Collaborative Research: Three-dimensional Onshore-Offshore MT Investigation of Cascadia Margin--Interpretation Phase

该项目的主要目标是了解北美西北边缘地球深处水的分布，在那里，胡安德富卡大洋板块俯冲到大陆之下，在过去的地质历史中曾产生过毁灭性的巨型逆冲地震。被困在俯冲板块和上覆大陆之间的水被认为对这些地震的发生和强度以及该地区地壳和地幔变形的其他方式起着重要的控制作用。这项研究使用了最近通过覆盖俄勒冈州和华盛顿州大陆边缘的陆上-近海传感器阵列收集的电磁地球物理数据，构建了地壳和地幔电导率的三维图像。由于这一物理性质对地球中流体的浓度很敏感，我们的结果将为流体的空间变化和物理状态提供独特的约束。这些信息将纳入根据该区域丰富的地球物理和地质研究得出的综合地震灾害模型。该项目的第二个目标是开发强大的建模软件，用于大规模陆上-近海电磁阵列数据的三维解释，这是迄今为止很少有人尝试的事情。通过该项目开发的工具和方法将广泛应用于未来用于构造成像的两栖电磁研究，以及用于能源和矿产勘探的两栖商业电磁调查。现在有相当多的证据表明，流体在相对较新发现的幕式地震和滑动现象中发挥了重要作用。至少，流体控制俯冲和俯冲板块中的流变性，并降低板块界面上的有效法向应力；更具推测的是，流体可能通过从渗透率泵送和水力压裂到依赖于速率和状态的摩擦滑动等机制，对幕式震颤和滑动动力学起关键作用。沿板块界面倾斜的低地震速度层，以及只有超压流体才能解释的低Vp/Vs比区域，进一步暗示了俯冲巨型逆冲旋回中的流体。大地电磁数据对流体的存在和连通性高度敏感，因此是成像系统这一关键组成部分的理想地球物理工具。美国国家科学基金会最近在俄勒冈州和华盛顿州南部边缘的一次活动中收集了大约150个大地电磁站点(一半在陆地上，一半在海上)，结合地球望远镜计划(可运输和FLEX阵列)的数据和该地区的遗留数据，形成了一个独特的数据集，用于详细研究俯冲带背景下的流体。该项目的主要重点是对这一合并的两栖大地电磁数据集进行完整的分析和三维反演，成像从传入板块到岩浆弧附近的流体分布。近海数据的可获得性将有助于更好地理解知之甚少但至关重要的细节，例如海沟处的流体输入，它们对锁定区域的影响？在两个板块之间，以及从下行板块释放的流体的分布，包括沿发生幕式地震和滑动的过渡带。相对密集的三维阵列覆盖将极大地完善我们对边缘分段的理解，解决沿Siletzia和Klamath省的结构和流体分布，以及这些与幕式地震和滑动的空间模式的关系。该项目还将改进两栖大地电磁数据集的三维解释方法和工具，这将对正在进行和计划中的大陆边缘研究举措很有价值，并将有助于支持对下一代跨学科电磁研究人员的培训。

The principal goal of this project is to understand the distribution of water deep in the Earth along the northwest margin of North America, where the subduction of the Juan de Fuca oceanic plate beneath the continent has produced devastating mega-thrust earthquakes in the geologic past. Water trapped between the subducting plate and overlying continent is hypothesized to play an important role in controlling the occurrence and magnitude of these earthquakes, as well as other modes of crust and mantle deformation in the region. This study uses electromagnetic geophysical data that was collected recently with an onshore-offshore array of sensors covering the Oregon and Washington continental margin, to construct a three-dimensional image of crust and mantle electrical conductivity. Since this physical property is sensitive to the concentration of fluids in the Earth, our results will provide unique constraints on spatial variations of fluids and physical state. This information will be integrated into comprehensive seismic hazard models derived from the wealth of geophysical and geological studies in this region. A secondary goal of this project is to develop robust modeling software for the three-dimensional interpretation of large-scale onshore-offshore electromagnetic array data, something that has rarely, if ever, been attempted to date. The tools and methods developed through this project will have broad application to future amphibious electromagnetic studies for tectonic imaging, as well as amphibious commercial electromagnetic surveys for energy and mineral exploration.There is now considerable evidence that fluids play an important role in the relatively recently discovered phenomenon of episodic tremor and slip. At the least, fluids control rheological properties in the subducting and overriding plates and reduce effective normal stress along the plate interface; more speculatively fluids may be critical to episodic tremor and slip dynamics through mechanisms which range from permeability pumping and hydrofracturing, to rate-and-state dependent frictional slip. Dipping low seismic velocity layers along the plate interface, and regions of low Vp/Vs ratios that can only be explained by over-pressured fluids, further implicate fluids in the subduction megathrust cycle. Magnetotelluric data are highly sensitive to the presence and connectivity of fluids, and thus represent an ideal geophysical tool for imaging this critical component of the system. Approximately 150 magnetotelluric sites (half onshore and half offshore) collected in a recent NSF funded campaign across the Oregon and southern Washington margin, combined with data from the EarthScope program (transportable and flex array) and legacy data from the region, represent a unique dataset for a detailed study of fluids in a subduction zone setting. The main focus of this project is a complete analysis and three dimensional inversion of this merged amphibious magnetotelluric dataset, imaging the distribution of fluids from the incoming plate to near the magmatic arc. The availability of offshore data will allow improved understanding of poorly understood but critical details, such as fluid inputs at the trench, their impacts on the ?locked zone? between the two plates, and the distribution of fluids released from the down-going slab, including along the transitional zone where episodic tremor and slip occurs. The relatively dense three dimensional array coverage will greatly refine our understanding of margin segmentation, resolving the structure and fluid distribution along the Siletzia and Klamath provinces and the relationship of these to the spatial pattern of episodic tremor and slip. The project will also lead to development of improved methodologies and tools for three dimensional interpretation of amphibious magnetotelluric datasets, which will be of value to ongoing and planned initiatives for studies of continental margins, and will help support training of the next generation of interdisciplinary electromagnetic researchers.