Postseismic deformation following the 2011 Tohoku-Oki earthquake has been observed by the Global Navigation Satellite System (GNSS) Earth Observation Network System (GEONET) and the Seafloor Geodetic Observation (SGO) over the past six and half years. Observed deformation at onshore sites exceeds 140 cm horizontally, there is uplift of 50 cm, and deformation tends eastward. However, offshore sites reveal complex patterns ranging from near-zero deformation in the northern part of Iwate-Oki, to westward in the southern part of Iwate-Oki, Miyagi-Oki, and the northern part of Fukushima-Oki regions, and eastward in the southern part of Fukushima-Oki and Ibaraki-Oki regions, respectively. The vertical deformation pattern is more complex than the horizontal. Offshore sites demonstrate subsidence but a large uplift is observed onshore along the Pacific coast. Subsidence is only observed along the Pacific coast in the northern part of Iwate, where there are variations in uplift or subsidence patterns. Many previous 2011 Tohoku-Oki event studies have used a primary model that considers only the afterslip effect. However, westward displacements observed by the SGO highlight the importance of viscoelastic relaxation, even during short-term deformation. It is thus considered that studies on postseismic deformation following the 2011 Tohoku-Oki earthquake should adopt a combined afterslip and viscoelastic model. Postseismic deformation following this event is estimated to continue for more than a few decades; therefore, assessing this effect is crucial for interpreting crustal deformation in Japan. Information on the status of interplate coupling or slip is also vital when assessing earthquake occurrence probability. The continued observation of postseismic deformation and careful monitoring of temporal and spatial changes in interplate coupling or slip will mitigate hazards from successive large megathrust earthquakes and improve understanding of crustal activity in Japan.

1. [1] H. Suito et al., “Co- and post- seismic deformation and fault model of the 2011 off the Pacific coast of Tohoku earthquake,” Zisin2, Vol.65, pp. 95-112, 2012 (in Japanese).
2. [2] M. Sato et al., “Displacement above the hypocenter of the 2011 Tohoku-Oki earthquake,” Science, Vol.332, p. 1395, 2011.
3. [3] M. Kido, Y. Osada, H. Fujimoto, R. Hino, and Y. Ito, “Trench-normal variation in observed seafloor displacements associate with the 2011 Tohoku-Oki earthquake,” Geophys. Res. Lett., Vol.35, L24303, 2011.
4. [4] Y. Hu, K. Wang, J. He, J. Klotz, and G. Khazaradze, “Three-dimensional viscoelastic finite element model for postseismic deformation of the great 1960 Chile earthquake,” J. of Geophys. Res., Vol.109, B12403, 2004.
5. [5] H. Suito and J. T. Freymueller, “A viscoelastic and afterslip postseismic deformation model for the 1964 Alaska earthquake,” J. of Geophys. Res., Vol.114, B11404, 2009.
6. [6] R. W. Briggs et al., “Deformation and slip along the Sunda megathrust in the great 2005 Nias-Simeulue earthquake,” Science, Vol.311, pp. 1897-1901, 2006.
7. [7] A. O. Konca et al., “Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence,” Nature, Vol.456, pp. 631-635, 2008.
8. [8] T. Lay et al., “The 2006-2007 Kuril Islands great eqrthquake sequence,” J. of Geophys. Res., Vol.114, B11308, 2009.
9. [9] H. Kanamori, “Mechanism of Tsunami earthquakes,” Phys. of the Earth and Planet. Interiors, Vol.6, pp. 346-359, 1972.
10. [10] M. Murakami, H. Suito, S. Ozawa, and M. Kaidzu, “Earthquake triggering by migrating slow slip initiated by M8 earthquake along Kuril Trench, Japan,” Geophys. Res. Lett., Vol.33, L09306, 2006.
11. [11] 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, 2014.
12. [12] F. Tomita, M. Kido, Y. Ohta, T. Iinuma, and R. Hino, “Along-trench variation in seafloor displacements after the 2011 Tohoku earthquake,” Sci. Adv., Vol.3, No.7, e1700113, 2017.
13. [13] S. Ozawa et al., “Preceding, coseismic, and postseismic slips of the 2011 Tohoku earthquake, Japan,” J. Geophys. Res., Vol.117, B07404, 2012.
14. [14] F. Diao et al., “Overlapping post-seismic deformation processes: afterslip and viscoelastic relaxation following the 2011 Mw9.0 Tohoku (Japan) earthquake,” Geophys. J. Int., Vol.196, pp. 218-219, 2014.
15. [15] S. Han, J. Sauber, and F. Pollitz, “Broadscale postseismic gravity change following the 2011 Tohoku-Oki earthquake and implication for deformation by viscoelastic relaxation and afterslip,” Geophys. Res. Lett., Vol.41, pp. 5797-5805, 2014.
