Collaborative Research: Variation of Incoming Plate Hydration and Faulting Along the Alaska Subduction Zone

Washington University, Saint Louis MO

Investigators

Abstract

Subduction zones are sites of important chemical exchanges between the solid earth, hydrosphere and atmosphere. Seawater that penetrates into the subducting oceanic plate is incorporated in water-bearing minerals and carried deep into the earth. As the subducting plate descends and heats up, water is released and can influence the properties of subduction zone faults, the generation of subduction zone earthquakes, and the generation of magmas that feed arc volcanoes. Some of the water is released back into the atmosphere through volcanic eruptions. A critical unknown in this cycle is the amount and distribution of water in the incoming tectonic plate. How much water is stored in the plate? To what depth in the plate does water penetrate? Does the amount of water in the plate vary along the subduction zone? If so, what controls this variation? This project will use seismic and bathymetric (seafloor-depth) data collected across the Alaska subduction zone between Kodiak Island and the Shumagin Islands to constrain the volume and distribution of water stored in the subducting oceanic plate. The results will be valuable for understanding changes in the occurrence of large earthquakes and the compositions of magmas feeding volcanoes in this part of the Alaska subduction zone. Graduate and undergraduate students will participate in data analysis and gain valuable training. The volume and distribution of water is thought to control a host of fundamental processes at subduction zones, including megathrust behavior, the generation of arc magmas, and intermediate depth earthquakes. However, the amount of water delivered into the subduction zone by the incoming oceanic plate remains controversial and poorly known, resulting in great uncertainties in Earth’s deep water budget. In addition, the along-strike variation of water input, and its influence on along-strike variations in subduction processes, are also poorly constrained. Newly acquired seismic and bathymetric data from the Alaska Amphibious Community Seismic Experiment (AACSE) will be combined with existing data off the Alaska Peninsula to characterize outer-rise faulting and possible hydration throughout the incoming oceanic lithosphere. This subduction zone is an excellent target for study because it exhibits along-strike variations in megathrust coupling and seismicity at a range of depths, and existing data suggest along-strike variations in faulting and hydration. The lateral and depth distribution of hydrous phases in the incoming crust and mantle will be determined using seismic surface wave, body wave, and active source methods, and the results will be compared with incoming plate seismicity and mapped faults from bathymetry. Seismic anisotropy will provide constraints on the distribution of hydrated minerals around fault zones. The results will provide a comprehensive estimate of the incoming-plate water budget as a function of location along the Alaska subduction zone, allowing the effects of water on other subduction processes to be quantitatively evaluated. New constraints on the incoming water budget can be used by multidisciplinary studies evaluating the influence of water on megathrust and intermediate depth seismicity characteristics, along-strike volcanic output changes, and on the global mantle water budget. Estimates of the maximum size of incoming plate normal faults will allow the possible tsunami hazards of such earthquakes to be evaluated. 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.