GeoPRISMS Postdoctoral Scholar: Refining GPS-Acoustic Processing to Measure Cascadia Subduction

有地质和历史证据表明，一场规模与2011年日本近海地震相当的9.0级地震可能袭击卡斯卡迪亚俯冲带，该地区紧邻俄勒冈州和华盛顿州，那里的海底正在缓慢地撞击并下沉到北美大陆之下。通常在评估陆地上的地震危险时，使用全球定位系统（GPS）测量地面的运动，该系统告知未来可能发生地震的地点以及可能发生的地震的大小。然而，全球定位系统不能用于卡斯卡迪亚俯冲带等近海地区，因为海底仪器不能通过水柱直接与轨道卫星通信。相反，使用称为GPS声学的类似技术。GPS-Acoustic将来自GPS的卫星信号与部署在海底的仪器发出的声波耦合起来，以测量海底的运动。然而，GPS声学仪器并不像陆上GPS站那样精确，因为很难预测声音在任何特定时间和地点在海洋中传播的速度。该项目力求利用改进的海洋学声速模型，改进对沿着卡斯卡迪亚俯冲带海底移动的测量。 这反过来将有助于更准确地评估该地区的地震危险。这项工作的更广泛影响包括：一名博士后学者的支助、俯冲带地震危险评估的潜在改进以及大地测量学和物理海洋学的潜在跨学科影响，该项目的目标是测试关于全球定位系统声学定位的静止和水平分层声速结构的假设。 海洋学后报模型将用于提供信息，以制定从GPS声学残差中滤除声速变化的策略。 这将允许实施方法，以更准确和有效地隔离在GPS声学部署期间收集的走时残差中的地质和海洋学信号。 这种方法将使用全球定位系统和声波测距数据先前收集的波滑翔机以上的卡斯卡迪亚俯冲带的网站进行评估。如果成功，GPS声学位移测量的精度将从厘米级提高到毫米级，并通过缩短所需的数据收集窗口来提高GPS声学的效率。 通过改进的GPS声学测量，将为沿着卡斯卡迪亚边缘的断层锁定模型生成精确的边界条件，并有助于监测地震周期，该奖项反映了NSF的法定使命，并通过利用基金会的智力价值和更广泛的影响进行评估，被认为值得支持审查标准。

There is geologic and historical evidence that a magnitude 9.0 earthquake, comparable in size to the earthquake off Japan in 2011, could strike the Cascadia subduction zone, the region immediately offshore Oregon and Washington State where the ocean floor is slowly crashing into and sinking below the North American continent. Normally when assessing seismic hazards on land, the movement of the ground is measured using the Global Positioning System (GPS), which informs where an earthquake may happen in the future and how large it may be. However, GPS can not be used in offshore regions like the Cascadia subduction zone because a seafloor instrument cannot directly communicate with an orbiting satellite through the water column. Instead, an analogous technique called GPS-Acoustic is used. GPS-Acoustic couples the satellite signals from GPS with sound waves from instruments deployed on the seabed to measure the movement of the seafloor. However, GPS-Acoustic instruments are not as accurate as onshore GPS stations because of difficulties in predicting how fast sound will travel through the ocean at any specific time and place. This project seeks to use improved oceanographic models of sound speed to improve measurements of seafloor movement along the Cascadia subduction zone. This will in turn allow a more accurate assessment of the seismic hazard in the region. Broader impacts of the work include the support of a postdoctoral scholar, potential improvements in subduction-zone seismic hazard assessments and potential cross-disciplinary impact for both geodesy and physical oceanography.The goal of the project is to test the assumptions of a stationary and horizontally layered sound velocity structure for GPS-Acoustic positioning. Oceanographic hindcast models will be used to inform a strategy to filter out the variations in sound velocity from the GPS-Acoustic residuals. This will allow implementation of methods to more accurately and efficiently isolate the geologic and oceanographic signal in the travel time residuals collected during GPS-Acoustic deployment. This approach will be evaluated using GPS and acoustic ranging data previously collected by a Wave Glider for sites above the Cascadia subduction zone. If successful, the accuracy of GPS-Acoustic displacement measurements will be improved from the cm-scale to the mm-scale, as well as the efficiency of GPS-Acoustic by shortening the required data collection window. With improved GPS-Acoustic measurements, refined boundary conditions will be generated for fault locking models along the Cascadia margin and aid in the monitoring of the seismic cycle, in line with the goals of the GeoPRISMS initiative to improve understanding of subduction cycles and deformation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.