Imagine a large mass of nonhomogeneous metal. Imagine that it is struck a severe blow by a large hammer. The shock wave of the impact will travel throughout the mass and arrive at different parts at different times. Furthermore, this shock wave will be reflected from boundaries and regions of nonhomogeneity. These reflected shock waves will affect some portions of the mass time and time again. Occasionally, shock waves reflected from several directions will converge on some internal region to give an aftershock as great (or greater than) the original shock. Such shock waves may persist for a considerable time.