Constraining the hydration state of the subducting mantle with converted seismic phases

• Applicant: Dr. Wasja Bloch
• Subject Area: Geophysics ; Mineralogy, Petrology and Geochemistry
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449279512

• Final Report Year: 2023

Final Report Abstract

The aim of the project was to estimate the amount of water stored in the subducting lithosphere in subduction zones. Water that enters the oceanic tectonic plate along the deep-sea trenches and is drawn into Earth’s mantle through plate tectonics plays an important role in a large number of earth processes, like volcanism, the deep element cycle, or seismogenesis. The presence of water in the subducting mantle was here imaged qualitatively using the receiver function method. The procedure allows to estimate the Earth’s structure and composition below a seismic station using elastic waves of earthquakes that occur at distances between 3,000 and 10,000 kilometers. To apply the method, a computer code to model seismic wave scattering was translated into the modern Python computing language. This allowed for a ten times accelerated processing speed, improved interpretation of the receiver function sections, and parallel application of the program on multiple computer cores. The application of the method to the first study area of the Cascade range in North America revealed that the structure of the subducting slab is more complex than previously thought. As a consequence, only a qualitative estimate about the presence of absence of water inside the subducting oceanic mantle beneath 37 stations located on the on-shore forearc was possible. It turned out that regions where the oceanic mantle is hydrated correlate with the occurrence of intermediate depth seismicity, as well as with the location of fracture zones within the oceanic plate. This correlation suggests that water that enters the oceanic mantle through fracture zones causes intermediate depth seismicity during subduction. The project then focused on the detailed structural modeling of the subducting plate within the Cascades; a requirement to arrive at a quantitative estimate of mantle hydration. The extension of the subsurface model to observations from 298 seismic stations, also with poorer data quality, resulted in a detailed, double-layered structural model of the Juan de Fuca plate beneath North America. The model shows how material from the top of the subducting plate is accumulated at the base of the overriding North American plate, and how the location of a stiff and competent basaltic province within the continental crust deforms the subducting plate. The image of the plate boundary and the oceanic Moho correlates well with recently acquired off-shore reflection seismic measurements. The model contributes to characterizing the rupture plane of a possible future magnitude 9 megathrust earthquake that may occur in the American Pacific Northwest.

Publications

• Estimating the amount of hydration of the subducting oceanic mantle using receiver function data. AGU Fall Meeting Abstracts. T25C-0178.
  Bloch, W.; Bostock, M. G.; Audet, P. & Brownlee, S.
  
• PyRaysum: Software for modeling ray-theoretical bodywave propagation. Zenodo
  Audet, P. & Bloch, W.
  
• A Cascadia Slab Model From Receiver Functions. Geochemistry, Geophysics, Geosystems, 24(10).
  Bloch, Wasja; Bostock, Michael G. & Audet, Pascal
  
• PyRaysum: Software for Modeling Ray-theoretical Plane Body-wave Propagation in Dipping Anisotropic Media. Seismica, 2(1).
  Bloch, Wasja & Audet, Pascal