Tsunamigenic structures in a creeping section of the Alaska subduction zone

Abstract

Segments of subduction zones that are capable of generating tsunamigenic earthquakes appear to have characteristic structural configurations. These structures include heterogeneous plate interfaces, a small wedge of deformed sediment at the toe of the overriding plate (the frontal prism), and splay faults in the crust of the overriding plate that root within the plate boundary megathrust. Here we use seismic reflection imaging to show that these features also exist within a creeping segment of the Alaska subduction zone, the Shumagin Gap. We identify an active crustal-scale normal fault system that dips landward and resembles that involved in the 2011 Tohoku-oki earthquake in Japan. We also find that the Shumagin Gap has a small frontal prism, a deep-water splay fault, and that the plate interface here is rough and thinly sedimented. We propose that lateral propagation of rupture from a neighbouring segment into the Shumagin Gap may explain a tsunamigenic earthquake that occurred there in 1788 and that tsunamigenic potential should be considered in hazard assessments for the region. Our results demonstrate that structural configurations similar to those in Tohoku may exist in other subduction zones, including within creeping segments or segments with no record of historical megathrust earthquakes, but are under-recognized. Identifying similar configurations globally may improve our ability to anticipate regions capable of generating large tsunamis.

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Figure 1: Tectonic setting and extent of the Sanak and Unimak basins.
Figure 2: Reflection image and bathymetry profile for ALEUT Line 5.
Figure 3: Lower slope of ALEUT Lines 5 and 6.

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Acknowledgements

We gratefully acknowledge the captain, technical staff and crew of the RV Marcus G. Langseth and the Scripps OBS team who made this data set possible. This work was supported by the US National Science Foundation grants OCE-0926614 and EAR-1347312.

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D.J.S., M.R.N. and S.C.W. obtained funding for the marine seismic programme. D.J.S., A.B., M.R.N., S.C.W. and M.D. collected marine data during the research cruise on RV Langseth. A.B., G.A.A., K.M.K., D.J.S., M.R.N. and D.M.S. obtained funding for seismicity and seismic data integration. A.B. conducted the MCS processing. D.J.S. performed the wide-angle reflection/refraction modelling. K.M.K. and G.A.A. analysed the seismicity data. All authors discussed the results and commented on the manuscript.

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Correspondence to Anne Bécel.

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Bécel, A., Shillington, D., Delescluse, M. et al. Tsunamigenic structures in a creeping section of the Alaska subduction zone. Nature Geosci 10, 609–613 (2017). https://doi.org/10.1038/ngeo2990

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