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

有地质和历史证据表明，一场规模相当于2011年日本近海地震的9.0级地震，可能会袭击卡斯卡迪亚俯冲带，该地区紧邻俄勒冈州和华盛顿州近海，那里的海底正在缓慢坠落到北美大陆以下。通常，在评估陆地上的地震风险时，地面的移动是使用全球定位系统(GPS)来测量的，该系统可以告知未来可能在哪里发生地震，以及地震的强度可能有多大。然而，GPS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与轨道卫星直接通信。取而代之的是，使用了一种类似的技术，称为GPS声学。GPS-声学将来自GPS的卫星信号与部署在海底的仪器的声波耦合起来，以测量海底的运动。然而，GPS声学仪器不像陆上GPS站那样准确，因为很难预测声音在任何特定时间和地点通过海洋的速度。该项目力求使用改进的声速海洋学模型来改进卡斯卡迪亚俯冲带海底运动的测量。这反过来将允许对该地区的地震危险进行更准确的评估。这项工作的更广泛影响包括博士后学者的支持，俯冲带地震危险性评估的潜在改进，以及对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用于GPS声学定位的固定和水平分层声速结构的假设。海洋学后播模型将被用来提供从GPS-声波残差中滤除声速变化的战略。这将允许采用各种方法，更准确和有效地分离在GPS声学部署期间收集的旅行时间残差中的地质和海洋信号。这一方法将使用全球定位系统和以前由波浪滑翔机收集的卡斯卡迪亚俯冲带上方地点的声波测距数据进行评估。如果成功，GPS-声波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-声波系统的效率。随着改进的GPS-声波测量，将为卡斯卡迪亚边缘的断层锁定模型生成精细的边界条件，并帮助监测地震周期，与GeoPRISMS倡议的目标一致，以提高对俯冲周期和变形的理解。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.