Research ArticleEARTH SCIENCES

Shallow slow slip events along the Nankai Trough detected by GNSS-A

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Science Advances  15 Jan 2020:
Vol. 6, no. 3, eaay5786
DOI: 10.1126/sciadv.aay5786
  • Fig. 1 Results of the SSE signal detection process off the Bungo Channel.

    (A) Seafloor crustal deformations detected in the SSE signal detection process. Vectors indicate seafloor crustal deformations detected in the GNSS-A data. Closed and open squares indicate the seafloor observation sites installed before and after 2011, respectively. Blue regions indicate deep SSEs detected by the onshore GEONET (24, 25). The yellow star indicates the epicenter of the 2016 Kumamoto earthquake. Blue dots indicate shallow VLF activities after 2013 (11). (B) Time series of seafloor crustal deformations at the sites in the green region. The maximum likelihood straight and piece-wise lines at the sites where SSE signals were not detected and were detected, respectively, are displayed. Each time series was plotted in the direction for the case of the maximum likelihood solution in the SSE signal detection process. Red lines indicate straight and piece-wise lines estimated as the maximum likelihood solutions. Gray histograms indicate Δc-AIC time series (bin range, 1 year) every 0.2 years. Each light blue bin indicates a period judged to be an SSE signal. Bottom red histograms are shallow VLF numbers (11) within green region in (A) every month. Purple dashed lines indicate the 2011 Tohoku-oki earthquake. (C) Time series of onshore crustal deformations near this region. The reference frame is ITRF2005 (28). The blue region and red line indicate deformations due to deep SSE and the 2016 Kumamoto earthquake, respectively.

  • Fig. 2 Results of the SSE signal detection process off Tosa Bay.

    (A) Seafloor crustal deformations detected in the SSE signal detection process. Blue regions indicate deep SSEs detected by the onshore GEONET (24, 25). Blue dots indicate shallow VLF activities after 2013 (11). (B) Time series of seafloor crustal deformations at the sites in the green region. (C) Time series of onshore crustal deformations near this region. Other depictions are the same as in Fig. 1.

  • Fig. 3 Results of the SSE signal detection process off the Kii Channel.

    (A, A′, and A″) Seafloor crustal deformations detected in the SSE signal detection process. Blue regions indicate deep SSEs detected by the onshore GEONET (25). Light blue regions indicate shallow short-term SSEs (14). Blue dots in (A), (A′), and (A″) indicate shallow VLF activities between 2008 and 2009, between 2013 and 2016, and after 2017, respectively (11). Green focal mechanisms indicate reanalyzed VLF events (33). (B) Time series of seafloor crustal deformations at the sites in the green region. (C) Time series of onshore crustal deformations near this region. Other depictions are the same as in Fig. 1.

  • Fig. 4 Results of the SSE signal detection process around Kumano-nada.

    (A) Seafloor crustal deformations detected in the SSE signal detection process. Blue regions indicate deep SSEs detected by the onshore GEONET (25, 37). Light blue regions indicate shallow short-term SSEs (14). The yellow star indicates the epicenter of the 2016 off-Mie earthquake (Mw 5.6). Blue dots indicate shallow VLF activities after 2013 (11). (B) Time series of seafloor crustal deformations at the sites in the green region. (C) Time series of onshore crustal deformations near this region. Other depictions are the same as in Fig. 1.

  • Fig. 5 Results of the SSE signal detection process around Enshu-nada.

    (A) Seafloor crustal deformations detected in the SSE signal detection process. Blue regions indicate deep SSEs detected by the onshore GEONET (37). The light blue region indicates shallow short-term SSE activity (14). Blue dots indicate shallow VLF activities after 2013 (11). (B) Time series of seafloor crustal deformations at the sites in the green region. (C) Time series of onshore crustal deformations near this region. Other depictions are the same as in Fig. 1.

  • Fig. 6 Spatial relationship of seafloor sites that detected SSE signals.

    (A) Spatial relationship between seafloor site that detected SSE signals (red squares) and some phenomena along the Nankai Trough. Red rectangle and vector are the 2017–2018 Kii Channel shallow SSE model and slip angle, respectively, estimated by the grid search when it was assumed to be due to single SSE. Pink circles indicate the shallow VLF catalog after 2006 (11). The light blue polygon indicates the Kumano-nada short-term SSE activity region (14). The gray contour map indicates high coupling rate distribution (rate: more than 0.5) (17). The light blue solid line indicates the most recent seismogenic region (slip, more than 2 m) (6). Blue regions indicate deep SSE regions (24, 25, 37) (total slip, more than 2 cm). Dashed lines indicate the depths of the plate boundary of (52, 53). (B) Shallow VLF time series compared with shallow and deep SSE timings. Black dots indicate shallow VLF activities (11). The red lines indicate the SSE timings detected at seafloor sites. Each yellow line indicates the longitude of the site. Blue regions indicate deep SSE periods (24, 25, 37). The light blue solid line indicates the Kumano-nada short-term SSE activity region (14).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/3/eaay5786/DC1

    Fig. S1. Schematic diagram of the GNSS-A seafloor geodetic observation system.

    Fig. S2. SSE detection capability maps.

    Fig. S3. Flow of the SSE signal detection process.

    Fig. S4. Time series of horizontal components of seafloor GNSS-A data.

    Fig. S5. Results of the SSE signal detection process for all time series.

    Fig. S6. Pseudo analysis test results for the SSE detection process.

    Fig. S7. Grid search results for estimating an SSE model off the Kii Channel around 2017–2018 and potential slip regions in other cases when deformations of onshore GNSS sites are 0 cm (<~5 mm).

    Data file S1. Time series of seafloor positions.

  • Supplementary Materials

    The PDFset includes:

    • Fig. S1. Schematic diagram of the GNSS-A seafloor geodetic observation system.
    • Fig. S2. SSE detection capability maps.
    • Fig. S3. Flow of the SSE signal detection process.
    • Fig. S4. Time series of horizontal components of seafloor GNSS-A data.
    • Fig. S5. Results of the SSE signal detection process for all time series.
    • Fig. S6. Pseudo analysis test results for the SSE detection process.
    • Fig. S7. Grid search results for estimating an SSE model off the Kii Channel around 2017–2018 and potential slip regions in other cases when deformations of onshore GNSS sites are 0 cm (<~5 mm).
    • Legend for data file S1

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    Other Supplementary Material for this manuscript includes the following:

    • Data file S1 (Microsoft Excel format). Time series of seafloor positions.

    Files in this Data Supplement:

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