Abstract: A strong earthquake is necessarily followed by a sequence of aftershocks and usually preceded by weak seismic activity of increasing magnitude. A sequence of weak earthquakes closely spaced in time is generally followed by an ensuing strong earthquake. In this paper, a possible criterion is derived on the basis of energy relation of the sequence of weak earthquakes for estimating the occurrence of the ensuing strong earthquake. Besides, it is also suggested that the possible geological structural condition with which the type of aftershock sequence may be related.