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Seafloor geodetic constraints on interplate coupling of the Nankai Trough megathrust zone

Nature volume 534, pages 374–377 (2016)Cite this article

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Abstract

Interplate megathrust earthquakes have inflicted catastrophic damage on human society. Such an earthquake is predicted to occur in the near future along the Nankai Trough off southwestern Japan—an economically active and densely populated area in which megathrust earthquakes have already occurred1,2,3,4,5. Megathrust earthquakes are the result of a plate-subduction mechanism and occur at slip-deficit regions (also known as ‘coupling’ regions)6,7, where friction prevents plates from slipping against each other and the accumulated energy is eventually released forcefully. Many studies have attempted to capture distributions of slip-deficit rates (SDRs) in order to predict earthquakes8,9,10. However, these studies could not obtain a complete view of the earthquake source region, because they had no seafloor geodetic data. The Hydrographic and Oceanographic Department of the Japan Coast Guard (JHOD) has been developing a precise and sustainable seafloor geodetic observation network11 in this subduction zone to obtain information related to offshore SDRs. Here, we present seafloor geodetic observation data and an offshore interplate SDR-distribution model. Our data suggest that most offshore regions in this subduction zone have positive SDRs. Specifically, our observations indicate previously unknown regions of high SDR that will be important for tsunami disaster mitigation, and regions of low SDR that are consistent with distributions of shallow slow earthquakes and subducting seamounts. This is the first direct evidence that coupling conditions might be related to these seismological and geological phenomena. Our findings provide information for inferring megathrust earthquake scenarios and interpreting research on the Nankai Trough subduction zone.

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Figure 1: Seafloor velocity field, based on seafloor geodetic observations at 15 seafloor sites along the Nankai Trough.
Figure 2: Interplate SDR distribution as indicated by onshore and seafloor geodetic data.

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Acknowledgements

We thank R. Burgmann and J.-P. Avouac for reviews of, and comments on, the manuscript. We thank O.L. Colombo of the NASA Goddard Space Flight Center for the kinematic GPS software IT (Interferometric Translocation)30, and the Geospatial Information Authority of Japan (GSI) for high-rate GPS data for kinematic GPS analysis, and for daily coordinates of the sites on the GSI website. T. Iinuma and T. Sun calculated the coseismic and postseismic deformations, respectively, of the 2011 Tohoku-oki earthquake. Comments from K. Wang and A. Kato improved our manuscript. We thank T. Yabuki of the JHOD for the Bayesian inversion software. Additionally, many among the staff of the JHOD—including the crew of the vessels Shoyo, Takuyo, Meiyo and Kaiyo—have supported our observations and data processing. Some figures were produced using GMT software, developed by P. Wessel and W. H. F. Smith.

Author information

Authors and Affiliations

  1. Hydrographic and Oceanographic Department, Japan Coast Guard, 3-1-1, Kasumigaseki, Chiyoda-ku, 100-8932, Tokyo, Japan

    Yusuke Yokota, Tadashi Ishikawa, Shun-ichi Watanabe & Toshiharu Tashiro

  2. Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, 153-8505, Tokyo, Japan

    Akira Asada

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  1. Yusuke Yokota
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  4. Toshiharu Tashiro
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Contributions

Y.Y. carried out the inversion analysis. T.I. designed the study and performed the statistical processing. Y.Y. and S.W. performed the resolution tests. Y.Y., T.I., S.W. and T.T. processed the GPS-A seafloor geodetic data. A.A. constructed the GPS-A system.

Corresponding author

Correspondence to Yusuke Yokota.

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Extended data figures and tables

Extended Data Figure 1 Our GPS-A seafloor geodetic observation system.

The system, comprising on-board and seafloor sensors, was developed as described in ref. 11 and improved as described in ref. 35. This diagram was modified from refs 15, 34, 37. CTD, conductivity temperature depth profiler; XCTD, expendable conductivity temperature depth profiler; XBT, expendable bathythermographs.

Extended Data Figure 2 Horizontal movements of seafloor sites over time.

