Intermediate-Depth Earthquakes and Seismotectonics beneath southern Kyushu, Japan

利用鹿儿岛大学和九州大学微震台网的高质量观测资料，研究了日本九州南部洼地-贝尼奥夫带中深地震的地震活动、震源机制解和构造。研究表明，Wadati-Benioff带构造复杂。最显著的特征是地震带水平滑动，即樱岛火山NNE区地震带比SSW区地震带向WNW移动约10km。在130km深度处，沿50km范围逐渐发生滑动，在70km深度处，沿80km范围逐渐发生滑动。有趣的是，作为该地区特征之一的Kaimon火山下约160km深度的极高地震活动发生在滑动区的边缘。地震带剖面在150 ~ 200km左右的深度表现出延续性，并改变了其曲率。具体来说，它具有波状特征，即在深度约65km处向上凸起，在深度约50km和100km处向下凸起。震源机制解表明，在垂直平面上，拉应力主轴平行于板坯下降方向，压应力主轴垂直于板坯下降方向。而在水平面上，纵应力轴与大洋板块辐合方向一致。根据这些事实，可以认为在几乎所有岛弧的下降过程中，都有一些力作用于九州南部下方的菲律宾海板块的下降。这种作用力作用于复杂的地震带构造和中深地震非下倾机制的产生。

Seismic activities and focal mechanism solutions of intermediate-depth earthquakes and the structure of Wadati-Benioff zone beneath southern Kyushu, Japan are investigated based on the high quality data observed by the micro-seismic network of Kagoshima University and Kyushu University. It is revealed that the Wadati-Benioff zone shows the complicated structure. The most characteristic feature is that the seismic zone slides horizontally, that is, the seismic zone in NNE region of Sakurajima volcano shifts to WNW by about 10km compared to that in SSW region. Sliding occurs gradually along about 50km extent at the depth of 130km and about 80km at the depth of 70km. It is interesting that the extreme high seismic activity at about 160km in depth beneath Kaimon volcano, which is one of the characteristic features in this region, occurs at the rim of the sliding region. The section of seismic zone shows to be continued to the depth of about 150-200km with changing its curvature. In detail, it has the wavelike feature, that is, it is convex upward at about 65km in depth and downward at the depths of about 50km and 100km. Focal mechanism solution indicates that the principal axes of tensional stresses are parallel to and those of compressional ones are perpendicular to the descending direction of slab in the vertical plane. However, in the horizontal plane, the axes of compressional stresses are aligned to the converging direction of oceanic plate. From these facts, it is considered that some force acts to the descending Philippine-Sea plate beneath southern Kyushu adding to the force produced by the descending process as seen in almost all island arc. And such force works the complicated structure of seismic zone and the generation of non-down-dip mechanism of intermediate-depth events.