Role of Silica Redistribution in the Evolution of Subduction Megathrusts, Shimanto Belt Japan

过去十年发生了两次有记录以来最具破坏性的地震，由于地面震动和海啸淹没沿海地区，近30万人丧生。在2011年的东北地震和最近发生的其他几次大地震中，地震事件发生在一种被称为“慢滑”的断层行为之前，因此也可能由这种断层行为引发——这种行为在20年前是未知的。因此，沿板块边界的断层要么是“蠕变”，要么是由于“粘滑”行为而经历地震的旧范式，并不能解释沿活动俯冲带的断层行为的全部范围——包括周期性的“慢地震”和与震颤同时发生的微地震活动。认识到滑动行为的多样性，需要一个新的范式来解释这些不同的行为在引发地震中可能发挥的作用。这项研究的核心原则是，关于板块边界滑动行为的非均质性的信息不仅记录在地震活动中，而且记录在我们今天可以在挖掘出的古代俯冲边界中观察到的脉状或矿化裂缝的分布和结构中。本项目专门研究天然水力压裂和碳酸钙和二氧化硅的局部再分配在与微震活动、慢滑和地震中弹性应变的积累和释放相关的滑动不稳定性演变中的作用。日本古代板块边界断裂带的脉系统将被检查，该断裂带记录了一系列反映地震产生深度和温度的条件。首席研究员和他的同事们提出了一个全面的假设，将断裂带中的二氧化硅再分布与地震动力学联系起来，例如在脉状结构和矿物学中。如果这一假设是正确的，那么俯冲带行为模型不仅要考虑断层的摩擦行为，还要考虑下盘水力压裂（即流体来源和渗透率的可预测函数）和硅重分布（即热激活过程）的作用，以解释俯冲带板块边界行为的非均质性。除了该项目的研究目标之外，该奖项还为宾夕法尼亚州立大学的一名西班牙裔女研究生的培训提供支持，从而有助于扩大科学、技术、工程和数学（STEM）学科中代表性不足的群体，并为一名本科生的参与提供机会，该本科生将在项目的第二年完成繁重的独立研究论文。学生将学习如何使用低温模型进行裂缝密封，这些模型已经应用于石油工业的被动边缘，同时将这些模型过渡到俯冲板块边界的高温岩石。该研究生还将参与与日本科学家的国际合作，这项对暴露的古代岩石的研究将补充正在进行的NantroSEIZE海上钻探实验，该实验旨在实地评估地震产生深度的板块边界特征过程。由于俯冲带具有发生大震级地震的巨大潜力，因此在这些环境中研究断裂带的行为对社会的健康和经济福祉具有潜在的重大意义。本研究旨在研究俯冲界面下盘发育的矿化矿脉和组构所记录的水力压裂和随后的愈合过程。假设在脉体中观察到的织构范围是沿会聚边缘观察到的非均匀板块边界滑移行为范围的表现。在靠近板块界面的逆冲沉积层中，裂缝的愈合可能发生在与发震区温度下的地震复发间隔重叠的时间。开放的充满流体的裂缝会影响有效应力和储存和释放弹性应变能的岩石的强度/弹性特性，因此裂缝的愈合可能是孕震带锁定行为的基础。本文将研究日本岛东带内6个区域广泛剪切带的脉系及其相关的鳞状结构，这些剪切带形成于俯冲期间，含有普遍存在的石英脉，代表了孕震带内的整个温度范围。主要研究人员将：1)根据岩性和相对于主要断层的位置对露头上的脉系进行表征；2)用岩石学和阴极发光技术评估脉的微观结构；3)绘制脉附近潜在硅源的元素图，包括鳞状结构。为了评估二氧化硅局部扩散的作用，我们将绘制确定潜在二氧化硅来源区域的主要元素浓度图（例如，鳞状结构和裂缝附近解理的围岩）。他们还将根据微观结构确定裂缝的孔径和裂缝的密封程度。显微结构信息将与扫描线测量的静脉间距、厚度和长度结合使用，以限制开放裂缝的间距和密封裂缝所需的时间。这项研究将使我们能够解决有关俯冲界面附近石英脉的基本问题：脉内二氧化硅的来源是什么？我们能否确定典型的局部二氧化硅再分配的二氧化硅枯竭带？裂缝孔径和裂缝间距如何随岩石类型和温度的变化而变化？在发震区内，脉体结构是否随温度和深度的增加而有系统的变化？日本的岛曼托带是本研究的理想选择，因为它暴露了被解释为古滑脱的区域断裂带（逆冲下盘的活动板块边界破坏带），并且为了进行比较，有适合地震滑动和现有增生棱镜并置岩石的非序列带状带的例子。

The last decade has seen two of the most destructive earthquakes ever recorded, with nearly 300,000 lives lost due to ground shaking and inundation of coastal areas by tsunami. In the case of the 2011 Tohoku earthquake and several other large recent earthquakes, the seismic event was preceded by, and therefore potentially triggered by, a type of fault behavior known as "slow slip" -- a behavior that was unknown two decades ago. Therefore, the old paradigm that faults along plate boundaries either "creep" or experience earthquakes due to "stick-slip" behavior does not explain the full range of fault behavior along active subduction zones-- behavior that includes periodic "slow earthquakes" and microseismicity that coincides with tremor. Recognition of this diversity of slip behavior demands a new paradigm that explains the role that these various behaviors may play in triggering earthquakes. The central tenet of this study is that information about heterogeneity in plate boundary slip behavior is not only recorded by seismicity, but also in the distributions and textures of veins, or mineralized cracks, that we can observe today in exhumed ancient subduction boundaries. This project is specifically designed to investigate the roles of natural hydrofracking and local redistribution of calcium carbonate and silica for the evolution of slip instabilities associated with microseismicity, slow slip, and the buildup and release of elastic strain in earthquakes. Vein systems will be examined in ancient plate boundary fault zones in Japan that record a range of conditions that reflect the depths and temperatures at which earthquakes are generated. The principal investigator and his colleagues have developed an over-arching hypothesis that relates silica redistribution in fault zones - as exemplified in vein textures and mineralogies - to earthquake dynamics. If the hypothesis is correct, models of subduction zone behavior must consider not just the frictional behavior of the fault but also the role of footwall hydrofracturing (i.e., a predictable function of fluid sources and permeability) and silica redistribution (i.e., a thermally activated process) as an explanation for the heterogeneity in plate boundary behavior in subduction zones. In addition to the research