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  • Articles  (30)
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  • 1
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    Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing (2009)
    Nature Publishing Group (NPG)
    Details
    Publication Date: 2009-01-06
    Description: Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction-and indeed plate tectonics-to occur. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson's ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson's ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Audet, Pascal -- Bostock, Michael G -- Christensen, Nikolas I -- Peacock, Simon M -- England -- Nature. 2009 Jan 1;457(7225):76-8. doi: 10.1038/nature07650.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada. paudet@berkeley.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/19122639" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Nature Publishing Group (NPG)
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  • 2
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    Split from slip and schist: Crustal anisotropy beneath northern Cascadia from non-volcanic tremor (2012)
    Wiley
    Details
    Publication Date: 2012-08-07
    Description: Polarization and splitting analyses of minute-long windows of non-volcanic tremor at 3-component stations in the vicinity of southern Vancouver Island provide new evidence for the presence of anisotropy within the crustal portion of the North American plate beneath Cascadia. As reported previously, tremor particle motions are predominantly horizontal and inferred to manifest an upgoing wavefield composed primarily of S-waves. Under this assumption, estimates of incidence angle and back azimuth can be deduced from orientation of the smallest principle direction of motion. When subjected to standard S-wave splitting analysis, most stations yield systematic and reproducible fast polarization directions. Estimates of original polarization direction are more scattered and splitting times often exhibit multimodal distributions. Numerical simulations, wherein band-limited synthetic seismograms comprising contributions from multiple sources are analyzed for splitting, neatly reproduce these characteristics and thereby provide a framework for interpretation. A number of stations in our study sit astride the San Juan fault which, based on reflection studies, dips approximately 60 to 70° to the north. Underlying these stations is the Leech River Complex consisting of strongly foliated greenschist facies phyllites with steeply dipping foliations striking parallel to fast polarization directions of split S-waves. Detailed laboratory velocity measurements at elevated pressures show that as little as 2 to 3 km of vertically dipping Leech River phyllite are required to produce the observed splitting. These phyllites have some of the highest S-wave anisotropies measured to date and are part of a once continuous belt of anisotropic crust extending over 2000 km along the western North American coast from southern Vancouver Island to southwest Alaska.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    On corner frequencies, attenuation and low-frequency earthquakes (2017)
    Wiley
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    Publication Date: 2017-01-12
    Description: We have recently suggested that the nearly constant duration of low frequency earthquakes (LFEs) (and, equivalently, the band-limitation of tectonic tremor) manifests a moment-duration scaling that is fundamentally different from regular earthquakes and is most easily explained as rupture on asperities of roughly constant dimension. In that work, we employed qualitative arguments against potential bias by attenuation. Here we examine the role of attenuation more quantitatively through an analysis that avoids specification of particular source ( e.g. , Brune) models and relies on the particle velocity spectral maximum as the definition of apparent corner frequency. The analysis leads to the formal definition of a saturation frequency as the limiting value of apparent corner frequency as the true corner frequency tends infinity. The saturation frequency, a formal equivalent to f m a x , can be used to set bounds on path-averaged quality factor Q . We apply these relations to deep crustal and intraslab earthquakes beneath Vancouver Island to estimate bulk crustal attenuation parameters that are subsequently used to correct apparent corner-frequency measurements of LFEs reported in our earlier work. The attenuation bias due to bulk crustal structure is shown to be small, with negligible effect on the principal conclusions of that study. However, a review of laboratory and seismic refraction measurements of attenuation in oceanic basalts and evidence for high P -to- S LFE corner frequency ratios raises the possibility that strong, highly localized, near-source attenuation accompanying high pore-fluid pressures could cause the bandlimited nature of LFEs through the depletion of high frequencies.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
    Electronic Resource
    Electronic Resource
    Seismic Anisotropy in the Lower Oceanic Crust (1972)
    [s.l.] : Nature Publishing Group
    Nature 237 (1972), S. 450-451 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Laboratory studies of seismic velocities in rocks7-8 show that many common crustal rocks are highly aniso^opic to compressional wave propagation, so it is plausible that in many regions the Earth's crust is anisotropic, and Dorman9 has recently presented seismic data which indicate anisotropy of ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/237450a0
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Shear Wave Propagation in Rocks (1971)
    [s.l.] : Nature Publishing Group
    Nature 229 (1971), S. 549-550 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Fig. 1 Oscilloscope traces for shear wave propagation through (a) slate from Poultney, Vermont, and (b) dunite from Twin Sisters Peaks, Washington. The transducers are oriented at 0 to receive the higher velocity shear wave. Rotation of the transducers through 90 emphasizes the first arrival of the ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/229549b0
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  • 6
    Electronic Resource
    Electronic Resource
    Q structure of the oceanic crust (1991)
    Springer
    Marine geophysical researches 13 (1991), S. 227-237 
    Details
    ISSN: 1573-0581
    Keywords: Q ; attenuation ; ophiolite ; oceanic crust ; velocity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Compressional wave attenuations and velocities have been measured as a function of confining pressure in ophiolite samples representing a cross-section of the oceanic crust and uppermost mantle. Data are presented for basalts, diabase dikes, gabbros and a suite of serpentinites and peridotites showing a range of serpentization. An ultrasonic pulse-echo spectral ratio technique was used to determine the attenuations to confining pressures of 500 MPa. From this data a Q profile for the oceanic crust and upper mantle is presented. Q is found to moderately increase with depth through the pillow basalts of the upper oceanic crust. The sheeted dike rocks of Layer 2C show an increase in Q with depth due to progressive metamorphism (from greenschist to amphibolite facies). Q drops abruptly from Layer 2C to Layer 3, though it is not clear why the gabbros have such low Q's. The crust-mantle boundary is a Q discontinuity; however, the Q contrast between Layer 3 and the upper mantle could be altered by upper mantle serpentinization, interlayered gabbros and peridotites at the boundary, or serpentinized peridotite diapirs intruding the gabbroic section. Q varies significantly with the percentage of serpentinization in the ultramafic samples, with the largest changes in Q being at the extremes of zero and full serpentinization. Q is sensitive to the overburden pressure for all of the samples.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00369151
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  • 7
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    The Northern Terminus of Cascadia Subduction (2020)
    American Geophysical Union
    Details
    Publication Date: 2020-06-01
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 8
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    Q structure of the oceanic crust (1991)
    Springer
    Details
    Publication Date: 1991-08-01
    Print ISSN: 0025-3235
    Electronic ISSN: 1573-0581
    Topics: Geosciences , Physics
    Published by Springer
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  • 9
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    A Double Difference Tomography Study of the Washington Forearc: Does Siletzia Control Crustal Seismicity? (2020)
    American Geophysical Union
    Details
    Publication Date: 2020-10-01
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 10
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    The Iceland Research Drilling Project (1982)
    American Geophysical Union
    Details
    Publication Date: 1982-08-10
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
    Published by American Geophysical Union
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