1976年松潘地震序列的小震震源机制

The focal mechanisms of small events of the 1976 Songpan earthquake sequence

利用区域地震台网的地震波资料，由垂直向记录Ｐ和Ｓ振幅比值的拟合，反演得到松潘地震序列１５８个中小余震的震源机制解．分析表明虽然小震机制解的空间取向比较离散，但其优势取向和区域应力场相符。集中在３个地点的８８个小震机制解与离散分布在全区的７０个小震机制解之间没有显著差异．据此推论序列中３次大震之所以具有较大的逆冲滑动分量，系予存软弱面的空间取向决定，而非应力场的原因。序列中的中小余震并没有表现出逆冲滑动的优势，表明它们主要是震源区应力场调整的产物，且分布在一定的震源体积之中。

The focal mechanisms of 158 small and moderate events of Songpan earthquake sequence are reversed based on amplitude ratio of P and S of vertical component observed at regional network. The results indicated that the orientations of focal mechanisms are scattered, but the dominant orientation is similar to that of the regional stress field. There are no obvious differences between the focal mechanisms of 88 events occurred in the three sites and that of 70 events distributed in the whole terrain. From mentioned results, we inferred that the three strong shocks with thrust faulting slide were caused by the orientations of three old weak planes but stress field. The small and moderate events with thrust faulting were not in the dominant position. Judging from this, these small events were the outcome of adjustment of the stress field on the source region and were distributed within some source body.