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

合作研究：卡斯卡迪亚边缘三维陆上-海上大地电磁勘探--解释阶段

• 批准号: 1459067
• 负责人: Kerry Key
• 金额: $ 6.06万
• 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: 2015
• 资助国家: 美国
• 起止时间: 2015-04-01 至 2018-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1459067&HistoricalAwards=false
• 关键词: Collaborative; Research; Three; dimensional; Onshore

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.

该项目的主要目标是了解北美洲西北边缘地球深处水的分布，胡安德富卡洋板块俯冲到大陆下方，在地质历史上曾产生过毁灭性的特大逆冲地震。 据推测，俯冲板块和上覆大陆之间滞留的水在控制这些地震的发生和震级以及该地区地壳和地幔变形的其他模式方面发挥着重要作用。 这项研究利用最近通过覆盖俄勒冈州和华盛顿州大陆边缘的陆上和海上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。由于这种物理性质对地球中流体的浓度敏感，因此我们的结果将为流体和物理状态的空间变化提供独特的约束。 这些信息将被整合到源自该地区丰富的地球物理和地质研究的综合地震灾害模型中。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-海上电磁阵列数据的三维解释，这是迄今为止很少尝试的事情。 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。现在有大量证据表明，流体在相对较新的发现中发挥着重要作用。阵发性震颤和滑倒现象。 至少，流体控制俯冲板块和俯冲板块的流变特性，并减少沿板块界面的有效法向应力；更推测的是，流体可能通过从渗透率泵送和水力压裂到速率和状态相关的摩擦滑移等机制，对阵发性震颤和滑移动力学至关重要。沿板块界面倾斜的低地震速度层，以及只能用超压流体解释的低 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，因此是对系统的这一关键组件进行成像的理想地球物理工具。 最近由 NSF 资助的一项横跨俄勒冈州和华盛顿州南部边缘的活动收集了大约 150 个大地电磁站点（一半陆上和一半海上），结合来自 EarthScope 计划（可移动和柔性阵列）的数据以及来自该地区的遗留数据，代表了一个独特的数据集对俯冲带环境中的流体进行详细研究。 该项目的主要重点是对合并的两栖大地电磁数据集进行完整分析和三维反演，对从传入板块到岩浆弧附近的流体分布进行成像。海上数据的可用性将有助于更好地了解人们知之甚少但关键的细节，例如沟槽的流体输入及其对“锁定区域”的影响？两个板块之间的分布，以及从下行板块释放的流体分布，包括沿着发生阵发性震颤和滑移的过渡区。相对密集的三维阵列覆盖范围将极大地完善我们对边缘分割的理解，解决沿西莱齐亚省和克拉马斯省的结构和流体分布以及它们与阵发性震颤和滑动的空间模式的关系。该项目还将开发改进的方法和工具，用于两栖大地电磁数据集的三维解释，这对于正在进行和计划中的大陆边缘研究举措具有价值，并将有助于支持下一代跨学科电磁研究人员的培训。