In general, then, the concentrated energy at the source is actually divided among the principal wave types, which are there added together. Within very short distances, however, these wave types begin to separate because of their different velocities. There are thus two factors working to reduce the maximum shaking to which the ground is subjected. One is the natural decay of each wave with distance, as internal friction exhausts its original energy. The other, which is usually the dominating effect at short distances, is this stringing out of the wave types, each carrying its portion of the initial energy, until there is no longer any concentration where two or more types join forces to produce additive amplitudes.