Research ArticleGEOPHYSICS

Earthquakes track subduction fluids from slab source to mantle wedge sink

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Science Advances  03 Apr 2019:
Vol. 5, no. 4, eaav7369
DOI: 10.1126/sciadv.aav7369
  • Fig. 1 Map of the Peloponnese region of the Hellenic subduction zone.

    The displayed area is indicated in the plate boundary map in the inset. The map shows the seismicity from 2006 to 2017 investigated in this study (colored circles), arc volcanoes (red triangles), and the depth contours of the top of the Ionian slab [black lines: inferred interface from (16) with small adjustments described in the Supplementary Materials], which subducts beneath the region from southwest to northeast. Seismograph stations (inverted triangles) with more than 100 observations are displayed (filled black: waveforms used in analysis; gray outline: only picks used). Beach balls depict the focal mechanisms of three major historic earthquakes that are believed to have occurred on the plate interface at depths that are unusually large for interplate earthquakes. Vertical profiles of seismicity across sections A-A′ to E-E′ are shown in Fig. 2.

  • Fig. 2 Distribution of slab earthquakes and seismic velocity structure beneath western Greece.

    (A) Number of mantle wedge earthquakes (located above the plate interface) and forearc slab earthquakes (located updip of line A-A′ in Fig. 1) along the subduction strike, calculated with a 5-km-wide moving window. The curves are anticorrelated, indicating that clusters of mantle wedge seismicity are associated with updip segments of reduced intraslab seismicity. (B to E) Body wave tomographic images from (16) along four cross sections (see Fig. 1 for locations) showing the low-velocity subducting crust and relocated seismicity with absolute or relative location errors of <5 km. Contour lines indicate areas with Vp/Vs ratio of >1.8 and <1.7 (see fig. S2 for full Vp/Vs ratio structure). High values indicate the presence of free fluid or melt, particularly in the subarc mantle, while low values point to quartz enrichment in the lower overriding crust (18).

  • Fig. 3 Seismic and thermal attributes of the Tripoli cluster of mantle wedge earthquakes.

    (A) Cross section (same location as Fig. 2D) of all earthquakes with location errors of <2.5 km superimposed on a teleseismic scattered wave image (13, 16), with temperature contours from the geodynamic model and main dehydration reactions outlined in color (see legend). The background color scale represents scattering potential in terms of relative S-wave speed (Vs) perturbations. The image resolves two main features: the hydrated subducting oceanic crust denoted by a low-velocity (red) dipping layer and the Moho of the overriding plate denoted by a downward slow-to-fast velocity (red-to-blue) contrast at a depth of ~25 km. (B) Water content at the top and bottom of a fully hydrated subducted crust calculated from thermal-petrologic models. No major dehydration reaction occurs before the crust reaches depths of >80 km. (C) Map view of the Tripoli cluster grouped by earthquake category and associated focal mechanisms (see fig. S3 for individual mechanisms). (D) Magnified cross section centered on the Tripoli cluster [see areas indicated D-D′ in (A) and (C)] showing earthquakes with relative location errors of <0.25 km.

  • Fig. 4 Clusters of mantle wedge seismicity.

    The map indicates the worldwide distribution of subduction zones and their calculated hydration state after 50 Ma of slab dehydration [data from (1) and (2)]. Observations of mantle wedge seismicity are shown in cross sections A to F. (A) Seismicity for the Tripoli cluster studied in this paper. (B) Seismicity for east Crete. (C) Seismicity for offshore Sanriku. (D) Seismicity for Raukumara. (E) Seismicity for Martinique in the Lesser Antilles. The exact location of cross sections and the data sources are listed in table S3 (description in Supplementary Text). A similar observation not shown here has been made onshore Chiba [east Japan, indicated as F on the map (10)]. The updip gap in seismicity is shaded in red. Where distinguishable, the vent marked by a cluster of seismicity on the interface (IF) is indicated, along with the cluster of mantle wedge (MW) seismicity.

  • Fig. 5 Schematic showing fluid migration paths between the sources and sinks of water in the Hellenic subduction zone.

    The fluids originate from dehydration reactions in the slab and flow either toward the melting zone in the mantle or updip below a sealed plate interface. Fluids that flow updip trigger earthquakes (EQ; black circles) as they migrate along the slab toward the overriding crust (where they precipitate into quartz) or if they escape via localized vents into the cold mantle wedge corner (where they cause unusual mantle wedge earthquakes). Reduced fluid flow in the subducting crust updip of these vents leads to reduced seismicity. Other subduction zones with cold interfaces and dry mantle wedges (see Fig. 4) exhibit similar fluid migration patterns.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/4/eaav7369/DC1

    Supplementary Text

    Mantle wedge seismicity in other subduction zones

    Episodicity of mantle wedge earthquakes and velocity of fluid migration

    Fig. S1. Along-trench profiles of mantle wedge seismicity and P-velocity structure, plotted as seen from the trench.

    Fig. S2. Temporal evolution of seismicity in the Tripoli cluster.

    Fig. S3. P-velocity to S-velocity (Vp/Vs) ratio structure beneath western Greece.

    Fig. S4. Focal mechanisms of the Tripoli cluster.

    Fig. S5. Estimates of completeness magnitude, Mc, for various catalogs of deep earthquakes (>35 km) below western Greece.

    Fig. S6. Comparison of seismic images and calculated seismic structure along the cross section of Fig. 1D.

    Fig. S7. Hypocenters displayed with their relative location errors.

    Fig. S8. Earthquake distribution and electric resistivity structure below the Peloponnese.

    Table S1. Seismograph networks from western Greece used in the waveform processing.

    Table S2. Focal mechanism solutions of deep earthquakes in the Western Hellenic subduction zone.

    Table S3. Locations of mantle wedge seismicity displayed in cross sections in Fig. 4.

    External Data file S1. Deep earthquake hypocenters in Greece.

    External Data file S2. Deep earthquake focal mechanisms in Greece.

    External Data file S3. Earthquake arrival time picks.

    External Data file S4. Model of the subduction plate interface.

    External Data file S5. Thermal structure model of the subduction zone.

    References (5981)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Mantle wedge seismicity in other subduction zones
    • Episodicity of mantle wedge earthquakes and velocity of fluid migration
    • Fig. S1. Along-trench profiles of mantle wedge seismicity and P-velocity structure, plotted as seen from the trench.
    • Fig. S2. Temporal evolution of seismicity in the Tripoli cluster.
    • Fig. S3. P-velocity to S-velocity (Vp/Vs) ratio structure beneath western Greece.
    • Fig. S4. Focal mechanisms of the Tripoli cluster.
    • Fig. S5. Estimates of completeness magnitude, Mc, for various catalogs of deep earthquakes (>35 km) below western Greece.
    • Fig. S6. Comparison of seismic images and calculated seismic structure along the cross section of Fig. 1D.
    • Fig. S7. Hypocenters displayed with their relative location errors.
    • Fig. S8. Earthquake distribution and electric resistivity structure below the Peloponnese.
    • Table S1. Seismograph networks from western Greece used in the waveform processing.
    • Table S2. Focal mechanism solutions of deep earthquakes in the Western Hellenic subduction zone.
    • Table S3. Locations of mantle wedge seismicity displayed in cross sections in Fig. 4.
    • External Data file S1. Deep earthquake hypocenters in Greece.
    • External Data file S2. Deep earthquake focal mechanisms in Greece.
    • External Data file S3. Earthquake arrival time picks.
    • External Data file S4. Model of the subduction plate interface.
    • External Data file S5. Thermal structure model of the subduction zone.
    • References (5981)

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