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Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing

Abstract

Water and hydrous minerals play a key part in geodynamic processes at subduction zones1,2,3 by weakening the plate boundary, aiding slip and permitting subduction—and indeed plate tectonics—to occur4. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5,6,7. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson’s ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson’s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

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Figure 1: Geographical map of northern Cascadia subduction zone.
Figure 2: Radial receiver function results beneath northern Cascadia.
Figure 3: Schematic interpretation of receiver function results.

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Acknowledgements

We thank E. Davis and M. Jellinek for discussions. Data used in this study come from the Canadian National Seismological Network and are distributed freely by the Geological Survey of Canada.

Author Contributions P.A. and M.G.B. designed the study, analysed the data and wrote the paper; N.I.C. and S.M.P. participated in the interpretation of results.

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Correspondence to Pascal Audet.

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Audet, P., Bostock, M., Christensen, N. et al. Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing. Nature 457, 76–78 (2009). https://doi.org/10.1038/nature07650

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