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.

自近十年前发现以来，幕式震颤和滑动(ETS)事件在日本边缘和美国太平洋西北部的卡斯卡迪亚俯冲带得到了广泛的研究。这些形变事件现在被认为发生在较浅、较冷、闭锁的孕震区和俯冲板块发生稳定滑动的下倾热区之间的过渡区。流体在ETS中可能扮演的角色引起了相当大的兴趣，从简单地控制俯冲和俯冲板块的流变性，到可能通过相互竞争的模型解释ETS动力学，从渗透率泵送/水力压裂到速率和状态摩擦滑移的连续体。观测到的ETS时间模式可以用低围压的数值模式复制，这与沿板块界面的孔压升高一致。然而，缺乏深部的高分辨率地球物理成像，阻碍了我们绘制显示ET的俯冲带上流体基线分布的能力。由于大地电磁(MT)数据对流体的存在和连通性高度敏感，它们为绘制俯冲带内的流体分布以及检验相互竞争的ETS假说(例如摩擦与流体)提供了最佳策略。使用巨型阵列对卡斯卡迪亚进行大地电磁观测(MOCHA)项目需要获得高分辨率的陆上和近海大地电磁测深资料，这些资料可用于成像深层电导率结构。由此获得的图像提供了与俄勒冈州和华盛顿州海岸的俯冲板块有关的流体分布和会聚边缘的分段的知识。MOCHA由60个海上和75个陆地MT站组成，覆盖从北到南Siletzia地体的400公里弧前部分。MOCHA数据最终将约束俯冲系统的地壳和上地幔的3D模型，从进入的板块到岩浆弧，以前所未有的细节揭示俯冲系统中流体的分布。这些模型将详细说明输入到系统的近海流体以及从下行板块释放的流体的分布，包括沿发生ETS的过渡带。