Subduction of rough seafloor and seamounts is thought to impact a broad range of geodynamic processes, including megathrust slip behaviour, forearc fluid flow and long-term structural evolution in the overriding plate. Although there are many conceptual models describing the effects of seamount subduction, our quantitative and mechanistic understanding of the underlying deformation and fluid processes remains incomplete. Here we investigate the interplay between sediment consolidation, faulting and the evolution of stress and pore fluid pressure in response to seamount subduction, using a numerical model that couples mechanical and hydrological processes and is constrained by laboratory and field observations. Our results show that subducting topography drives marked spatial variations in tectonic loading, sediment consolidation and megathrust stress state. Downdip of a subducting seamount on its leading flank, enhanced compression and drainage lead to large fault-normal stress and overconsolidated wall rocks. A stress shadow in the seamount’s wake leads to anomalously high sediment porosity. These variations help explain observed patterns of megathrust slip, with earthquakes and microseismicity favoured at the downdip edge of seamounts and aseismic or slow slip in the updip stress shadow.
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All the data of earthquake and slow earthquake catalogues and slow slip distributions used in this study have been published by other researchers as referenced in this paper.
The algorithm and the part of code for coupling mechanical and hydrological computation is accessible from the corresponding author upon request.
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Constructive comments from Jonas Ruh and an anonymous reviewer are greatly appreciated. This study was funded by NSF EAR Award no. 1616664 to D.S., and an IODP Exp 380 post-cruise award was provided to T.S. by USSSP. S.E. was supported by core funding to GNS Science and by a MBIE Endeavour fund. The mechanical code SULEC was modified from an existing version jointly written by S.E. and Susanne Buiter. We thank Kelin Wang, Earl Davis and members of the SHIRE project for helpful discussions.
The authors declare no competing interests.
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Extended Data Fig. 1 Consolidation curves and corresponding compaction loading efficiency of the sediment matrix (black) and the oceanic slab (blue).
a, Small black dots represent laboratory and shipboard experimental results using sediment samples from Nankai off-Kumano56. Larger grey dots show field-based (borehole logging) measurements from Nankai off-Muroto and Barbados57. The four numerical digits, sometimes with a “C” in the front, indicate site numbers of the International Ocean Drilling Program. Measurements made on the sedimentary rock samples of Nias Sumatra outcrop are from ref. 59. b, Compaction loading efficiency is computed using Eqs. (2) and (3) in Methods, assuming a constant fluid compressibility of 5 × 10−10 Pa−1 and incompressible solid grain.
a, Log-linear relationship between matrix permeability and porosity is assigned for the sediment matrix (black thick line) and the oceanic slab (blue thick line). For sediment matrix, this porosity dependence is supported by the results of laboratory experiments on various types of sediments, including argillaceous sediments62 (grey box) and mudstones and sandstones56,65 (other symbols). b, To account for the hydraulic effects of permeable fault damage zones, permeability is enhanced by a factor that linearly scales with the shear strain (see Methods).
Three models with different functions of enhancement factor but otherwise identical input parameters show similar model-predicted patterns of fluid overpressure, fault structure, and sediment consolidation. Grey dashed lines indicate the plate interface. See Fig. 2 caption for definition of parameters. See Extended Data Table 1 for input model parameters.
Extended Data Fig. 5 Empirical relationship between seismic P wave velocity (VP) and sediment porosity (ref. 43).
This is originally based on field measurements for sediments at the Nankai Trough subduction zone (offshore of Cape Muroto). This function is used to obtain the synthetic VP profile shown in Fig. 3a. Data shown here for comparison include shipboard measurements for sediments at Nankai (offshore of the Kii Peninsula56), results of laboratory consolidation tests and measurements66 (dots in colors), and measurements for Shimanto Belt shales from Nankai67.
A subducting sediment layer is assigned between the décollement and the subducting seamount. Grey dashed lines indicate the plate interface. See Fig. 2 caption for definition of parameters. See Extended Data Table 1 for input model parameters. Compared with upper-plate sediment subjected to horizontal compression, modeled porosity values of underthrust sediment are higher owing to the reduction of compressive stress below the plate interface.
Extended Data Fig. 7 Map view of distribution of fault slip behaviour around subducting seamounts at two margins.
a, Central Nankai, and b, Central Ecuador. Grey dashed contours show plate interface depth (in km). In insert, black lines delineate plate boundaries; grey box encloses the zoom-in map area. In a, blue dots show two Ocean Drilling Program boreholes (Holes 808 and 1173) in which fluid pressure records suggest the occurrence of trench-breaching slow slip50. VLFE, very-low-frequency earthquake; SSE, slow slip event. Full summary of the data shown here is in Extended Data Table 3.
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Sun, T., Saffer, D. & Ellis, S. Mechanical and hydrological effects of seamount subduction on megathrust stress and slip. Nat. Geosci. 13, 249–255 (2020). https://doi.org/10.1038/s41561-020-0542-0
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