Since the sliding of either the upstream or downstream slopes in an earthquake can only
occur downhill, if sliding occurs at all there will be a lowering of the crest of the dam caused
by a cumulative slip on both slopes, always downhill. The net motions of either slope can
be determined only after assumptions are made of: (a) the input motions of the ground;
(b) the effective number of spikes of accelerations similar to one of the single spikes considered
in Fig. 17 and in the derivation of equation (23) ; and (c) the resistance of the sliding elements.