| The fundamental causation of an earthquake occurrence is action of the stress field. Therefore, studying on the regional stress fields which come from the small and intermediate earthquakes is quite important effect to know the course of an earthquake occurrence. Exploring the epicenter and looking after the target of earthquake forecast, as a goal, geophysicists and seismologists are all along hardworking.In this paper, we use the data of ML≥3.0 earthquakes which have occurred since 2001 in northeast margin of Qinghai-Tibet Plateau, recorded by11broad frequency stations of GanSu Province seismic digital station network. We have calculated focal mechanism solutions of 242 ML≥3.0 earthquakes by moment tensor inversion. We have statistically analyzed the earthquakes types and characteristics of the stress fields in northeast edge of Qinghai-Tibet Plateau using the results which we have calculated. We traverse characteristics of the regional stress fields in 4 and 8 areas which depended on the subarea activity characteristics of northeast edge of Qinghai-Tibet Plateau. As a case Minle earthquake 2003, we analyze the local stress fields before and after Minle earthquake and discuss its generant mechanism. Through contrast analysis on the inversion results and existing results of 4 strong earthquakes, we investigate the characteristics of the strong earthquakes .Summing up the analysis and study from above, we get following conclusions:1. Statistical analysis on the P ,B ,T axes of focal parameters of ML≥3.0 earthquakes, we get the characteristics of the stress fields in northeast margin of Qinghai-Tibet Plateau: in the middle part of Qilanshan , the preponderant orientation of the principal compressive stress P axis is in NE ;in middle east part, its orientation changes from NE to approximately EW and circumrotates some angles in the same direction as hour hand in interspace. In the southeast region of GanSu, the preponderant orientation of the principal compressive stress P axis is in NWW which is consistent to the way of the faults. In the whole, the distribution of the principal compressive stress has obvious divisional characteristics, from west to east, regional principal compressive stress direction changes from NNE-NE-NEE to EW presenting sector distribution.2. Analysis on the elevation of P,T axes in the focal mechanism, we get that the elevation βof P axis less than 30o is 82.23%, the elevationγof T axis less than 45o is 68.59%, the type of earthquakes gives priority to strike-slip fault; when the elevationγof T axis more than 45o happens oblique fault is 31.41%. Based on the results we get that the properties of faults in the investigative area are mostly strike-slip fault, concurrently reverse fault and normal fault .3. Analysis on the mean stress field we get that in the Qilanshan earthquake belt, the elevation ofσ1 andσ3 axes are close to horizontal direction, the stress present in horizon and the principal axis is in NNE and NE. In southeast region of GanSu, the azimuth and elevation are 123o,153o,109o,9o,34o,15o , separately , NW or near NS , the stress show itself in horizon .4. We think that the direction of the principal compressive stress axes appears changeable process by phases before the main shock, the middle part of Qilianshan belt where the main shock occurred was in stress-accumulation state. The most pressure stress axis rotated some angles before and after MinLe earthquake in its zone.With the development of seismic digital station network and improvement of monitor ability about some areas, study on the focal mechanism solutions in location is important scientific meaning for earthquake prediction.
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