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Abstract Oceanic plate seamounts are believed to play an important role in megathrust rupture at subduction zones, although consistent relationships between subducting seamounts and plate interface seismicity patterns are not found. While most studies focus on impacts linked to their topography, seamounts are also sites of heterogeneity in incoming plate sediments that may contribute to megathrust properties. Here, we characterize incoming plate sediments along the Cascadia subduction zone using new high‐resolution seismic images and compressional wave (Vp) models from the CASIE21 multi‐channel‐seismic experiment. Nine fully‐to‐partially buried seamounts are identified seaward of the deformation front within a region of thick Plio‐Pleistocene sediment where the Juan de Fuca plate is bending into the subduction zone. Anomalously highVpsediment blankets two seamounts offshore Washington‐Central Oregon, with wavespeeds reaching 36% and 20% higher than adjacent sediment. Fluid seepage and temperatures warm enough for smectite diagenesis extending to shallow depths are inferred from heat flow studies and we attributeVpanomalies to sediment cementation linked primarily to smectite dehydration. Signatures of fluid seepage above seamounts are also identified offshore Vancouver Island, but anomalously lowVpsediment below distinct reverse polarity reflections are found, indicating trapped fluids, and cooler basement temperatures are inferred. Landward of one seamount, a zone of enhanced sediment compaction is found, consistent with the predicted stress modulating effects of seamount subduction. These new findings of variations in sediment diagenesis and strength around seamounts prior to subduction may contribute to the diverse megathrust frictional properties and seismicity patterns evident at subducting seamounts.
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This content will become publicly available on July 4, 2026
Effects of incoming polygonal fault systems on subduction zone and slow slip behavior
The physical properties of subduction inputs profoundly influence megathrust slip behavior. Seismic data reveal extensive polygonal fault systems (PFSs) in the input sequences of the Hikurangi Margin and Nankai Trough. The mechanical and hydrological effects of these incoming PFSs on subduction zones are potentially substantial. Here, we investigate their effects following transport into the accretionary wedge by integrating discrete-element modeling with three-dimensional seismic interpretation. We find that the typical dips of the incoming PFSs overlap with modeled dips prone to reactivation and confirm that subducting PFSs can be reactivated and gradually evolve into major thrust faults. Comparisons with electromagnetic data indicate that PFSs may provide conduits for fluid leakage along the plate interface, coincide with disrupted strata and decreased shear stress, and enhance geometric and stress heterogeneity along the megathrust. These suggest that PFSs may play a previously unrecognized role in contributing to shallow slow earthquake phenomena in subduction zones.
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- Award ID(s):
- 2123255
- PAR ID:
- 10653686
- Publisher / Repository:
- AAAS
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 11
- Issue:
- 27
- ISSN:
- 2375-2548
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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