DISCUSSION ABOUT GROUPING OCCURRENCES OF RECENT STRONG EARTHQUAKES IN CHINA FROM BLOCK VIEWPOINT

The thinking of earthquake research in China should be shifted from the viewpoint of fault to active block (MA Jin, 1999). ZHANG Pei-zhen et al. divide the active blocks in Chinese mainland and its adjacent area into two degrees: the first degree refers to active-block regions and the second degree refers to active blocks. The former contains 6 block regions, e.g. the Qingzang (Qinghai-Tibet) region, etc. and the latter contains more than 20 active blocks, e.g. the Lhasa block and so on. We attempt to analyze the characteristics of geological structure and focal mechanism of group strong earthquakes that occurred recently in Chinese mainland from the block viewpoint on the basis of the two-degree active blocks. The strong earthquakes (M≥7 in the west and M≥6 in the east) occurring in China of 1977—2003 can be roughly divided into 9 groups. In summary, the strong earthquakes occurring in the recent 10 years still have the grouping feature and most of them are located in the boundary zones between active-block regions or active blocks. Moreover, their focalmechanism solutions are quite similar to each other, except for the earthquakes in the 4th group (the earthquakes that occurred in the Beibu Gulf and the Taiwan Straits can be considered as an individual case) and in the 5th group (the earthquakes that occurred in Mandalay-Diannan block near the plate boundary are not regarded as intraplate earthquakes). Based on the study of horizontal strain field in Chinese mainland and its surroundings with GPS data, we point out in the paper that group strong earthquakes have their own genesis for the similar motion pattern and dynamic origin. From the above analysis, we conclude: (1) The rule of strong earthquake occurrences in groups is still effective after more than 10 years practice, and it is an applicable method for locating earthquakes in the medium and short-term earthquake predictions. (2) The regional characteristics of group strong earthquakes enable us to predict the location of earthquakes in a smaller range on the boundary zone between the second-degree blocks in a first-degree block region or between two first-degree block regions. (3) Except plate-margin area, the group strong earthquakes have consistent focalmechanism solutions. This indicates that they have similar kinetic mechanisms and dynamic processes, or perhaps, we can say, that they develop monolithically and occur successively.