Collaborative Research: Assessing the State of Locking on the Frontal Thrust of the Cascadia Subduction Zone With Seafloor Geodesy

卡斯卡迪亚俯冲带位于沿着北方加州、俄勒冈州和华盛顿的海岸，是一个重大的地质灾害，可以产生大地震和海啸。 卡斯卡迪亚俯冲带是由海洋胡安德富卡构造板块向下移动，并在覆盖大陆的北美构造板块之下形成的。 两个板块相互作用或摩擦的界面沿着称为巨型逆冲断层。 当胡安·德富卡板块向下移动时，巨型逆冲断层上的摩擦力使上方的北美板块弯曲和收缩。 应变的累积速度是每年一厘米左右，但最终这些储存的能量会释放出来，引起大地震和海啸。 最后一次大地震发生在1700年，其威力足以在日本记录到海啸波。 从那时起，美国海岸人口稠密，对社会构成了巨大的危害。 陆基GPS测量可以测量应变积累的缓慢积累，但沿海站点距离北美板块的淹没大陆架太远，无法提供可靠的离岸估计。 正是在这一近海地区，海啸的产生可能是最大的。 该项目使用在一个小型机器人平台上在海面测量的全球定位系统，结合从平台到海底传感器的声学测距。这种技术被称为GPS声学，可以测量海底厘米级的运动。 该项目的目标是更好地记录海底移动的程度，并帮助评估未来海啸的潜在规模。该项目将寻找卡斯卡迪亚俯冲带变形前缘外趾沿着的锁定。将在北纬44.4度和北纬46.7度两个最近建立的海底全球定位系统声学站点阵列中增加北纬43.0度和北纬45.3度两个新的海底全球定位系统声学站点，并将允许确定它们相对于北美板块的运动。所有这些地点都位于海沟内侧几公里处，它们的运动将限制锋面冲断层最浅部分的运动学。 该项目将使用新的低成本方法，包括从波浪滑翔机而不是昂贵的船只收集的GPS声学数据。还将安装永久性海底基准，以无限期地延长位置时间序列，并利用可重复使用的商业转发器。具体而言，将通过回收和重新部署现有的一套转发器来演示海底转发器的重新用途。在为期三年的计划结束时，位于两个新址的六个转发器将被回收，以供日后建议的公益项目使用。这两个新地点的基准保持不变，可在未来（数年至数十年）重新使用，以更新测量时间序列。 为了解释从GPS-A观测中推断出的运动，这些海上数据将与现有的陆上GPS和水准观测相结合，这将允许探索一系列锁定模型。 该项目还将比较四个沿走向的观测结果，并与地质和结构模式的沿走向变化相关联。

The Cascadia Subduction Zone lies along the coast of Northern California, Oregon and Washington and is a significant geohazard that can generate great earthquakes and tsunamis. The Cascadia subduction zone is formed by the oceanic Juan de Fuca tectonic plate moving downward and beneath the overriding continental North American tectonic plate. The interface surface along which the two plates interact or rub is called the megathrust fault. As the Juan de Fuca plate moves downward, friction on the megathrust bends and contracts the overriding North American plate. The rate of buildup of strain is at the level of a centimeter or so per year, but eventually this stored energy is released causing a large earthquake and tsunami. The last great event occurred in 1700 and was powerful enough that the tsunami waves were recorded in Japan. Since that time the US Coast has become heavily populated posing a large hazard to society. Land-based GPS measurements can measure the slow accumulation of strain buildup, but the coastal sites are too far from the submerged shelf of the North American plate to provide reliable estimates far offshore. It is this offshore region where the tsunami generation may be greatest. This project uses GPS measured at the sea surface on a small robotic platform, combined with acoustic ranging from the platform to sensors on the seafloor. This technique is called GPS-Acoustic and can measure the centimeter-level motion of the seafloor. The project goal is to better document how much the seafloor is displacing and aid assessment of the potential size of the future tsunamis. This project will look for locking along the outer toe of the deformation front on the Cascadia Subduction Zone. Two new seafloor GPS-Acoustic sites at 43.0N and 45.3N will be added to the array of two recently established sites at 44.4N and 46.7N, and will allow for a determination of their motion relative to the North American plate. All of these sites are located several km inboard of the trench, and their motions will constrain the kinematics of the shallowest section of the frontal thrust. The project will use new lower cost methods that include GPS-Acoustic data that are collected from a Wave Glider rather than from an expensive ship. Permanent seafloor benchmarks will also be installed to extend the position time series indefinitely, and utilize commercial transponders that are reusable. Specifically, the re-purposing of seafloor transponders will be demonstrated by recovering and re-deploying an existing set of transponders. At the end of the three-year project, the six transponders at the two new sites will be recovered for reuse in future proposed projects of community interest. The benchmarks at these two new sites remain and can be re-occupied in the future (years to decades) to update the measurement time series. To interpret the motions inferred from the GPS-A observations, these offshore data will be integrated with existing onshore GPS and leveling observations, which will allow for a range of locking models to be explored. The project will also compare the four along-strike observations to each other, and correlate with along-strike variations in geologic and structural patterns.