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Abstract Shear attenuation provides insights into the physical and chemical state of the upper mantle. Yet, observations of attenuation are infrequent in the oceans, despite recent proliferation of arrays of ocean‐bottom seismometers (OBSs). Studies of attenuation in marine environments must overcome unique challenges associated with strong oceanographic noise at the seafloor and data loss during OBS recovery in addition to untangling the competing influences of elastic focusing, local site amplification, and anelastic attenuation on surface‐wave amplitudes. We apply Helmholtz tomography to OBS data to simultaneously resolve array‐averaged Rayleigh wave attenuation and maps of site amplification at periods of 20–150 s. The approach explicitly accounts for elastic focusing and defocusing due to lateral velocity heterogeneity using wavefield curvature. We validate the approach using realistic wavefield simulations at the NoMelt Experiment and Juan de Fuca (JdF) plate, which represent endmember open‐ocean and coastline‐adjacent environments, respectively. Focusing corrections are successfully recovered at both OBS arrays, including at periods <35 s at JdF where coastline effects result in strong multipathing. When applied to real data, our observations of Rayleigh wave attenuation at NoMelt and JdF revise previous estimates. At NoMelt, we observe a low attenuation lithospheric layer (> 1,500) overlying a highly attenuating asthenospheric layer (∼ 50 to 70). At JdF, we find a broad peak in attenuation (∼ 50 to 60) centered at a depth of 100–130 km. We also report strong local site amplification at the JdF Ridge (>10% at 31 s period), which can be used to refine models of crust and shallow mantle structure.
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Comparison of Biotite Elastic Properties Recovered by Spherical Nanoindentations and Atomistic Simulations — Influence of Nano‐Scale Defects in Phyllosilicates
Abstract Phyllosilicate minerals, due to their sheets structure and morphology, are known to cause anisotropy in bulk rock properties and make the bulk rock more compliant. Accurately characterizing the micromechanical behavior of phyllosilicate minerals from laboratory observations, which eventually translates to the bulk rock behavior, is still challenging due to their fine‐grained nature. Recent advances in atomistic simulations open the possibility of theoretically investigating such mineral mechanical behavior. We compare the elastic properties of biotites recovered by spherical nanoindentation with those predicted from density functional theory (DFT) simulations to investigate to what extent theoretical predictions reproduce actual phyllosilicate properties. Spherical nanoindentation was conducted using schist rocks from Poorman Formation, South Dakota, USA, to recover continuous indentation stress‐strain curves. Loading in the layer‐normal orientation shows an average indentation modulus () of about 35 GPa, while loading in the layer‐parallel orientation gives a higher average of about 95 GPa. To facilitate comparison, the elastic stiffness constants (cij) determined from DFT were converted to indentation modulus () using solutions proposed in this study. The majority of the nanoindentation modulus results are below the values inferred from the simulation results representing ideal defect‐free minerals. We suggest that crystal defects present at the nano‐scale, potentially ripplocations, are the dominant cause of the lower indentation modulus recovered from nanoindentation compared to those inferred from DFT simulations. Results highlight the importance of acknowledging the defects that exist down to the nano‐scale as it modifies the mechanical properties of phyllosilicates compared to its pure defect‐free form.
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- PAR ID:
- 10444995
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 126
- Issue:
- 8
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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