16. [16] Y. Hu, R. Bürgmann, J. T. Freymueller, P. Banerjee, and K. Wang, “Contributions of poroelastic rebound and a weak volcanic arc to the postseismic deformation of the 2011 Tohoku earthquake,” Earth Planets and Space, Vol.66, Article No.106, 2014.
17. [17] T. Sun et al., “Prevalence of viscoelastic relaxation after the 2011 Tohoku-oki earthquake,” Nature, Vol.514, pp. 84-87, 2014.
18. [18] Y. Hu, R. Bürgmann, N. Uchida, P. Banerjee, and J. T. Freymueller, “Stress-driven relaxation of heterogeneous upper mantle and time-dependent afterslip following the 2011 Tohoku earthquake,” J. of Geophys. Res., Vol.121, pp. 385-411, 2016.
19. [19] 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 Planet. Sci. Lett., Vol.459, pp. 279-290, 2017.
20. [20] H. Suito, “Importance of rheological heterogeneity for intepreting viscoelastic relaxation caused by the 2011 Tohou-Oki earthquake,” Earth Planets and Space, Vol.69, No.21, 2017.
21. [21] S. Jónsson, P. Segall, R. Pedersen, and G. Björnsson, “Post-earthquake ground movements correlated to pore-pressure transients,” Nature, Vol.424, pp. 179-183, 2003.
22. [22] G. Peltzer, P. Rosen, F. Rogez, and K. W. Hudnut, “Poroelastic rebound along the Landers 1992 earthquake surface rupture,” J. of Geophys. Res., Vol.103, pp. 30131-30145, 1998.
23. [23] K. Heki, S. Miyazaki, and H. Tsuji, “Silent faults slip following an interplate thrust earthquake at the Japan Trench,” Nature, Vol.386, pp. 595-598, 1997.
24. [24] S. Ozawa, M. Kaidzu, M. Murakami, T. Imakiire, and Y. Hatanaka, “Coseismic and postseismic crustal deformation after the M_w8 Tokachi-oki earthquake in Japan,” Earth Planets and Space, Vol.56, pp. 675-680, 2004.
25. [25] A. M. Freed, R. Bürgmann, E. Calais, and J. T. Freymueller, “Stress-dependent power-law flow in the upper mantle following the 2002 Denali, Alaska, earthquake,” Earth Planet. and Sci. Lett., Vol.252, pp. 481-489, 2006.
26. [26] F. Silverii, D. Cheloni, N. D’Agostino, G. Selvaggi, and E. Boschi, “Post-seismic slip of the 2011 Tohoku-Oki earthquake from GPS observations: implications for depth-dependent properties of subduction megathrusts,” Geophys. J. Int., Vol.198, 2014.
27. [27] K. M. Johnson, J. Fukuda, and P. Seagall, “Challenging the rate-state asperity model: Afterslip following the 2011 M9 Tohoku-oki, Japan earthquake,” Geophys. Res. Lett., Vol.39, L20302, 2012.
28. [28] E. L. Evans and B. J. Meade, “Geodetic imaging of coseismic slip and postseismic afterslip: Sparsity promoting methods applied to the great Tohoku earthquake,” Geophys. Res. Lett., Vol.39, L11314, 2012.
29. [29] T. Iinuma et al., “Seafloor observations indicate spatial separation of coseismic and postseismic slips in the 2011 Tohoku earthquake,” Nat. Commun., Vol.7, 13506, 2016.
30. [30] T. Sun and K. Wang, “Viscoelastic relaxation following subduction earthquakes and its effects on afterslip determination,” J. of Geophys. Res., Vol.120, 2015.
31. [31] M. Tobita, “Combined logarithmic and exponential function model for fitting postseismic GNSS time series after 2011 Tohoku-Oki earthquake,” Earth Planets and Space, Vol.68, Article No.41, 2016.
32. [32] “Active deformation processes in Alaska, based on 15 years of GPS measurements,” J. T. Freymueller, P. J. Haeussler, R. L. Wesson, and G. Ekström eds., Active Tectonic and Seismic Potential of Alaska, Geophysical Monograph, Vol.179, pp. 1-42, 2009.
33. [33] S. E. Barrientos, G. Plafker, and E. Lorca, “Postseismic coastal uplift in southern Chile,” Geophys. Res. Lett., Vol.19, pp. 701-704, 1992.
34. [34] S. C. Cohen and J. T. Freymueller, “Deformation of the Kenai Peninsula, Alaska,” J. of Geophys. Res., Vol.102, pp. 20479-20487, 1997.
35. [35] P. Wessel and W. H. F. Smith, “New, Improved Version of Generic Mapping Tools Released,” EOS Trans., Vol.79, pp. 579, 1998.