Research ArticleGEOPHYSICS

Deep slab seismicity limited by rate of deformation in the transition zone

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Science Advances  27 May 2020:
Vol. 6, no. 22, eaaz7692
DOI: 10.1126/sciadv.aaz7692
  • Fig. 1 Subduction zones exhibit strong variability of seismicity and strain rate in depth.

    (A) Global seismicity in subduction zones for earthquakes greater than 100-km depth for Mw > 4 (blue) and Mw > 5 (gray) for the period 1964–2014 from the ISC-EHB catalog (52). (B to I) Regional seismicity rate and strain rate versus depth. Tonga (B), Kermadec (C), and Java-Sumatra (D) all have regional seismicity depth profiles that mimic the global profile in (A). The Kuriles (F), Japan (G), and Marianas (H) do not exhibit the characteristic increase in seismicity rate within the transition zone (400 to 660 km). Chile (E) and Peru (I) have a distinct lack of seismicity from 300 to 500 km but do have earthquakes in the transition zone from 500 to 660 km.

  • Fig. 2 Seismicity and strain rate exhibit continuous changes in peak depth and width along adjacent profiles.

    Examples from the Tonga (1 to 5) and Kermadec (7 to 11) slabs. From profiles 1 through 5, there is a continuous change in the depth and width of the transition zone peak and the emergence of a narrow strain rate peak at 400-km depth. From profiles 11 to 7, the transition zone peak disappears, being replaced by two smaller peaks, and then a shift to a broad peak centered at 400-km depth. Locations of profiles are shown in fig. S2 (A and B).

  • Fig. 3 In simulations of subduction, phase transitions cause folding and buckling of the slab, leading to localized peaks of high strain rate that change in time.

    Cross sections of subducted slab with strain rate (color) within the 1000°C contour. Red bars indicate the pattern of shortening (compression) directions. (A) Model 1 with no phase transitions shows little strain rate variation in the slab. (B) Model 2 with phase transitions has strong variations in strain rate both spatially and temporally as the slab changes shape. (B, e to f) Depth profiles of the maximum strain rate occurring below a specified temperature in the slab interior for model 2 at the times shown in (B, a to d). Colder temperature limits show the strain rate at the interior of the slab. These strain rate depth profiles are similar to the observed strain rate profiles calculated from slab seismicity. (C) Model 3, a younger slab (40 Ma), is a warmer and therefore weaker slab and deforms at higher strain rates but shows similar folding and buckling behavior to model 2.

  • Fig. 4 Time evolution of the maximum strain rate in the slab for model 2 demonstrates spatial and temporal variability.

    The color is the maximum strain rate occurring in the slab at temperatures less than 900°C as a function of depth and simulation time. The depth and width of strain rate peaks migrate in time following bending regions and folds. There is also a peak centered at 600-km depth that occurs at three times, independent of a major fold in the slab (see Fig. 5).

  • Fig. 5 Strain rate peak below 600-km depth is associated with garnet-to-bridgmanite transition in the harzburgite layer.

    (A) Zoom-in on the strain-rate in the slab for model 2 at 13.9 Ma (Fig. 3E) showing the location of the phase transitions (gray), and temperature contours (black). (B) shows the profiles of the strain rate peak at 1000°C (black) and 900°C (blue). This strain rate peak occurs for periods of 5 to 10 Ma (see Fig. 4) in the absence of appreciable folding or bending of the slab. gt, garnet; il, ilmenite; rw, Ringwoodite; fp, ferropericlase; brg, bridgmanite.

  • Fig. 6 Comparisons of model snapshots and observations show similarities between slab shape and strain rate distribution.

    Comparison for (A) the Chilean slab, (B) the Marianas slab, and (C) the Bonin slab. (a) Snapshots from model 2 at times indicated. Colors and contours are the same as in Fig. 3. (b) Maximum strain rate profiles. Line colors are the same as in Fig. 3. (c) Earthquake histogram and strain rate for profile 7 in Chile, profile 7 in the Marianas, and profile 9 in Bonin. (d) Cross section (depth versus distance) of earthquakes for the same profile as the histograms. Colors indicate depth and are the same as used in fig. S2.

Supplementary Materials

  • Supplementary Materials

    Deep slab seismicity limited by rate of deformation in the transition zone

    Magali I. Billen

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    • Sections S1 to S4
    • Figs. S1 to S3
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