(a) This is a location map showing fault planes of the 2016 earthquake (red) and the 1944 Tonankai earthquake (blue). (sciencedaily.com)
Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock. (squaremouth.com)
In particular, uncertainties relating to (a) the fault plane, (b) the rate at which seismic energy is diffused, and (c) the vertical ground movement suggest that JAEA's estimate is too low.4 (cnic.jp)