Large seamounts and basement relief cause permanent deformation when they collide with the overriding plate at subduction zones. The resulting structural and compositional heterogeneities have been implicated as controlling factors in megathrust slip behaviour. Subducting seamounts may temporarily lock plates, favouring subsequent large earthquakes. Alternatively, seamounts may redistribute stress, reducing seismic slip. Here we present three-dimensional seismic data from the seamount-studded subducting Hikurangi Plateau along New Zealand’s North Island. We find that one well-imaged seamount, the Pāpaku Seamount, locally uplifts the overriding plate and leaves a tube-shaped lens of sediment trailing in its wake. Anomalously low seismic velocities within and below the Pāpaku lens and along the megathrust fault are consistent with the presence of unconsolidated, overpressured fluid-rich sediments. Similar observations from an older sediment lens, which corresponds to the location of a 2014 slow-slip rupture event, suggest that such overpressures can persist along the megathrust due to delayed drainage out of the subducting plate. The collocation of the 2014 slow-slip earthquake with this sediment lens suggests that these fluid-rich regions define zones that enable slow slip. We hypothesize that sediment lenses left behind by subducting seamounts can create low-effective-stress patches within transitionally stable marine sediment along the megathrust that are conducive to slow slip.
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We thank the captains, crew and onboard science parties of the RV Tangaroa and the RV Marcus G. Langseth for their contribution to the data acquisition. We thank Paradigm Geophysical for providing software. We thank NIWA for providing bathymetry data. The academic NZ3D project was financially supported by US NSF (award #s 1559298 (N.L.B., S.H. and A.C.G.), 1901645 (N.L.B. and S.H.), 1559008 (G.F.M. and H.L.T.), 2023186 (G.F.M. and H.L.T.), 1558574 (H.J.T.) and 1558440 (J.H.E.)), NERC NE/M021203/1 (R.E.B., R.D. and L.F.), JAMSTEC (R.A. and S.K.), NZ MBIE and GNS Science (D.H.N.B., S.A.H., D.B., R.K., V.S. and B.F.).
The authors declare no competing interests.
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Map directly overlaps with Fig. 1 and shows the seamounts and sediment lenses in NZ3D survey area (black rectangle), and slip during the 2014 slow slip event21. Barbed black line marks the deformation front. The NZ3D survey is 15 × 65 km2. Red dashed lines (with green shading) are seamount locations imaged within the 3D volume. Dark blue shading shows the outline of an inferred seamount from Barker et al.25) based on a two-dimensional seismic profile and magnetic anomalies. Cyan shading shows the position of a sediment lens in the wake of the Pāpaku seamount and beneath the Pāpaku fault, and second sediment lens believed to have formed from an older, subducted seamount. Slip contours are in mm.
This view looks north and shows the seismic data volume with the sediment lens (light shading) updip from the Pāpaku seamount. The Pāpaku sediment lens formed in the wake of the subducting Pāpaku seamount. The yellow surface is the Pāpaku fault, which forms a lateral ramp that intersects the décollement in an arcuate pattern. It extends farthest downdip above the Pāpaku seamount peak. The sediment lens beneath the Pāpaku fault fills in the space updip from the Pāpaku seamount. The peak of the Pāpaku seamount lies directly updip from the thickest section of the sediment lens, which thins to the SW updip of the flank of the Pāpaku seamount. A second sediment lens lies beneath Tuaheni ridge and is believed to have formed from an older subducted seamount.
Extended Data Fig. 5 Perspective view showing the sediment lens beneath the Pāpaku fault and underlying basement seismic velocities.
Light shading shows the geometry of the sediment lens beneath the Pāpaku fault. The colored surface lies 500 m below the top of the subducting basement (red line) and shows the average velocity within the uppermost 500 m of subducting basement. Basement velocities are consistently lower beneath the sediment lens associated with the Pāpaku seamount than regions outside of the influence of the sediment lens to the southwest. This correlation suggests that the sediment lens inhibits drainage from beneath it.
Extended Data Fig. 6 Inline 480 from the NZ3D seismic data volume with and without the velocity model.
Top image is the seismic profile. Bottom section has an overlay of the velocity model. Inline 480 is located mid-way between Inlines 188 and 774, which are located in Fig. 1. This line lies ~ 350 m south of the IODP Expedition 372/375 drilling transect.
Full-waveform inversion models of streamer and OBS records. Surface streamer shot record (top) and a portion of an OBS shot record (bottom) with an FWI model from inversion up to 7 Hz superimposed on top. The positive lobe of the signal is shaded in black and the negative left unshaded to show where fit is good (blue showing) and poor (red showing). Models of both the reflections in the shot records and refractions in the OBS records fit the data well, and were further improved in subsequent inversions.
Extended Data Fig. 9 OBS20 geophone record showing first arrivals and the corresponding model result.
Shown is one record from one of 97 OBS used in the FWI and tomographic inversion. a) OBS20 record with first arrivals picked (blue line) to compare with synthetic model. b) Synthetic model produced from FWI and tomographic inversion of OBS 20. First arrival picked (red line) to compare with data. c) Comparison of first arrivals from OBS 20 and synthetic seismogram derived from FWI and tomographic inversion by CGG Services (Singapore) showing excellent agreement at all offset ranges out to ~25 km. FWI was limited to 25 km offsets due to previous shot noise and weak signals at higher offsets. ε and δ are Thomsen anisotropy parameters52. See Arai et al.30 for location.
The initial (top) and the final (bottom) images for Inline 500 produced with initial and final velocity models to show improved continuity of major reflections, reduced interference from reflections crosscutting primary reflections, higher reflection amplitudes, and more geologically realistic structures.
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Bangs, N.L., Morgan, J.K., Bell, R.E. et al. Slow slip along the Hikurangi margin linked to fluid-rich sediments trailing subducting seamounts. Nat. Geosci. 16, 505–512 (2023). https://doi.org/10.1038/s41561-023-01186-3