ANALYSIS OF THE STRESS FIELD IN SOURCE REGION FOR THE YONGSHAN—DAGUAN, EARTHQUAKE SEQUENCE WITH MAINSHOCK MS 7.1

On the basis of the focal mechanisms of 11 evevts with M_s ≥ 4 and 98 small events in the Yongshan-Daguan, Yunnan,earthquake (Magnitude of the mainshock is M_s 7.1)sequence,the general patterns and variation of the stress field in source region have been discussed.The results show that the stress field in source region corresponds to the tectonic stress field.The orientation of the stress field in source region didn't change obviously when the mainshock (M_s 7.1) took place.The dominant orientation of the focal mechanisms of many small events accord with that of the mainshock.The small events with scattered orientation of the focal mechanisms appeared mostly in the earlier period, and seldom in the later period, of the sequence.The orientations of the focal mechanisms of the small events vary from scattering to concentration.Judging from this, a contribution made by a host of small earthquakes can not be ignored.