Research ArticleSeismology

High seismic attenuation at a mid-ocean ridge reveals the distribution of deep melt

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Science Advances  24 May 2017:
Vol. 3, no. 5, e1602829
DOI: 10.1126/sciadv.1602829
  • Fig. 1 Example of S waves recorded at JdF OBS stations from an event on 3 April 2014 at a distance of ~84°, from a back azimuth of ~129°.

    T-component displacement seismograms are aligned by cross-correlation, colored by age, arranged by distance to ridge, and plotted with a 0.05- to 2-Hz fourth-order Butterworth filter. Dashed lines, data window for calculation of spectra. DF, deformation front.

  • Fig. 2 S-wave Δt* (left) and δT (right) recorded at OBS stations.

    Radial spokes show individual arrivals at their incoming azimuth, whereas central circles show least-squares station average terms. Open circles show land stations used to link JdF and Gorda arrays. Boxes show three areas: north JdF (blue), south JdF (red), and Gorda (yellow).

  • Fig. 3 Station-averaged S-wave Δt* and δT as a function of crustal age, relative to the mean value for 4- to 8-My seafloor.

    A representative west-northwest–east-southeast bathymetric profile at 46.8°N (top) includes deformation front and isochrons reflecting an irregular age-distance relationship. Colors relate to geographic area (Fig. 2) and point size scales with the number of individual observations contributing to the average. Superimposed lines show predictions of differential attenuation due to the effect of temperature alone, assuming half-space cooling and using laboratory-derived anelastic scaling relationships (see the Supplementary Materials). 2σ uncertainties for Δt* are shown where they exceed the symbol size. δT uncertainties are ~0.3 s.

  • Fig. 4 Schematic of MOR showing several contributions to seismic structure, including temperature (green lines are 200°C isotherms except where stated), strain rate [solid flow streamlines in brown, modified after the work of Braun et al. (41)], and melt regimes.

    Profiles of QS and VS along rays incident at stations on 0-My (solid line) and 10-My (dashed line) crust are shown on the right. Melt fraction in the carbonated melt region is 0.01%, melt fraction in the hydrous melt region is 0.01 to 0.2%, and melt fraction in the dry melt region is 0.2 to 2%. See Materials and Methods for details.

  • Table 1 Predicted maximum differential attenuation and travel time for custom MOR models using the method of Faul and Jackson (30).

    Columns are as follows: (1) model conditions; (2) viscosity reduction between 0- and 10-My mantle, averaged between depths of 50 and 100 km; (3) maximum differential Embedded Image between the ridge axis and the farthest flank; (4) same as (3), but for P waves; (5) maximum differential S wave travel time between the ridge axis and the farthest flank; (6) same as (5), but for P waves.

    ModelEmbedded ImageEmbedded ImageEmbedded ImageδTSδTP
    Observations~1.7~0.4~2.0~0.7
    Temperature only1.10.210.050.690.36
    φ = 2% box (120 km)261.21.670.423.111.10
    ① Temp. + Embedded Image3.60.340.090.800.39
    ② + H2O4.60.420.110.880.41
    ③ + dry melt41.91.020.261.760.66
    ④ + hydrous melt135.21.460.372.260.78
    ⑤ + carbonated melt186.41.900.482.710.89

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/5/e1602829/DC1

    fig. S1. Example of a station-pair differential attenuation measurement using amplitude and phase spectra.

    fig. S2. Example of an attenuated S wave recorded across the OBS array, with amplitude and phase spectra computed as a function of age.

    fig. S3. S-wave attenuation measurements for the event shown in fig. S2.

    fig. S4. S-wave ray paths and attenuation for a southern JdF transect.

    fig. S5. Best-fitting attenuation frequency exponent (α).

    fig. S6. S-wave attenuation for across the array, including results on land.

    fig. S7. Differential attenuation and travel time for P waves as a function of age.

    fig. S8. Trends of Formula versus Formula and δTS versus δTP.

    fig. S9. Example of absolute S-wave amplitudes to test long-period focusing.

    table S1. Forward models for differential attenuation and travel time at an MOR using anelastic scaling relationships from Jackson and Faul (16).

    References (6269)

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Example of a station-pair differential attenuation measurement using amplitude and phase spectra.
    • fig. S2. Example of an attenuated S wave recorded across the OBS array, with amplitude and phase spectra computed as a function of age.
    • fig. S3. S-wave attenuation measurements for the event shown in fig. S2.
    • fig. S4. S-wave ray paths and attenuation for a southern JdF transect.
    • fig. S5. Best-fitting attenuation frequency exponent (α).
    • fig. S6. S-wave attenuation for across the array, including results on land.
    • fig. S7. Differential attenuation and travel time for P waves as a function of age.
    • fig. S8. Trends of Δt*S versus Δt*p and δTs versus δTp .
    • fig. S9. Example of absolute S-wave amplitudes to test long-period focusing.
    • table S1. Forward models for differential attenuation and travel time at an MOR using anelastic scaling relationships from Jackson and Faul (16).
    • References (62–69)

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