This paper is an application of the theory of 'chaos' to geophysics. It is pointed out that interactions between seismic events may play an important role in causing the apparently random behavior of earthquake occurrence. The great earthquake belt is modeled by a number of large-scale fault blocks having the same strength and the same interaction parameters. Simulated seismic events show spatio-temporal random behavior if the blocks interact with each other in such a way that when an earthquake occurs in one block, the energy of blocks in the nearest region increases while the energy in the distant blocks decreases. A simple model composed of only two blocks also can exhibit nonperiodic behavior, if the blocks mutually interact in such a way that when an earthquake occurs in one block, the rate of energy increase decreases in the other block. An electrical analog model of the coupled fault blocks showing nonperiodic oscillation is presented.