Abstract: The features of subsurface fluid anomalies before Zhang bei earthquakes with M_S 6. 2 and M_S 5. 6 have been com paratively re searched. Combining the focal mechanisms and the isoseismal contours of the two events, with the pattern of the small tomoderate seismic activity before the two events, some re search results are obtained as follows. ① M_S 5. 6 earthquake is a strong after shock of M_S 6. 2 earthquake, and these two events might be caused by the alternating slip of two intersecting tectonic belts; ② the distribution of long-term subsurface fluid anomalies was controlled by the tectonic belts; ③ the short-term subsurface anomalies were located along Zhangjiakou-Bohai tectonic belt trending EW, which might imply that the Zhangjiakou-Bohai tectonic belt would be more active than the others; ④ the reason that the typical shortterm anomalies in some stations occurred repeatedly before the two events might be the similar focal mechanisms; ⑤ the difference of the intensity of the shorttermanomalies and of the iso seismal contours might be caused by difference of the seismic intensity of the two events.

Key words: Subsurface fluid, Field of anomalies, Alternating movement of two intersecting belts, Zhang bei earthquake