Inversion of Plate boundary Tectonic Stress Field and its Effect on Earthquakes in North China

Based on focal mechanism solutions of global earthquakes,spatial tectonic stress field on plate boundaries around China mainland was inverted by the method of average stress tensor.The results indicate that maximal principal stress axis azimuth is in accordancs with the direction of plate movement from the GPS results,and the maximal principal stress axis dip angle between the western boundary of the Pacific plate and the Ryukyu island arc segment of the Philippine plate is comparable with the plate subduction angle,and therefore the tectonic stress field inverted with this method is reliable.The maximal principal stress axis azimuth of the Japan Honshu segment of the Pacific plate approched turning point in 1999,2005and 2011.The summary of earthquake cases shows that these turning points often correspond to larger than M5.0even M6.0earthquakes in North China.However,the turning change in 2011only corresponded to about M5.0earthquakes in North China area.According to segmentation research on the western boundary of the Pacific plate,we estimated that the turning change in 2011was mainly caused by the turning of tectonic stress field from 42°to 50°N segment.Furthermore,in view of geographical location and subduction direction of this segment,the Northeast region was mainly affected,so North China area was less affected and the corresponding magnitude was lower.There was substantial and long-time counter clockwise deflection of maximal principal stress axis azimuth of the Ryukyu island arc segment in the Philippine plate from 1992to 2000.This perhaps is the principal reason that more than one earthquakes with typical tectonic stress field of South China occurred in the same period in the south of the North China.