Research ArticleGEOPHYSICS

Slab temperature controls on the Tonga double seismic zone and slab mantle dehydration

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Science Advances  11 Jan 2017:
Vol. 3, no. 1, e1601755
DOI: 10.1126/sciadv.1601755
  • Fig. 1 Map of relocated epicenters (red circles).

    Straight lines indicate the cross sections with a width of 133 km in this study. Earthquake depths range from 50 to 450 km. Dashed rectangle highlights the seismic belt discussed in the main text. Black arrows with numbers show the Global Positioning System site velocities in a Pacific-fixed reference frame (18). Blue triangle represents station FONI in Fig. 4. Bathymetry of a depth of 1 km is contoured to outline the Tonga Ridge, Tofua Arc, Lau Ridge, and Fiji Plateau, and contours of 7, 8, 9, and 10 km are also shown to delineate the Tonga Trench. Inset displays the study region in a global map.

  • Fig. 2 Cross sections of hypocenters superposed on thermal models.

    (A to E) Hypocenters with thermal models for each cross section. The temperature contour interval is 200°C. The gray curve shows the slab surface fit to the seismicity, and the red triangle on the top represents the volcanic arc. Earthquakes are sorted into the categories of downdip compression (yellow dots), downdip tension (magenta dots), and null (black dots) on the basis of the directions of the principal axes shown in fig. S5. Small black dots illustrate the events without a CMT solution. Superposed on the left is the slab curvature along each cross section. (F) Enlarged view of cross section C-C′ to show the seismic belt. Yellow and black dots are the actual hypocenters. The solid and dashed gray curves illustrate the slab surface and Moho, respectively.

  • Fig. 3 Histograms of earthquake depths below the slab surface in the slab-normal direction.

    (A) Earthquakes at vertical depths of 100 to 300 km along each cross section. Downdip compressional and tensional events are shown in yellow and magenta, respectively. In the south, the seismogenic zone becomes a single layer of downdip compressional earthquakes along cross section E-E′. (B) All earthquakes along cross sections B-B′, C-C′, and D-D′ sorted into five subsets according to their vertical depths. The DSZ lower plane disappears below the vertical depth of ~300 km.

  • Fig. 4 Waveform similarity analysis along cross section C-C′.

    (A) Locations for events recorded by both global stations and the 2009–2010 local deployment. Two of the events have CMT solutions, with the upper plane event showing downdip compression and the lower plane event showing downdip tension. Gray curve shows the slab surface fit by the seismicity, and the red triangle on the top represents the volcanic arc. We compare waveforms of these events recorded at station FONI in Tonga (black triangle), because the raypaths are roughly parallel with the slab. Events right beneath FONI [gray dots in (A)] are excluded to avoid near-vertical raypaths. Blue, red, and black dots indicate three groups of earthquakes sorted in (B). (B) Seismograms recorded at FONI filtered at 1 to 5 Hz. The bold blue waveform shows the seismogram of the downdip compressional earthquake near the slab surface, whereas the seismogram of the downdip tensional event is shown with a bold red curve. These two waveforms are shifted so that the peak amplitudes are aligned to 0 s. Seismograms of other events are aligned on the basis of waveform cross-correlation. All events are sorted into three groups according to their cross-correlation coefficients: blue events similar to the downdip compressional CMT event, red events similar to the downdip tensional CMT event, and black events that show ambiguous similarities. (C) Histogram of all earthquake depths in the slab-normal direction (black). Blue and red histograms correspond to the blue and red events sorted in (B), respectively.

  • Fig. 5 Depth of the maximum downward extent of the DSZ versus thermal parameter for different subduction zones.

    The thermal parameter is defined as the product of plate age, convergence velocity, and the sine of the slab dip angle, and it is proportional to the depth at which a given isotherm would be subducted (11). Thermal parameters are given by Syracuse et al. (48), except for Mariana, which shows rapid lateral variations in thermal parameter along strike. In this case, we calculated the thermal parameter for 18°N, where the DSZ is observed (9). The maximum depth of the Tonga DSZ is quantitatively estimated on the basis of histograms in Fig. 3B. The maximum DSZ depths in other subduction zones are visually estimated on the basis of previous studies for eastern Aleutians (4), New Britain (5), Ryukyu trench (6), northern Chile (7), central Marianas (9), and northeastern Japan (8).

  • Fig. 6 Pressure-temperature conditions of earthquakes in Fig. 2.

    (A to E) Modeled pressures and temperatures (P-T) of earthquakes along each cross section. Black dots indicate the earthquake P-T conditions with thermal uncertainties (gray error bars). The downdip tensional events are shown in magenta. The P-T paths of the slab surface and the Moho are illustrated by the red and blue curves, respectively. Bold gray curves show the P-T conditions of the dehydration reactions (1) antigorite = forsterite + enstatite + H2O, (2) antigorite = phase A + enstatite + H2O, and (3) phase A = forsterite + H2O from Schmidt and Poli (32). (F) Modeled P-T paths for the seismic belt (ellipses) and the Moho (color curves) along all cross sections. Open ellipses represent the P-T conditions of the seismic belt, whereas solid ellipses indicate the adjacent Moho P-T paths. The ellipses are quantitatively defined in figs. S9 and S10. Gray solid, gray dashed, and black solid curves show phase boundaries from studies by Komabayashi et al. (30), Hilairet et al. (31), and Schmidt and Poli (32), respectively.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/1/e1601755/DC1

    fig. S1. Cross sections of hypocenters.

    fig. S2. Tectonic map and seismic stations.

    fig. S3. Map view of relocated Global CMT solutions.

    fig. S4. Cross sections of earthquake at vertical depths of 50 to 300 km with Global CMT solutions.

    fig. S5. Cross sections of earthquake principal axes based on Global CMT solutions.

    fig. S6. Cross sections of hypocenters from different inversions.

    fig. S7. Histograms of earthquake depths along each cross section.

    fig. S8. 3D view of error ellipses for all events, viewed normal to the trench.

    fig. S9. Histograms of earthquake P-T conditions along each cross section.

    fig. S10. Histograms of P-T conditions of the Moho adjacent to the seismic belt along each cross section.

    table S1. Thermodynamic parameters used in thermal modeling.

    table S2. Slab parameters for each cross section.

    References (54)

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Cross sections of hypocenters.
    • fig. S2. Tectonic map and seismic stations.
    • fig. S3. Map view of relocated Global CMT solutions.
    • fig. S4. Cross sections of earthquake at vertical depths of 50 to 300 km with Global CMT solutions.
    • fig. S5. Cross sections of earthquake principal axes based on Global CMT solutions.
    • fig. S6. Cross sections of hypocenters from different inversions.
    • fig. S7. Histograms of earthquake depths along each cross section.
    • fig. S8. 3D view of error ellipses for all events, viewed normal to the trench.
    • fig. S9. Histograms of earthquake P-T conditions along each cross section.
    • fig. S10. Histograms of P-T conditions of the Moho adjacent to the seismic belt along each cross section.
    • table S1. Thermodynamic parameters used in thermal modeling.
    • table S2. Slab parameters for each cross section.
    • Reference (54)

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