High-resolution Rayleigh-wave tomography constraints on hydration, faulting, and mantle fabric in the incoming plate along the Alaska subduction zone

Yakubu, Tawfic (2023) High-resolution Rayleigh-wave tomography constraints on hydration, faulting, and mantle fabric in the incoming plate along the Alaska subduction zone. Masters thesis, Northern Arizona University.

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Abstract

Subduction zones are known to drive the deep-water cycle on our planet, and water that penetrates the subduction zone controls fundamental geological processes like the occurrence of large earthquakes and the generation of arc magmas. However, the initial water content of many subducting slabs is poorly known. Here we provide new constraints on the distribution of hydration in the incoming Pacific plate offshore of the Alaska Peninsula. We image crust and shallow-mantle shear velocity and anisotropy using short-period Rayleigh waves derived from 15 months of continuous ambient seismic noise recorded by 32 ocean-bottom seismometers (OBS) that were deployed from outboard of the outer rise to just inboard of the trench during the Alaska Amphibious Community Seismic Experiment. After correction for tilt and compliance effects, we observe first-overtone Rayleigh waves with high signal-to-noise ratio within a period range of 5-10s. These data are particularly useful for constraining velocity variations of the incoming oceanic crust and uppermost mantle. We estimate Rayleigh-wave phase velocities utilizing spectral-amplitude Bessel-function fitting, and we invert for 2-D phase velocity maps and 1-D and 2-D azimuthal anisotropy. Phase-velocity maps exhibit significant (>5%) lateral variations on the incoming plate along the outer rise, with localized low velocities outboard of the Shumagin gap, the Semidi segment, and the Kodiak region. We invert the phase-velocity observations for 3D models of shear-wave velocity in the incoming plate crust and shallow mantle. We find slower velocities than expected for the oceanic mantle and crust. We find that the Shumagin gap has lower velocities than the Semidi segment. These observations are consistent with effect of hydration, both in the crust and mantle, with more in Shumagin gap than Semidi. Where it is well constrained, we find relatively weak azimuthal anisotropy with peak-to-peak amplitudes of 0-2%, and fast directions that are oblique to the orientations of both expected seafloor-spreading mantle fabric and observed bending faults. These observations may be consistent with the competing anisotropic effects of fault-induced structural heterogeneity, and seafloor-spreading-induced olivine fabric.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Water; Subduction zones; Alaska Amphibious Community Seismic Experiment; Rayleigh-wave phase velocities
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	07 Aug 2026 17:41
Last Modified:	07 Aug 2026 17:41
URI:	https://openknowledge.nau.edu/id/eprint/6337

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