Abstract: In this paper, the focal mechanisms of 20 earthquakes with ML ≥ 4.0, including the fo reshock, mainshock a nd af tershock of Xiuya n-Haicheng M = 5.4 ea rthquake are obtained by using th e sign data of P wavefi rst mo tion. The analysis shows that the principal compressional st ress axes P a re mo stly in the no rtheast-east di rection and the extension nal axes Tin the north-nor thw est di rection, both P and T dipang les less than 30°are nearly ho ri zo ntal. The null axes N are most ly rather steep. The st rike of the two no dal planes i s a long the north-no rtheast and nor thw est-west directions respectively and most Xiuya n-Haicheng ea rthquakes belo ng to the strike slip type. Focal mechani sms of aftershocks are most ly similar to that of the mainshock and result is relatively stable. Source stress field of the mainshock cont rols the stress field ofaf tershocks.

Key words: Seismic sequence, Focal mechanism, Focal stress field, Xiuyan-Haicheng earthquakes