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Forearc seismogenesis in a weakly coupled subduction zone influenced by slab mantle fluids

A Publisher Correction to this article was published on 09 October 2023

This article has been updated

Abstract

The role of fluids in earthquake rupture is key to understanding seismic hazards, particularly at subduction zones. The Shumagin Gap, Alaska, is notable due to a paucity of large earthquake nucleation and weak coupling between the overriding and subducting plates. Fluids have been hypothesized to explain these observations, but the source of the fluids remains unclear. Here we present an image of the subsurface electrical resistivity derived from marine magnetotelluric data collected in the Shumagin segment. The model reveals an approximately 50-km-wide conductive (that is, fluid-rich) zone near the plate interface with fluids sourced from the dehydration of slab mantle (15–25 km beneath the crust–mantle boundary). We find that the July 2020 megathrust earthquake—which nucleated near the Semidi segment and propagated westwards into the Shumagin segment—only ruptured the conductive portion of the plate interface. This suggests that slab mantle fluids can influence the seismogenic zone by, for example, creating patches that are prone to dynamic rupture. In contrast, updip of the slip patch is simultaneously resistive and weakly coupled, suggesting that fluids alone are not responsible for weak coupling and that plate roughness plays a role. More broadly, these results suggest that slab mantle fluids could be an underappreciated fluid source in the water budgets of forearc subduction zones.

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Fig. 1: Map of the Shumagin segment study area.
Fig. 2: Magnetotelluric modelling results and interpretation.
Fig. 3: Hydrous mineral dehydration and slab PT paths for the Shumagin segment.
Fig. 4: Conceptual model.

Data availability

Magnetotelluric impedance data and inversion model files are available via Figshare at https://doi.org/10.6084/m9.figshare.21751634.

Change history

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Acknowledgements

We thank the captain and crew of RV Sikuliaq expedition SKQ201914S, the Scripps Marine EM Lab (C. Armerding, J. Lemire, J. Perez and J. Souders) and the EMAGE science party (T. Acquisto, J. Alvarez-Aramberri, J. Andrys, C. Armerding, E. Attias, G. Boren, B. Chase, C. Chesley, P. K. Miller, J. Perez, L. Wei and J. Zhu). This work was supported by the National Science Foundation under grant numbers OCE-1654652 to K.K., S.C., S.N. and D.S., and OCE-1654619 to R.E.

Author information

Authors and Affiliations

Authors

Contributions

K.K., R.E, S.C. and S.N. designed the experiment. K.K., S.C. and S.N. collected the data. S.N. processed the magnetotelluric responses. D.C. modelled the data, produced the figures and wrote the initial manuscript. All authors discussed the results and contributed to writing the manuscript.

Corresponding author

Correspondence to Darcy Cordell.

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The authors declare no competing interests.

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Nature Geoscience thanks Yasuo Ogawa, Bo Yang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Stefan Lachowycz and James Super, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Dimensionality analysis using polar diagrams.

Dimensionality analysis using polar diagrams for the profile data.

Extended Data Fig. 2 Phase tensor ellipse pseudo-section.

Phase tensor ellipse pseudo-section along profile. Blue circular ellipses denote 1-D structure, whereas blue or yellow ellipses denote 2-D or pseudo-2-D structure. Red ellipses indicate 3-D structures. The data included in the inversion is indicated by the red boxes.

Extended Data Fig. 3 Dimensionality analysis using phase tensor.

Dimensionality analysis for the profile data using phase tensor. (a) Rose histogram showing phase tensor geoelectric strike angles for raw, un- edited dataset at all frequencies indicating no coherent strike direction. (b) Rose histogram showing phase tensor geoelectric strike angles for edited dataset at all frequencies indicating coherent strike direction of approximately 75°E of N. (c) Histogram of phase tensor skew angles for un-edited data at all frequencies with long tails to ±80°. (d) Histogram of phase tensor skew angles for edited data set at all frequencies with tighter distribution near 0°.

Extended Data Fig. 4 Convergence for the preferred inversion.

Convergence for the preferred inversion run40. Top left: r.m.s. misfit as a function of iteration. Top right panel: model roughness. Bottom left panel: iteration time. Bottom right panel: Lagrange multiplier search at each iteration step.

Extended Data Fig. 5 Data fits for the 30th iteration of run40.

Data fits for the 30th iteration of run40 with an overall r.m.s. misfit of 1.00. Red dots and blue dots are TM and TE mode data, respectively. Curves are the inversion response. Error bars on the data points are smaller than the data marker.

Extended Data Fig. 6 Breakdown of data misfit as a function of site, period, and impedance component.

Breakdown of data misfit as a function of: site NW to SE (top), period (middle), and impedance component (bottom) for the 30th iteration of run40 with an overall r.m.s. misfit of 1.00.

Extended Data Fig. 7 Inversion model which enforces a no smoothing constraint across the plate interface.

The model converged to an r.m.s. of 1.0 after 86 iterations.

Extended Data Fig. 8 Full model space of the preferred inversion model.

Full model space of the preferred inversion model (run40) showing the deeper conductor at 100 km depth.

Extended Data Fig. 9 Sensitivity tests for features C2, C3, and C4.

Sensitivity tests for features: (A) C2, (B) C3, and (C) C4. Each feature is replaced with 100 Ωm anomaly and then a forward simulation is run.

Extended Data Fig. 10 Results of sensitivity tests.

Results of sensitivity tests showing r.m.s. misfit changes as a function of site (top), period (middle), and impedance component (bottom).

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Cordell, D., Naif, S., Evans, R. et al. Forearc seismogenesis in a weakly coupled subduction zone influenced by slab mantle fluids. Nat. Geosci. 16, 822–827 (2023). https://doi.org/10.1038/s41561-023-01260-w

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