Abstract: The clustering seismicity identification method based on nearest- neighbor distance proposed by Zaliapin et al, is adopted in this paper. Firstly, different type's data are used to test its validity; then, the method is applied to regional seismicity in Jiaodong peninsula and Yishu fault belt. The results show that,small earthquake activities in Yishu fault belt from 1996 satisfy Poisson stochastic process approximatively, which can be seen as background seismicity; while activities in J iaodong peninsula area display obvious clustering features. Finally, the method is applied to the 1976 Tangshan M_s7.8 earthquake sequence. Comparing different stages of the Tangshan earthquake sequence, the results indicate that, with the elapse of time, events affected by the main shock and previous large aftershocks become lesser and lesser, the clustering features of the T angshan sequenceare gradually fading out, and more and more smnall shocks in the sequence maybe treated as background seismicity.

Key words: Earthquake cluster, Background seismicity, ETAS model, Tangshan earth-quake sequence