Abstract: By analysing a number of cases of moderate and strong earthquakes, it is pointed out in this paper that seismic gaps can be classified into two categories. Besides, background gaps formed by large earthquakes in a large area with a long time duration, there can also be preparatory gaps formed by smaller earthquakes in a smaller area and with a shorter time duration. A comparison has been made between these two kinds of seismic gaps. It has been also found that at the later period of preparatory gaps, one or a group of rather outstanding earthquakes occur on their edges, called imminent earthquakes. Some characteristic features of such earthquakes and their relation to the forthcoming main earthqaukes for their possibility in predicting earthquakes are preliminarily studies. Finally, we also made a preliminary analysis of the physical process causing the preparatory gaps and imminent earthquakes with the point of view of fracture mechanics.