Collaborative Research: Onshore-offshore MT Investigation of Cascadia Margin 3D Structure, Segmentation and Fluid Distribution

合作研究：卡斯卡迪亚边缘3D结构、分段和流体分布的陆上和海上MT研究

• 批准号: 1053207
• 负责人: Kerry Key
• 金额: $ 43.45万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-10-01 至 2014-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1053207&HistoricalAwards=false
• 关键词: Collaborative; Research; Onshore; offshore; MT

Since their discovery nearly a decade ago, Episodic Tremor and Slip (ETS) events have been intensively studied along the Japan margin and in the Cascadia subduction zone of the US Pacific Northwest. These deformation events are now understood to occur in a transitional zone between the shallower, colder, locked seismogenic region and the down-dip, hot region where stable sliding of the subducting plate occurs. There is considerable interest in the role that fluids may play in ETS, from simply controlling rheological properties in the subducting and overriding plates to, possibly, explaining ETS dynamics through competing models ranging from permeability pumping/hydrofracturing to a continuum of rate and state frictional slip. Observed ETS temporal patterns can be replicated by numerical models with low confining pressures, consistent with elevated pore pressure along the plate interface. However, a lack of high-resolution geophysical imaging at depth has inhibited our ability to map the baseline distribution of fluids along subduction zones exhibiting ETS. Since magnetotelluric (MT) data are highly sensitive to the presence and connectivity of fluids, they offer the best strategy for mapping the fluid distribution within the subduction zone, and for testing the competing ETS hypotheses (e.g. friction vs. fluids). The Magnetotelluric Observations of Cascadia using a Huge Array (MOCHA) project entails acquiring high-resolution onshore and offshore MT soundings that can be used to image deep electrical conductivity structure. Images thus derived impart knowledge of the fluid distribution and the segmentation of the convergent margin that is associated with the subducting slab along the coasts of Oregon and Washington. MOCHA consists of 60 marine and 75 land MT stations covering a 400 km section of the forearc along the north to south Siletzia terranes. The MOCHA data will ultimately constrain 3D models of the crust and upper mantle of the subduction system, from the incoming plate to the magmatic arc, revealing the distribution of fluids in the subduction system in unprecedented detail. These models will detail the offshore fluid input to the system and the distribution of fluids released from the down-going slab, including along the transitional zone where ETS occurs.

自从近十年前发现以来，人们对日本边缘和美国太平洋西北部卡斯卡迪亚俯冲带的阵发性震颤和滑动（ETS）事件进行了深入研究。现在人们知道这些变形事件发生在较浅、较冷、锁定的震源区域和俯冲板块发生稳定滑动的下倾热区域之间的过渡带。人们对流体在 ETS 中可能发挥的作用非常感兴趣，从简单地控制俯冲板块和上覆板块的流变特性，到可能通过从渗透率泵送/水力压裂到速率和状态摩擦滑移的连续体等竞争模型来解释 ETS 动力学。观察到的 ETS 时间模式可以通过低围压的数值模型来复制，这与沿板块界面升高的孔隙压力一致。然而，由于缺乏深度高分辨率地球物理成像，我们无法绘制沿俯冲带显示 ETS 的流体基线分布图。由于大地电磁 (MT) 数据对流体的存在和连通性高度敏感，因此它们提供了绘制俯冲带内流体分布图以及测试竞争 ETS 假设（例如摩擦与流体）的最佳策略。使用巨型阵列 (MOCHA) 项目的卡斯卡迪亚大地电磁观测需要获取高分辨率的陆上和海上大地电磁探测数据，可用于对深层导电结构进行成像。由此得出的图像提供了与俄勒冈州和华盛顿州海岸的俯冲板片相关的流体分布和会聚边缘分割的知识。 MOCHA 由 60 个海洋 MT 站和 75 个陆地 MT 站组成，覆盖从北到南 Siletzia 地体的弧前 400 公里的部分。 MOCHA 数据最终将约束俯冲系统的地壳和上地幔的 3D 模型，从传入板块到岩浆弧，以前所未有的细节揭示俯冲系统中流体的分布。这些模型将详细介绍系统的海上流体输入以及从下行板片释放的流体分布，包括沿着发生 ETS 的过渡区。