2011年日本东北大地震( M W =9.0)震间与震前变形场特征及其对强震预测的启示

The research on the preseismic dynamic process is of great significance for earthquake prediction. However, due to the limitations of observational data, the current understanding of the mechanical state and its evolution process in the seismogenic zone before a strong earthquake is still very limited. The 2011 Tohoku - Oki great earthquake with a magnitude of 9.0 in northeastern Japan occurred in the area with the densest GPS observation stations, providing a rare opportunity to study the interseismic and preseismic deformation states of a great earthquake. In this paper, continuous GPS observation data before the Tohoku - Oki great earthquake in Japan will be used to calculate the velocity field and deformation field during the interseismic and preseismic periods respectively. Through comparative analysis, it is found that the strain state before the earthquake in the Tohoku region is very different from that during the interseismic period. The deformation during the interseismic period is mainly controlled by the northwesterly subduction and compression of the Pacific plate towards the Japan Trench, and its principal compressive strain is mainly in the near east - west direction of compression. The movement direction of the Tohoku region in Japan is generally consistent with that of the Pacific plate. However, the movement direction of the Tohoku region in Japan changed greatly before the approaching earthquake (preseismic). The continuous GPS observation results 30 days before the earthquake show that the dominant direction of the velocity field often changes and intermittently appears to be opposite to the movement direction of the Pacific plate. This means that the mechanical state in the seismogenic zone before the earthquake has changed greatly. This change may be related to processes such as preseismic rupture nucleation or slow slip and slow earthquakes. These processes will accelerate or promote the occurrence of a great earthquake, thus preparing the mechanical conditions for the occurrence of a great earthquake. It is particularly emphasized that these phenomena are all abnormal phenomena before a great earthquake that can be discovered through direct observation. It can be seen that densifying GPS stations for continuous observation, searching for preseismic deformation abnormal areas and exploring the physical mechanism of the abnormalities are of important scientific significance for earthquake prediction and forecasting.

强震震前（preseismic）动力学过程的研究对于地震预测具有十分重要的意义，但由于观测资料的限制，目前对强震前孕震区力学状态及其演化过程的认识还非常有限.2011年日本东北9.0特大地震（Tohoku-Oki）发生在GPS观测台站最为密集的地区，为研究特大地震震间（interseismic）与震前的变形状态提供了难得的机会.文中将利用日本东北大地震之前连续的GPS观测资料，分别计算震间与震前的速度场与变形场.通过对比分析发现，日本东北地区（Tohoku）震前的应变状态与震间的有很大的不同，震间的变形主要受到太平洋板块向日本海沟北西西向的俯冲挤压作用所控制，其主压应变以近东西向压缩为主，日本东北地区的运动方向与太平洋板块的运动方向大体一致.但是，临近地震前（震前）日本东北地区的运动方向发生了很大变化，震前30天的连续GPS观测结果显示，速度场的优势方向经常变换，间歇性地出现与太平洋板块运动方向相反的情况.这意味着震前孕震区的力学状态发生了很大的改变.这种变化可能与震前破裂成核或慢滑移及慢地震等过程有关，这些过程将加速或促进大地震的发生，从而为大地震的发生准备了力学条件.值得特别强调的是，这些现象都是可以通过直接观测能够发现的大地震之前的异常现象.由此可见，加密GPS站点进行连续观测，寻找震前变形异常区以及探索异常的物理机制对于地震预测预报有重要的科学意义.