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    Vp/Vs ratio of incoming sediments off Cascadia subduction zone from analysis of controlled-source multicomponent OBS records

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    Date
    2020-05-28
    Author
    Zhu, Jian  Concept link
    Canales, J. Pablo  Concept link
    Han, Shuoshuo  Concept link
    Carbotte, Suzanne M.  Concept link
    Arnulf, Adrien F.  Concept link
    Nedimovic, Mladen R.  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/26270
    As published
    https://doi.org/10.1029/2019JB019239
    DOI
    10.1029/2019JB019239
    Keyword
     Vp/Vs; sediments; ocean bottom seismometer; Juan de Fuca plate; Cascadia 
    Abstract
    P‐to‐S‐converted waves observed in controlled‐source multicomponent ocean bottom seismometer (OBS) records were used to derive the Vp/Vs structure of Cascadia Basin sediments. We used P‐to‐S waves converted at the basement to derive an empirical function describing the average Vp/Vs of Cascadia sediments as a function of sediment thickness. We derived one‐dimensional interval Vp/Vs functions from semblance velocity analysis of S‐converted intrasediment and basement reflections, which we used to define an empirical Vp/Vs versus burial depth compaction trend. We find that seaward from the Cascadia deformation front, Vp/Vs structure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevated Vp/Vs. These zones with elevated Vp/Vs are likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic pressure and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneous Vp/Vs functions of Cascadia Basin sediments.
    Description
    Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 125(6), (2020): e2019JB019239, doi:10.1029/2019JB019239.
    Collections
    • Geology and Geophysics (G&G)
    Suggested Citation
    Zhu, J., Canales, J. P., Han, S., Carbotte, S. M., Arnulf, A., & Nedimovic, M. R. (2020). Vp/Vs ratio of incoming sediments off Cascadia subduction zone from analysis of controlled-source multicomponent OBS records. Journal of Geophysical Research: Solid Earth, 125(6), e2019JB019239.
     

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