goals of the project, this award provides support for the training of an Hispanic female graduate student at Penn State thus contributing to broadening of underrepresented groups in a Science, Technologoy, Engineering and Math (STEM) discipline, as well as providing opportunities for the participation of an undergraduate student who will complete research-heavy independent senior thesis during year two of the project. Students will be trained in the use of lower temperature models for crack sealing that have been used along passive margins with application to the oil industry while bridging these models toward application to higher temperature rocks from subduction plate boundaries. The graduate student will also be involved in an international collaboration with Japanese scientists, and this study of exposed ancient rocks will complement the ongoing NantroSEIZE offshore drilling experiment designed to evaluate in situ the processes that characterize the plate boundary at depths where earthquakes are generated. Because subduction zones have significant potential for large magnitude earthquakes, studies of fault zone behavior in these settings have potentially significant implications for the health and economic well being of society.This study is designed to investigate the hydrofracturing and subsequent healing recorded by mineralized veins and fabrics that develop in the footwall of subduction interfaces. It is hypothesized that the range of textures observed in veins is a manifestation of the range of heterogeneous plate boundary slip behavior that is observed along convergent margins. The healing of the fractures within the underthrusting sediments adjacent to the plate interface can occur in times that overlap with earthquake recurrence intervals at the temperatures of the seismogenic zone. Open fluid-filled cracks could impact the effective stress and the strength/elastic properties of the rocks that store and release elastic strain energy, so the healing of cracks could be fundamental to the locking behavior of the seismogenic zone. Vein systems and related scaly fabrics from six regionally extensive shear zones within the Shimanto Belt in Japan that formed during subduction and contain pervasive quartz veins representing the full range of temperatures within the seismogenic zone will be studies. The principal investigators will: 1) characterize the vein systematics on the outcrop both as a function of lithology and position relative to major faults, 2) evaluate vein microstructures petrographically and with cathodoluminescence, and 3) develop elemental maps of potential silica sources adjacent to veins, including scaly fabrics. To evaluate the role of local diffusion of silica, we will map major element concentrations in areas that define potential silica sources (e.g., scaly fabrics and wall rock with cleavage adjacent to fractures). They will also determine the aperture of cracks and the degree to which fractures are sealed based on microstructures. Microstructural information will be used in conjunction with scanline surveys of vein spacings, thicknesses, and lengths to put constraints on the spacings of open fractures and the times needed to seal fractures. This study will enable us to address fundamental questions about quartz veins adjacent to the subduction interface: What are the sources of silica within veins? Can we identify silica depletion zones typical of local silica redistribution? How does crack aperture and crack spacing vary as a function of rock type and temperature? Is there a systematic variation in vein textures with increasing temperature and depth within the seismogenic zone? The Shimanto belt of Japan is ideal for this study as it exposes regional fault zones that are interpreted as paleo-decollements (active plate boundary damage zones in the underthrusting footwall) and for comparison, examples of out-of sequence splays that accommodated seismic slip and juxtaposed rocks of the existing accretionary prism.