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Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone

Nature volume 428, pages 163–167 (2004)Cite this article

Abstract

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust1. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together2. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent3,4,5,6,7. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, ∼110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there8,9.

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Figure 1: Map of the northern Cascadia subduction zone showing the distribution of inslab earthquakes (filled black circles) along the margin.
Figure 2: Composite seismic cross-section across the Cascadia forearc region in the vicinity of southern Vancouver Island.
Figure 3: Composite seismic section near the west coast of Vancouver Island.
Figure 4: Schematic cross-section of the northern Cascadia convergent margin at southern Vancouver Island.

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Acknowledgements

I thank K. Ramachandran for making available to me the 3D velocity model and the positions of the relocated earthquakes; T. Brocher for comments and suggestions that helped to improve the final manuscript; and H. Kao and M. Nedimović for discussions. This project was funded by the Natural Sciences and Engineering Research Council of Canada.

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  1. Department of Earth Sciences, Simon Fraser University, 8888 University Drive, British Columbia, Burnaby, V5A 1S6, Canada

    Andrew J. Calvert

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Correspondence to Andrew J. Calvert.

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Calvert, A. Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone. Nature 428, 163–167 (2004). https://doi.org/10.1038/nature02372

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