This paper describes earthquake and tsunami scenarios as basic information for preparing for the next Nankai megathrust earthquakes. Models to clarify the size of the Nankai megathrust earthquake and changes in occurrence intervals, simulations using such models, and simulations of crustal deformations and tsunamis based on the simulations were employed. This paper re-examines past earthquakes and tsunamis, the possibility of slightly larger earthquakes and tsunamis, their sizes, the necessity of countermeasures against subsidence caused by earthquakes in the Inland Sea, the possibility of the Nankai earthquake occurrence before the Tokai (Tonankai) earthquake, and the possibility of the triggering of the Nankai earthquake by the Hyuga-nada earthquake.

1. [1] M. Hyodo, T. Hori, and Y. Kaneda, “A possible scenario for earlier occurrence of the next Nankai earthquake due to triggering by an earthquake at Hyuga-nada, off southwest Japan,” Earth, Planets and Space, Vol.68, No.6, 2016.
2. [2] Headquarters for Earthquake Research Promotion, “Evaluation of occurrence potentials for subduction-zone earthquakes. Nankai Trough. rev.2,” 2013.
3. [3] T. Hori, “Mechanisms for Variation in Size and Occurrence Interval of Interplate Earthquakes,” Jishin, Vol.2, No.61, S391-S402, 2009 (in Japanese).
4. [4] M. Hyodo and T. Hori, “Re-examination of possible great interplate earthquake scenarios in the Nankai Trough, southwest Japan, based on recent findings and numerical simulations,” Tectonophysics, Vol.600, pp. 175-186, 2013.
5. [5] M. Hyodo, T. Hori, K. Ando, and T. Baba, “The possibility of deeper or shallower extent of the source area of Nankai Trough earthquakes based on the 1707 Hoei tsunami heights along the Pacific and Seto Inland Sea coasts, southwest Japan,” Earth, Planets and Space, Vol.66, No.123, 2014.
6. [6] N. Kato and T. E. Tullis, “A composite rate- and state-dependent law for rock friction,” Geophys Res Lett, Vol.28, pp. 1103-1106, 2001.
7. [7] M. Nakatani, “Conceptual and physical clarification of rate and state friction: frictional sliding as a thermally activated rheology,” J Geophys Res, Vol.106, pp. 13347-13380, 2001.
8. [8] M. Shishikura, H. Maemoku, T. Echigo, Y. Namegaya, and A. Nagai, “History of multi segment earthquake along the Nankai Trough, deduced from tsunami boulders and emerged sessile assemblage,” Japan Geoscience Union Meeting 2011. Makuhari Messe international conference hall, Chiba, pp. 22–27, May 2011.
9. [9] T. Hori, N. Kato, K. Hirahara, T. Baba, and Y. Kaneda, “A numerical simulation of earthquake cycles along the Nankai Trough in southwest Japan: lateral variation in frictional property due to slab geometry controls the nucleation position,” Earth Planet Sci Lett, Vol.228, pp. 215-226, 2004.
10. [10] S. Yoshioka and Y. Matsuoka, “Interplate coupling along the Nankai Trough, southwest Japan, inferred from inversion analyses of GPS data: Effects of subducting plate geometry and spacing of hypothetical ocean-bottom GPS stations,” Tectonophysics, Vol.600, pp. 165-174, 2013.
11. [11] S. Yoshioka, Y. Matsuoka, and S. Ide, “Spatiotemporal slip distributions of three long-term slow slip events beneath the Bungo Channel, southwest Japan, inferred from inversion analyses of GPS data.,” Geophys. J. Int., Vol.201, pp. 1437-1455, 2015.
12. [12] K. Obara and A. Kato, “ Connecting slow earthquakes to huge earthquakes.,” Science, Vol.353, No.6296, pp. 253-257, 2016.
13. [13] T. Furumura, K. Imai, and T. Maeda, “A revised tsunami source model for the 1707 Hoei earthquake and simulation of tsunami inundation of Ryujin Lake, Kyushu, Japan,” J. Geophys. Res., Vol.116, B02308, doi:10.1029/2010JB007918, 2011.
14. [14] T. Ichimura, R. Agata, T. Hori, K. Hirahara, C. Hashimoto, M. Hori, and Y. Fukahata, “An elastic/viscoelastic finite element analysis method for crustal deformation using a 3-D island-scale high-fidelity model,” Geophys. J Int., Vol.206, No.1, pp. 114-129, 2016.
15. [15] T. Yamaguchi, T. Ichimura, Y. Yagi, R. Agata, T. Hori, and M. Hori, “Fast crustal deformation computing method for multiple computations accelerated by a graphics processing unit cluster,” Geophys. J Int., Vol. 210, No.2, pp. 787-800, 2017.