Abstract
Earth’s largest earthquakes occur in subduction zones, along the boundary between the subducting and overriding plates1. Non-volcanic tremor generated by slow slip between the plates is thought to originate on, or near, this boundary2,3. Earthquakes also occur in the down-going plate as fluids are released4, and zones of anomalously low seismic velocities observed beneath several subduction zones are interpreted to be the subducting oceanic crust5,6,7,8,9,10. Yet, the exact location of the plate boundary remains uncertain5. Here we interpret a three-dimensional seismic tomography model from the northern Cascadia subduction zone in the northwest USA. We find that the low-velocity zone varies considerably along the Cascadia margin. In places, we observe the low-velocity zone to crop out at the surface and separate from the descending plate at depths of 35–40 km. We argue that the low-velocity zone here cannot represent oceanic crust as previously suggested, and instead the zone mostly represents sediments that have been subducted and underplated beneath the North American continent. We also find that tremor signals correlate with the position of the low-velocity zone, implying that slow slip and tremor may be facilitated by trapped fluids and high pore fluid pressures in subducted sedimentary rocks at, or close to the plate boundary. Our results also imply that the plate boundary beneath Cascadia is much deeper than previously thought.
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Acknowledgements
We thank G. Abers for providing the geometry of the CAFE teleseismic profile and H. Kao for providing some tremor locations in Canada. This project was funded by the Natural Sciences and Engineering Research Council of Canada and US Geological Survey through grant 06HQGR0029. The acquisition of the various field data was supported by the US Geological Survey, the Geological Survey of Canada, the National Science Foundation, and several US and Canadian universities. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration, under contract DE-AC04-94AL85000.
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L.A.P. created the tomographic velocity model and interpreted seismicity within the model; A.M.F. computed the tremor locations; A.J.C. interpreted velocity variations within the model and wrote the paper.
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Calvert, A., Preston, L. & Farahbod, A. Sedimentary underplating at the Cascadia mantle-wedge corner revealed by seismic imaging. Nature Geosci 4, 545–548 (2011). https://doi.org/10.1038/ngeo1195
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DOI: https://doi.org/10.1038/ngeo1195
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