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From focal mechanism solutions of the earthquakes in the northern part of North China during the period of 2002∼2006, the directions of principal stress axes in 4 stress sub-districts are analyzed using a grid test method. The characteristic of present crustal stress directions is discussed. Based on this result and on the focal mechanism solutions calculated for some events in the period of 1977∼1998, in combination with some other study results, the temporal variation of present crustal stress directions in the northern part of North China is investigated. The results confirm that the direction of crustal principal stress in some regions had somewhat rotated after the 1976 Tangshan M7.8 earthquake. The mean P axes of the focal mechanism solutions rotated clockwise not only in Tangshan sub-district, but also in Beijing and Xingtai sub-districts after the Tangshan earthquake. In Beijing and Xingtai sub-districts the orientations of principal stress axes in the period of 2002∼2006 are consistent with that before the Tangshan earthquake, implying that the stress orientations has rotated back to the state before the Tangshan earthquake in these two sub-districts. The directions of the mean P axes are nearly E-W in Tangshan sub-district since the M7.8 earthquake. The present stress field in the sub-district northwest to Beijing, or in the western part of the Zhangjiakou-Bohai fault zone, is relatively stable during the time period concerned in this study. Because of the limitation of data, this paper only states a possible variation of stress field in the northern part of North China in the recent decades.

Temporal and spatial variation characteristics of present stress field in the northern area of North China