从块体中心论看中国近期强震成组活动的特征

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

从块体中心论的角度出发,根据中国活动地块研究的最新成果,对1977—2003年中国大陆强震成组发生的构造和震源机制特征进行了分析。结果表明,这些成组发生的强震大都位于活动地块区(Ⅰ级地块)的边界带上或活动地块区内活动地块(Ⅱ级地块)的边界带上。根据哈佛大学提供的CMT(矩张量)解,同组的强震大都具有相似的震源机制。事实表明活动地块区具有整体活动的特点,而且在动力学与运动学方面存在统一性。这些特征对认识强震成组活动的机理,缩小成组强震预测的空间范围具有现实意义。

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.