Abstract: The foreshocks, mainshock and aftershocks of the 1999 Xiuyan earthquake sequence have been relocated using a relative location method. The results show that: mainshock is located at the 40.538°N, 123.026°E, and the focal depth is 6.958 km; the difference between the long axis and short one of the epicenter distribution of foreshocks relocated is small, the long axis is about 1.38 km, the short one is about 1.23 km and the depth is between 6~11 km. The epicenter distribution of M_L≥4.0 foreshocks is mainly NW orientation, and mainshock is located at the southeastern end of NW direction. The distribution of aftershocks relocated is nearly NW orientation, the long axis is 3.26 km, the short one is 0.79 km, and the depth is 6~12 km. The NW distribution of the 1999 Xiuyan earthquake sequence is consistent with that of the 1975 Haicheng sequence, it is generally identical to NW strike of nodal plane of focal mechanism of Haicheng and Xiuyan earthquakes, and is correlated to the NW faulting of the epicenter area, and authors consider that it is possible that the occurrence of the Xiuyan mainshock is triggered by foreshock action. It gives one example of the earthquake nucleation process.

Key words: Earthquake location, Foreshock feature, Earthquake nucleation, Xiuyan earthquake sequence