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

自近十年前发现以来，沿沿着日本边缘和美国太平洋西北部卡斯卡迪亚俯冲带的幕式震颤和滑动（ETS）事件已被深入研究。这些变形事件现在被理解为发生在一个过渡带之间的浅，冷，锁定孕震区和下倾，热的地区，稳定的俯冲板块滑动发生。有相当大的兴趣，流体可能在ETS中发挥的作用，从简单地控制俯冲和覆盖板的流变性能，可能的话，解释ETS动态通过竞争的模型，从渗透率泵送/水力压裂的连续率和状态摩擦滑动。观察到的ETS时间模式可以复制的数值模型与低围压，符合升高孔隙压力沿着板界面。然而，缺乏高分辨率的地球物理成像深度抑制了我们的能力，以地图的基线分布的流体沿着俯冲带展示ETS。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和测试竞争ETS假设（例如摩擦与流体）的最佳策略。卡斯卡迪亚大地电磁观测使用一个巨大的阵列（MOCHA）项目需要获得高分辨率的陆上和海上MT探测，可用于成像深电导率结构。由此得出的图像传授知识的流体分布和分割的会聚边缘，这是与俯冲板沿着海岸的俄勒冈州和华盛顿。MOCHA由60个海洋MT站和75个陆地MT站组成，覆盖了沿着北到南Siletzia山脉的400 km弧前段。MOCHA数据最终将限制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。这些模型将详细描述系统的海上流体输入和下行板释放的流体分布，包括沿着ETS发生的过渡区。

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.