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.

该项目的主要目的是了解北美西北边缘的地球深处的水分布，在那里，大陆下方的胡安·德福卡海洋板的俯冲在地质过去造成了毁灭性的巨型地震。 假设被困在俯冲板和上覆的大陆之间的水在控制这些地震的发生和大小以及其他地壳和地幔变形的模式中起着重要作用。 这项研究使用了电磁地球物理数据，这些数据最近用覆盖俄勒冈州和华盛顿大陆边缘的陆上岸岸阵列收集，以构建外壳和地幔电导率的三维图像。由于这种物理特性对地球中流体的浓度敏感，因此我们的结果将对流体和物理状态的空间变化提供独特的限制。 该信息将被整合到该地区的地质和地质研究丰富的全面地震危险模型中。该项目的次要目标是开发强大的建模软件，以对大型陆上岸电磁阵列数据的三维解释进行三维解释，这是很少（如果有的话）尝试约会的。 通过该项目开发的工具和方法将广泛应用于未来的两栖电磁研究，用于构造成像，以及用于能源和矿物勘探的两栖商业电磁调查。现在，有很多证据表明，流体在相对最近发现的情节震颤现象中相对较新的现象中起着重要作用。 至少，流体控制俯冲和压倒板中的流变特性，并沿板界面降低有效的正常应力。更具投机性的流体可能对发作性震颤和滑动动力学至关重要，这些机制范围从渗透性泵送和氢反骨压裂到速率和状态依赖的摩擦滑动。沿板界面的低地震速度层和低VP/VS比率的区域只能通过过度压力的流体来解释，进一步暗示了俯冲的巨型循环中的流体。磁铁性数据对流体的存在和连通性高度敏感，因此代表了用于对系统的关键组成部分进行成像的理想地球物理工具。 最近在俄勒冈州和华盛顿南部的NSF资助的广告系列中收集的大约150个磁铁站点（一半的陆上和半近海），结合了Earthscope计划（可运输和弹性阵列）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，代表了一个独特的数据集，以详细研究分支区域的流体研究。 该项目的主要重点是完整的分析和该合并的两栖磁铁数据集的三维反转，将流体从传入板到岩浆弧附近的流体分布。离岸数据的可用性将可以提高人们对知识渊博的了解，但关键细节，例如在沟槽中的流体输入，它们对锁定区域的影响？在这两个板之间，以及从向下的平板释放的流体分布，包括沿着发生情节震颤和滑移的过渡区。相对密集的三维阵列覆盖范围将大大完善我们对边缘分割的理解，解决沿Siletzia和Klamath省的结构和流体分布，以及这些省份与情节震颤和滑移的空间模式的关系。该项目还将导致改进的方法和工具开发用于三维磁铁数据集的三维解释，这将对正在进行的和计划的大陆边缘研究有价值，并将帮助支持下一代跨学科的跨学科电磁研究人员的培训。