Time series for east–west (left column) and north–south (right column) displacements of seafloor sites TOK1, TOK2, TOK3, KUM1, KUM2 and KUM3 from 2006 onwards. The position reference is the Amur plate. Blue circles indicate raw observations before deductions of the coseismic and postseismic effects that resulted from the magnitude-9.0 Tohoku-oki earthquake of 11 March 2011. Red circles indicate the corrected final results. The linear trends and the 95% two-sided confidence intervals are shown with red solid and dashed lines, respectively. Grey lines show the calculated coseismic and postseismic deformations following the Tohoku-oki earthquake18,19.

Extended Data Figure 3 Horizontal movements of seafloor sites over time.

Time series showing east–west (left column) and north–south (right column) displacements of seafloor sites SIOW, MRT1, MRT2, TOS1, TOS2 and ASZ1. Colours are as in Extended Data Fig. 2.

Extended Data Figure 4 Horizontal movements of seafloor sites over time.

Time series showing east–west (left column) and north–south (right column) displacements of seafloor sites ASZ2, HYG1 and HYG2. Colours are as in Extended Data Fig. 2.

Extended Data Figure 5 Comparison of calculated SDR distributions obtained using onshore data only versus onshore plus seafloor data.

a, b, SDR distributions (for SDRs of more than 2 cm year−1) calculated using only onshore data (a) or using onshore and seafloor data (b). Black and white vectors indicate the observed data and the calculated velocities, respectively. Grey shading indicates areas with resolution values lower than 0.05 (calculated in Extended Data Fig. 6d, e). α is the hyperparameter of smoothness.

Extended Data Figure 6 Checkerboard resolution tests and distributions of resolution values.

a–c, Results of checkerboard resolution tests for the SDR inversions. a, Input checkerboard-like SDR distribution. b, c, The checkerboard distributions calculated from using onshore data only (b) or using both onshore and seafloor data (c). d, e, Resolution values as diagonal elements of the resolution matrix, calculated using onshore data only (d) or using onshore and seafloor data (e). Black and blue dots denote the onshore and seafloor sites, respectively, used in each calculation. α is the hyperparameter of smoothness.

Extended Data Figure 7 Examination of boundary condition effects.

a–c, Estimated SDR distributions (for SDRs of more than 2 cm year−1) calculated with a, the free condition; b, zero backslip at all boundaries; and c, a 6.5 cm year−1 constraint at the south edge (the other boundaries being in the free condition). Grey shading indicates areas with resolution values lower than 0.05, calculated in Extended Data Fig. 6e. α is the hyperparameter of smoothness.

Extended Data Figure 8 Events determined using analysis of automatic VLFE detection24.

Purple and blue dots denote events in the periods 1 June 2003 to 10 May 2015, and 1 August 2008 to 10 May 2015, respectively. Grey stars indicate epicentres of the 2004 earthquakes off the Kii Peninsula. Pale green lines indicate the depths of the plate boundary determined using the CAMP model42.

Supplementary information

Supplementary Table 1

This file contains estimated relative site positions. The reference frame is ITRF2005. Eastward and Northward components indicate the data corrected the coseismic and postseismic effects due to the Tohoku-oki earthquake. Eastraw and Northraw components are raw data after March 2011. (XLSX 18 kb)

Supplementary Table 2

This table shows the position and calculated SDR value of each subfault in the inversion analysis. First and second columns show location (longitude and latitude) of each subfault. Third and fourth columns show angle (from the east to the counterclockwise direction) and absolute SDR value (m/year) calculated for each subfault in the inversion. (XLSX 19 kb)

Supplementary Table 3

This table contains observed and calculated vectors of each site in the inversion analysis. First and second columns show location (longitude and latitude) of each site. Third and fourth columns show angle (from the east to the counterclockwise direction) and absolute velocity value (m/year) observed in each site. Fifth and sixth columns show angle and absolute velocity value calculated for each site in the inversion. (XLSX 21 kb)

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Yokota, Y., Ishikawa, T., Watanabe, Si. et al. Seafloor geodetic constraints on interplate coupling of the Nankai Trough megathrust zone. Nature 534, 374–377 (2016). https://doi.org/10.1038/nature17632

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