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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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.
Extended data figures
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.
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.
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.
About this article
Nature Geoscience (2018)