板块边界构造应力场的反演及其对华北地震的影响研究

利用全球震源机制解资料，采用力轴张量计算法，反演中国大陆附近板块边界线上的构造应力场空间分布，其最大主压应力轴的方位角与GPS研究得到的板块运动方向一致，太平洋板块西边界和菲律宾板块琉球岛弧段的最大主压应力轴的倾角与板块俯冲倾角基本相当，因此认为该方法反演的构造应力场真实可靠。1999年、2005年和2011年太平洋板块日本本州段的最大主压应力轴方位角存在转折现象，震例总结显示该转折现象往往对应华北地区5级以上，甚至6级左右地震，但2011年的转折变化对应华北地震的震级在5级左右。根据对太平洋板块西边界的分段研究，认为2011年的转折变化主要是由42°~50°N段的构造应力场转折引起的，而该段从地理位置结合俯冲方向来看，影响的主要地区是东北地区，而对华北的影响相对较小，因此导致对应地震的震级偏低。1992—2000年菲律宾板块琉球岛弧段的最大主压应力轴方位角存在大幅度、长时间的逆时针偏转现象，分析认为是造成同期华北南部地区发生多次具有典型华南应力场特征地震的原因。

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, 2005 and 2011. The summary of earthquake cases shows that these turning points often correspond to larger than M5. 0 even M6. 0 earthquakes in North China. However, the turning change in 2011 only corresponded to about M5. 0 earthquakes in North China area. According to segmentation research on the western boundary of the Pacific plate, we estimated that the turning change in 2011 was 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 1992 to 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.