段产生破坏性地震。这一预期是现实的，因为天文台的非活动岩石被认为持有更深、更高温度下断层运动的信息。该提案使用传统的信息收集工具，如X射线探头和电子显微镜，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)，以实现项目规定的目标。使用地理信息系统软件还可以通过基于网络的地图应用程序和可视化，更容易地将结果在线提供给公众。这项研究的主要目的是探索研究较少的SAFOD损伤带岩石的信息含量，以建立圣安德烈亚斯断裂带(CSAF)中部无震蠕变发育的微观结构-成分模型。这项研究将把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射线衍射在内的传统数据收集方法与地理信息科学(GIS)技术的非传统使用相结合，以产生更易于可视化和分析、存储和传播的结果。大多数基于SAFOD样品的研究表明，软弱粘土矿物相(摩擦系数1-0.01)上的剪切局部化是CSAF上部抗震蠕变的原因。目前的模型还指出，在SAFOD以下3公里深处存在蛇纹岩来源的粘土矿物。然而，鉴于SAFOD现场附近深部低速带的存在，CSAF中反复发生的微震，以及与活动蠕变带接壤的破坏带泥的证据，我们认为以下问题值得回答：1.什么累积和重新激活的变形过程和模式定义了SAF破坏带，我们可以了解到什么关于SAFOD以下深处的变形过程？2.破坏带和活跃的蠕变核之间可能相互作用的空间范围是什么，以及是什么过程(例如变形机制)驱动了这种相互作用？3.这种相互作用对CSAF的抗震行为有什么潜在的影响？这项拟议的研究为目前关于加州中部圣安德烈亚斯断层蠕动段是否会像历史记录所表明的那样保持稳定的抗震模式，或者未来可能发生强烈地震事件的讨论提供了物质基础。CSAF的一个直接基于证据的无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动断层段。这项研究的结果不仅对构造地质学家感兴趣，而且还为构造物理学、地震学和岩土工程的研究提供了物理约束。该项目通过将地理信息系统的方法引入固体地球和微构造地质学，为跨学科科学做出了贡献。研究生和本科生将接受使用这两个学科的方法和概念的培训。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NSF的法定使命，并通过使用基金会的智力优势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.