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

自近十年前发现以来，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幕式震颤和滑动（ETS）事件进行了深入研究。这些变形事件现在被理解为发生在较浅、较冷、锁定的发震区和俯冲板块稳定滑动的下倾热区之间的过渡地带。人们对流体在ETS中可能发挥的作用非常感兴趣，从简单地控制俯冲和覆盖板块的流变特性，到可能通过相互竞争的模型（从渗透率泵注/水力压裂到连续的速率和状态摩擦滑移）来解释ETS动力学。观测到的ETS时间模式可以用低围压下的数值模型复制，这与沿板界面的孔隙压力升高相一致。然而，由于缺乏高分辨率的地球物理成像技术，我们无法绘制出显示ETS的俯冲带沿线流体的基线分布。由于大地电磁（MT）数据对流体的存在和连通性高度敏感，因此它们为绘制俯冲带内流体分布以及测试相互竞争的ETS假设（例如摩擦与流体）提供了最佳策略。利用巨型阵列（MOCHA）项目进行的Cascadia大地电磁观测需要获取高分辨率的陆上和海上MT测深数据，这些数据可用于成像深层电导率结构。由此获得的图像提供了关于流体分布和与俄勒冈和华盛顿海岸俯冲板块相关的收敛边缘分割的知识。MOCHA由60个海洋MT站和75个陆地MT站组成，覆盖沿北至南西莱齐亚地形的前弧400公里区域。MOCHA数据最终将约束俯冲系统地壳和上地幔的三维模型，从进入板块到岩浆弧，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海上流体输入系统和从下行板块释放的流体分布，包括沿ETS发生的过渡带。

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.