Collaborative Research: Near-Trench Community Geodetic Experiment

Collaborative Research: Near-Trench Community Geodetic ExperimentThe largest earthquakes (and the resulting tsunamis) occur offshore, but the instruments traditionally used to study the ground deformation associated with these events can only be used on land. Therefore, traditional surveying cannot quantify the offshore seismic hazards. Studying these hazards requires tools designed to collect data offshore. One such tool is the GNSS-Acoustic method, an offshore GPS (Global Positioning System) method with sets of instruments set up on the seafloor in circular groupings. These instruments are positioned relative to a remote-controlled sea surface robot using sound pulses. This ‘Wave Glider’ can use GPS to determine its position, allowing the seafloor instruments to be located with cm-level accuracy. Tracking these positions over time allows researchers to learn how the deforming seafloor builds up stress that ultimately results in earthquakes. This project follows recommendations by a large group of scientists who are interested in knowing how seafloor deforms in subduction zones as the seafloor crust gradually slides beneath the continental crust. The project aims to detect the motions of the seafloor at six stations in the offshore Cascadia region and six in the offshore Alaska region. This will aid the scientific community in assessing the seismic and tsunami hazards in these regions. The data will all be open access and the project will contribute to training the next generation of geodetic scientists.The 2011 Tohoku earthquake demonstrated that coseismic rupture can extend to shallow depths along a subduction megathrust, where large vertical displacements of the seafloor can generate damaging tsunamis. This project will clarify whether a similar event can happen in the offshore Cascadia and Alaska regions by using seafloor geodetic observations to infer the level of elastic strain stored in the accretionary prisms of Cascadia and Alaska subduction zones. The researchers will deploy GNSS-Acoustic instruments from the seafloor geodesy community pool, which consists of 51 GNSS-Acoustic transponders (enough for 17 new GNSS-Acoustic sites) and 3 Wave Gliders, following recommendations from 165 scientists and students who attended a seafloor geodesy community workshop on how best to make use of the instrument pool. Twelve new GNSS-Acoustic sites will be established during this experiment, with six along each of the Cascadia and Alaska subduction zone trenches, which were identified by workshop attendees as high-value science targets. The researchers will survey these new sites as well as the seven sites already established in these regions annually throughout the five years of the project. These positions will be used to infer interseismic velocities from which the rate of elastic strain buildup in the accretionary prisms can be determined, with implications for the earthquake and tsunami hazard in Alaska and the Pacific Northwest. The data will all be open access and the project will contribute to training the next generation of geodetic scientists.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.