In Japan, interplate megathrust earthquakes frequently occur in subduction zones where oceanic plates subduct beneath continental plates, and it is important to elucidate the physical mechanism involved in such earthquakes to prevent associated disasters. Crustal movement data provide essential information to understand plate motion and earthquake source processes. We developed a system that combines GNSS measurements and acoustic ranging techniques to detect seafloor crustal movement. This paper reports the acquisition of recent seafloor crustal movements obtained during campaign observations with a survey vessel, from 2013 to 2016.

1. [1] A. Asada and T. Yabuki, “Centimeter-level positioning on the seafloor,” Proc Jpn. Acad., Vol.77, No.1, pp. 7-12, 2001.
2. [2] F. N. Spiess, “Suboceanic geodetic measurements,” IEEE Trans. Geosci. Remote Sens., GE-23, pp. 502-510, 1985.
3. [3] M. Fujita et al., “GPS/acoustic seafloor geodetic observation: method of data analysis and its application,” Earth Planets Space, Vol.58, pp. 265-275, doi:10.1186/BF03351923, 2006.
4. [4] O. L. Colombo, “Long-distance kinematic GPS,” GPS for Geodesy, 2nd ed., pp. 537-567, Springer, 1998.
5. [5] Japan Coast Guard web site: Datalist of GPS-A, http://www1.kaiho.mlit.go.jp/KOHO/chikaku/kaitei/sgs/datalist_e.html [accessed: May 14, 2018]
6. [6] K. Wang, Y. Hu, and J. He, “Deformation cycles of subduction earthquake in a viscoelastic Earth,” Nature, Vol.484, pp. 327-332, doi:10.1038/nature11032, 2012.
7. [7] S. Watanabe et al., “Evidence of viscoelastic deformation following the 2011 Tohoku-oki earthquake revealed from seafloor geodetic observation,” Geophys. Res. Lett., Vol.41, pp. 5789-5796, doi:10.1002/2014GL061134, 2014.
8. [8] T. Sun et al., “Prevalence of viscoelastic relaxation after the 2011 Tohoku-oki earthquake,” Nature, Vol.514, pp. 84-87, doi:10.1038/nature13778, 2014.
9. [9] T. Sun and K. Wang, “Viscoelastic relaxation following subduction earthquakes and its effects on afterslip determination,” J. Geophys. Res., Vol.120, pp. 1329-1344, doi:10.1002/2014JB011707, 2015.
10. [10] H. Suito, “Importance of rheological heterogeneity for interpreting viscoelastic relaxation caused by the 2011 Tohoku-Oki earthquake,” Earth, Planets and Space, Vol.69, No.21, doi:10.1186/s40623-017-0611-9, 2017.
11. [11] S. Yamagiwa et al., “Afterslip and viscoelas-tic relaxation following the 2011 Tohoku-oki earthquake (Mw9.0) inferred from inland GPS and seafloor GPS/Acoustic data,” Geophys. Res. Lett, Vol.42, pp. 66-73, doi:10.1002/2014GL061735, 2015.
12. [12] A. M. Freed et al., “Resolving depth-dependent subduction zone viscosity and afterslip from postseismic displacements following the 2011 Tohoku-oki, Japan earthquake,” Earth and Planetary Science Letters, 459, 279-290, doi:10.1016/j.epsl.2016.11.040, 2017.
13. [13] Y. Yokota et al., “Seafloor geodetic constraints on interplate coupling of the Nankai Trough megathrust zone,” Nature, Vol.534, pp. 374-388, doi:10.1038/nature17632, 2016.
14. [14] T. Hori, “Earthquake and tsunami scenarios as basic information to prepare next nankai megathrust earthquakes,” J. Disaster Res., Vol.12, No.4, pp. 775-781, doi:10.20965/jdr.2017.p0775, 2017.
15. [15] K. Obara, “Contribution of slow earthquake study for assessing the occurrence potential of megathrust earthquakes,” J. Disaster Res., Vol.9, No.33, pp. 317-329, doi:10.20965/jdr.2014.p0317, 2014.
16. [16] K. Obara and A. Kato, “Connecting slow earthquakes to huge earthquakes,” Science, Vol.353, pp. 253-257, doi:10.1126/science.aaf1512, 2016.
17. [17] N. Takahashi et al., “Real-time tsunami prediction system using DONET,” J. Disaster Res., Vol.12, No.4, pp. 766-774, doi:10.20965/jdr.2017.p0766, 2017.
18. [18] E .Araki et al., “Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust,” Science, Vol.356, pp. 1157-1160, doi:10.1126/science.aan3120, 2017.