Abstract: The strong earthquake usually occur in groups in China. With plenty of historic earthquake data in North China, this article analyses the correlation between earthquakes with M≥5.0 and movement of tectonic blocks and discovers the preparatory process of strong earthquakes in groups or the climax seismic activity period in this area. The conclusion is that the pattern of large earthquakes in groups is related with the evolution of block movement. A long seismic activity period can be divided into two phases. During the first phase, the movement of each part of block is rather lopsided. A group of earthquakes are mainly result from ruptures of several locked segments and interaction among ruptures in local region. When the ruptures that goes along boundaries of tectonic block has reached a certain level, the block surrounded the boundary will be changed into the state based on the movement in whole, meanwhile the movements of the boundaries of the block should be synchronous in a certain level. Furthermore, after one locked region has ruptured, it will not be locked again, which leads to such rupture process from weak to strong along the tectonic block boundaries till a group of strong locked regions complete rupture and come into a seismic activity climax period. This is the essential reason for resulting in the strong earthquakes in groups. In addition the induced effect among large earthquake is another factor in earthquakes in groups. Finally, this article summaries how to forecast the appearance time, average intensity and occurrence location before a group of strong earthquakes according to the model described above.

Key words: Strong earthquakes in group, Preparatory process, Long- and Mid-term prediction, North China region