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，地面震动和海啸淹没沿海地区，造成近 30 万人丧生。 就 2011 年东北地震和最近发生的其他几场大地震而言，地震事件发生之前是一种被称为“慢滑移”的断层行为，因此可能是由这种断层行为引发的，这种行为在二十年前是未知的。 因此，沿板块边界的断层要么“蠕变”，要么因“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“慢地震”和与地震一致的微震活动。 认识到这种滑动行为的多样性需要一种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。 这项研究的中心原则是，有关板块边界滑移行为非均质性的信息不仅由地震活动记录，而且还记录在矿脉或矿化裂缝的分布和纹理中，我们今天可以在挖掘出的古代俯冲边界中观察到这些信息。 该项目专门设计用于研究天然水力压裂以及碳酸钙和二氧化硅的局部重新分布对于与微震、慢滑移以及地震中弹性应变的积累和释放相关的滑移不稳定性的演变的作用。 将在日本古代板块边界断层带中检查脉状系统，记录一系列反映地震发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总体假设，将断层带中的二氧化硅重新分布（如静脉结构和矿物学所示）与地震动力学联系起来。 如果假设正确，俯冲带行为模型不仅必须考虑断层的摩擦行为，还必须考虑下盘水力压裂（即流体来源和渗透率的可预测函数）和二氧化硅重新分布（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外，该奖项还为宾夕法尼亚州立大学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，并为将在项目第二年完成研究型独立高级论文的本科生提供参与机会。学生将接受培训，了解如何使用低温模型进行裂缝密封，这些模型已沿被动边缘应用于石油工业，同时将这些模型应用于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，该实验旨在现场评估表征地震发生深度的板块边界的过程。由于俯冲带具有发生大规模地震的巨大潜力，因此对这些环境中断层带行为的研究对社会的健康和经济福祉具有潜在的重大影响。本研究旨在调查俯冲界面下盘发育的矿脉和织物记录的水力压裂和随后的愈合。假设在静脉中观察到的纹理范围是沿会聚边缘观察到的异质板块边界滑移行为范围的表现。 邻近板块界面的下冲沉积物内裂缝的愈合可能会在与地震发生带温度下的地震复发间隔重叠的时间内发生。开放的充满流体的裂缝可能会影响存储和释放弹性应变能的岩石的有效应力和强度/弹性特性，因此裂缝的愈合可能是地震带锁定行为的基础。将研究来自日本四万十带内六个区域性广泛剪切带的脉系统和相关鳞片结构，这些剪切带在俯冲过程中形成，并含有代表地震带内全部温度范围的普遍石英脉。 主要研究人员将：1）将露头的矿脉系统特征描述为岩性和相对于主要断层位置的函数，2）通过岩相学和阴极发光评估矿脉微观结构，3）开发矿脉附近潜在二氧化硅来源的元素图，包括鳞片状织物。 为了评估二氧化硅局部扩散的作用，我们将绘制定义潜在二氧化硅来源的区域（例如，与裂缝相邻的解理的鳞状织物和围岩）中的主要元素浓度。 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。 微观结构信息将与静脉间距、厚度和长度的扫描线测量结合使用，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？我们能否识别局部二氧化硅重新分布的典型二氧化硅贫化区？ 裂纹孔径和裂纹间距如何随岩石类型和温度变化？随着地震带内温度和深度的增加，脉纹结构是否存在系统性变化？ 日本的四万十带是这项研究的理想选择，因为它暴露了被解释为古断层的区域断层带（逆冲下盘中的活动板块边界损伤区），并且作为比较，无序展开的例子容纳了地震滑移和现有增生棱柱的并列岩石。

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.