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Links between sediment consolidation and Cascadia megathrust slip behaviour

Abstract

At sediment-rich subduction zones, megathrust slip behaviour and forearc deformation are tightly linked to the physical properties and in situ stresses within underthrust and accreted sediments. Yet the role of sediment consolidation at the onset of subduction in controlling the downdip evolution and along-strike variation in megathrust fault properties and accretionary wedge structure is poorly known. Here we use controlled-source seismic data combined with ocean drilling data to constrain the sediment consolidation and in situ stress state near the deformation front of the Cascadia subduction zone. Offshore Washington where the megathrust is inferred to be strongly locked, we find over-consolidated sediments near the deformation front that are incorporated into a strong outer wedge, with little sediment subducted. These conditions are favourable for strain accumulation on the megathrust and potential earthquake rupture close to the trench. In contrast, offshore Central Oregon, a thick under-consolidated sediment sequence is subducting, and is probably associated with elevated pore fluid pressures on the megathrust in a region where reduced locking is inferred. Our results suggest that the consolidation state of the sediments near the deformation front is a key factor contributing to megathrust slip behaviour and its along-strike variation, and it may also have a significant role in the deformation style of the accretionary wedge.

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Fig. 1: Palaeo-earthquake rupture segments and current megathrust locking status of the Cascadia subduction zone, and representative forearc structures offshore Washington and Central Oregon.
Fig. 2: Comparison of sediment consolidation and in situ stress state between the Washington transect (47.4° N) and the Oregon transect (44.6° N).
Fig. 3: Along-strike variation in sediment consolidation state in relation to underthrust sediment thickness and forearc structure from 44.3° N to 47.8° N.

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Acknowledgements

We thank the captain, crew and technical staff of R/V Marcus G. Langseth for their efforts, which made the success of cruise MGL1211 possible. Seismic data processing and interpretation was conducted using the Paradigm processing software packages Echos and Geodepth. We thank A. Arnulf for providing the velocity profile from the Hikurangi margin. This research was supported by the National Science Foundation through a GeoPRISMS Postdoctoral Fellowship (Award 1457221) to S.H. and Award 1029411 to S.M.C.

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S.H. participated in the data collection and processed the seismic data. S.M.C. conceived of the project and led the data collection. S.H., N.L.B. and S.M.C interpreted the seismic data. S.H. and D.M.S. conducted the porosity and effective stress analysis. J.C.G. participated in the data collection and provided the starting models for velocity analysis. S.H. wrote the paper with contributions and edits from all other authors.

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Correspondence to Shuoshuo Han.

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Supplementary data and analyses to support the proposed links between sediment consolidation and Cascadia megathrust slip behaviour

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Han, S., Bangs, N.L., Carbotte, S.M. et al. Links between sediment consolidation and Cascadia megathrust slip behaviour. Nature Geosci 10, 954–959 (2017). https://doi.org/10.1038/s41561-017-0007-2

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