中文摘要：

日本本州及其邻近区域的应力状态以及弧后盆地的演化机制一直是人们所关注的问题.本文对2011年3月11日东日本大地震地震序列（2011年3月11日至2012年3月15日）的哈佛双力偶解进行了聚类分析,得到五种类型的震源机制解：与主震类型一致的低倾角逆断层型地震;主张应力方向垂直于日本海沟走向的正断层型地震;主张应力方向平行于日本海沟走向的正断层型地震;主压应力方向平行于日本海沟走向的逆断层型地震;包括走滑型地震在内的其他类型地震.东日本大地震地震序列中发生在弧前增生楔地震的震源机制解与大地震发生之前地震的震源机制解特征有显著区别,反映出该地区的应力状态与震前相比有较大改变.东日本大地震及其前震释放了附近区域的累积弹性应力,主震破裂区附近太平洋板块和其上覆板块接近完全解耦,降低了日本海盆地、中国东北地区的近东西向挤压应力水平.不过,整个本州岛东部区域太平洋板块和其上覆板块并没有完全解耦,但应力水平并不高.我们认为,日本海及中国东北应力水平的降低会使该区域的近东西向挤压型地震的危险性降低,而使NNE-SSW走向的走滑型地震活动性增强.同时,火山活动性也会增强.尤其是本州岛地区,存在近期火山爆发的可能性.东日本大地震地震序列的震源机制解特征还提示我们,日本海的应力状态及日本海的演化可能在一定程度上取决于太平洋板块和上覆板块的耦合状态.持续的弱耦合将不仅使得弧后大范围的地区保持岩浆上涌所必须的拉伸应力环境,而且还会因弧前隆起区发育大量正断层型地震而向深部提供促使岩浆生成所必须的水,因而造成日本海的再次扩张.

英文摘要：

The stress state of Japan Sea region and its evolution are both concerned greatly by geoscientists. In this paper, we study Harvard CMT solutions of the March 11, 2011 Tohoku- Oki Earthquake sequence from March 11th, 2011 to March 15th, 2012 by cluster analysis and find that they might he roughly sorted to four groups. The first group are low-angle thrust events,which are the same as the main shock and are mostly interplate events. The second group events are normal-fault earthquakes with principal extensional direction roughly pointing to E-W direction. Most of these earthquakes were located in the fore-arc uplift region of the Pacific plate, while some others occurred in the fore-arc accretionary wedge. The third group events are also normal-fault events mainly occurred in the fore-arc accretionary wedge though their principal extensional directions roughly parallel the Japan Trench. The forth group are reverse earthquakes with principal compressional directions roughly along N-S direction. We observe that Harvard CMT solutions of 2011 Tohoku-Oki earthquake sequence are obviously different from before. They show that Tohoku-Oki Earthquake sequence have released most of elastic energy in the main shock area. As a result, Pacific Plate and North America Plate have decoupled in the main rupture zone, which lowered the E-W compressional stress in Japan Sea and Northeastern China. However, Japan Sea Subduction Zone has not completely decoupled with North American Plate, even though the stress level in this region is lowered now. We claim that releasing of accumulated stress in the region of Japan Sea and Northeast China could lower the seismic hazard of roughly E- W direction compressive events, while tend to increase the activity of NNE-SSW strike-slip events as well as the volcanism, especially in Honshu. The characteristics of the Focal Mechanism Solutions of the Tohoku-Oki Earthquake Sequence remind us that stress state as well as evolution of Japan Sea might depend to some extent on coupling state between Pacif