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Title: The seismic velocity structure of the Wadati-Benioff Zone : insights from guided waves
Author: Garth, Thomas
ISNI:       0000 0004 5350 8502
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2014
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Low velocity hydrous minerals in the subducting plate deliver water to the mantle and are thought to cause intermediate depth Wadati-Benioff zone (WBZ) seismicity through dehydration embrittlement. High frequency seismic energy (> 2 Hz) from intermediate depth earthquakes that occur within this low velocity oceanic crust is retained and delayed by the crustal waveguide while lower frequency (< 0.5 Hz) energy travels at the faster velocities of the surrounding mantle. These guided waves therefore spend longer interacting with the low velocity oceanic crust than any other seismic phase, and have the potential to reveal a large amount about the velocity structure of the WBZ. Dispersive arrivals recorded in the forearc of Northern Japan are directly compared to synthetic waveforms produced from full 2D and 3D waveform simulations. Comparing the relative amplitude and arrival time of a given frequency using the velocity spectra and spectrogram respectively, allows the full dispersive P-waveform to be constrained. Analysis of dispersive arrivals from upper plane WBZ events at 150 – 220 km depth place the first observational constraints on the metamorphic reactions occurring before full eclogitisation of the subducting oceanic crust. I show that blueschist and lawsonite bearing rocks may persist well beyond the depths inferred from established thermo-petrological subduction zone models, and that full eclogitization may occur at much greater depths than is inferred by receiver function studies. The persistence of meta-stable hydrous minerals explains the occurrence of WBZ seismicity at 200 - 250 km depth, and may be due to the partially hydrated oceanic crust. Dispersion from events that occur well below the upper plane of WBZ seismicity can be explained by the occurrence of low velocity hydrated outer rise normal faults at intermediate depths. At depth, these faults are inferred to be 2 - 3 km thick and 12 - 15 % slower that the surrounding mantle, suggesting they are 50 - 71 % serpentinised. We suggest that the extended P-wave coda observed at stations close to the trench in Northern Japan are explained by low velocity dipping faults of a range of scale lengths forming a scattering medium. This scattering medium is simulated using a von Kármán function, and the synthetic waveforms produced are compared to the observed P-wave coda, that decays in amplitude with distance from the trench. The magnitude of this spatial coda decay is sensitive to the average bulk velocity of the scattering medium and provides a constraint on the hydration of the lithospheric mantle subducted beneath Japan. This first in-situ constraint on the degree of slab mantle hydration at intermediate depth suggests that 170.4 - 318.7 Tg/Myr/m of water is subducted beneath Northern Japan by the slab mantle. In summary we have shown that up to 94 % of the water subducted beneath Northern Japan is transported by the lithospheric mantle, and that upper and lower planes of WBZ zone seismicity are directly related to hydrous mineral assemblages, and so may occur through dehydration embrittlement. This work shows that guided waves have the potential to resolve new details of the WBZ velocity structure and the techniques developed here can be applied to other subduction zone settings.
Supervisor: Rietbrock, Andreas; Kusznir, Nick Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QE Geology