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

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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.