Focal mechanism solutions and spatio-temporal variations of the present tectonic stress field in Capital Circle region

Based on 23 293 P-wave polarities of 8 061 earthquakes occurred during the period from January 1, 2009 to November 30, 2017, we calculate the focal mechanism solutions of single earthquakes and composite fault plane solutions with improved grid point test method. On the basis of preliminary analysis on these data, using obtained focal mechanism solutions and the focal mechanism solutions of historical earthquakes, we discuss the spatio-temporal variation of the present tectonic stress field in Capital Circle region by linear inversion method. The result shows that the focal mechanism solutions are mainly of strike-slip type in the studied region and the second is normal-faulting type. The P axis azimuths of these focal mechanism solutions mostly trend ENE or almost EW, which is consistent with the tectonic stress field in the region; except that P axis azimuths of a few earthquakes are in NNW direction. The results by Michael’s inversion method show that tectonic stress field of Capital Circle region present good consistency and continuity. The orientation of maximum principal stress axis is in ENE to nearly EW direction from west to east. On a whole, the stress is of strike-slip type, which is consistent with the results of previous scholars. Comparison with the previous research results indicates that the present stress field in northwest of Beijing is in relatively stable state, and the maximum principal stress axis has no obvious variation. But for Tangshan region and Beijing region, the tectonic stress field probably exhibits some variations before and after the 1976 Tangshan earthquake. Whether there is any variations of maximum principal stress axis since the Tangshan earthquake （1977—2017） in the studied region is unknown according to present calculation results, which need to be further verified.