日本海沟俯冲带MW9.0地震震源区应力场演化分析

于2011年3月11日发生在日本东北部的M_W9.0级逆冲型板间地震是日本有地震记录以来震级最大的一次地震.本研究基于NIED F-net矩张量解目录中的震源机制解，选取两个长轴相互垂直的矩形区域进行应力场2D反演，获取了日本海沟俯冲带地区应力场的空间及时间分布图像.结果表明：主震前，俯冲带地区应力状态在空间上大体趋于一致，即应力轴（P轴、σ₁轴及S_Hmax轴）系统性地倾向板块汇聚方向，P轴、σ₁轴倾角整体偏缓（<30°），且远离震源区及日本海沟东侧区域内的应力轴倾角普遍大于主震震源区内应力轴倾角；主震前，受2003年5月26日在宫城县北部发生的M_W7.0地震影响，位于M_W9.0地震震源区西北侧的应力场出现明显扰动，σ₁轴倾向顺时针偏转150°~180°，并于之后大体恢复至震前状态，同期其他地区没有明显变化，这种情况可能和主震断层局部（深部）的前兆性滑动有关；主震后，距离震源区较远处应力场变化不大，主震震源区内应力场发生显著改变，P轴及σ₁轴均以大角度（>60°）倾伏于板块汇聚方向，S_Hmax轴顺时针偏转60°~90°且在日本海沟附近普遍平行于海沟轴.这项研究以时空图像的方式展示了大地震前应力场变化的特点，反映了大地震孕震过程中构造与地震的相互作用，对于理解大地震孕震过程有重要意义.

The evolutionary analysis of the stress field in the seismic focal zone of the great Tohoku-Oki earthquake (MW=9.0) in the Japan Trench subduction zone

The great Tohoku-Oki earthquake (M_W=9.0) which occurred on 11 March, 2011 and ruptured the plate boundary between the subducting Pacific plate and the overlying continental plate was the best scientifically recorded earthquake in Japan so far. Focal mechanisms were collected from NIED CMT solutions, and then we selected two rectangle regions to perform 2D stress inversion to acquire spatial and temporal stress patterns of the seismic focal zone in the Japan Trench subduction zone. Before the mainshock, it could be inferred from the spatial distribution images that the stress state in the subduction zone was roughly uniform in space:P-axes, σ₁-axes and S_Hmax-axes almost plunge with a low angle (<30°) in the direction of plate convergence, and the plunges of P-axes and σ₁-axes away from the seismic focal zone and at the east side of the Japan Trench are generally bigger than those in the seismic focal zone. Meanwhile, the stress field at the northwest side of the seismic focal zone (off the Miyagi Prefecture) is obviously disturbed by the 2003 Miyagi earthquake (M_W=7.0) with an evident deflection at nearly 150° to 180° of the trend of σ₁-axes, and then it returns to its previous state. Nevertheless, there is no significant change of the stress state elsewhere before the mainshock, which may be related to the precursory sliding of the mainshock fault at the deep front. After the mainshock, the stress field away from the seismic focal zone shows no evident change, while the change of the stress state in and around the seismic focal zone is remarkable. Stress axes including P-axes and σ₁-axes plunge at a high angle (>60°) in the direction of plate convergence, and the S_Hmax-axes rotate clockwisely nearly 60° to 90° in the seismic focal zone and parallel to the trench axis around the Japan trench. This study shows the characteristics of the stress field variation before giant earthquakes in the form of temporal and spatial images, and reflects the interaction between the structure and earthquakes in the seismogenic process of great earthquakes as well, which is very significant for understanding the seismogenic process of great earthquakes.