The Mid-Niigata Prefecture Earthquake did not rupture the earth's surface and occurred along a fault hidden well under folded terrain. In order to evaluate the seismic activity of such hidden earthquakes beneath fault-related folds, a new method to analyze self-affinities is introduced. Based on this analysis, the geometry of a fold is shown to be self-affine and can be variously scaled in different directions. The size and temporal properties, b-value in Gutenberg - Richter's law and p-value in the modified Omori's law, express aftershock activity. Then, from the analyzed data of the folds, we estimate the bvalue and p-value of the hidden earthquake under the fault-related folds. These estimated b-values and p-values are concordant with seismologically obtained ones in the aftershock sequence of the Mid-Niigata Prefecture Earthquake.