Intermediate-Depth Earthquakes and Seismotectonics beneath southern Kyushu, Japan

根据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和田-贝尼奥夫带的中深地震活动和震源机制解。结果表明，瓦达蒂-贝尼奥夫带具有复杂的结构特征。最具特点的是地震带水平滑动，即樱岛火山NNE区的地震带比SSW区向西西偏移约10公里。滑动深度约50公里，深度约130公里，深度约80公里，深度约70公里。有趣的是，凯蒙火山下方约160公里处的极高地震活动发生在滑动区的边缘，这是该地区的特征之一。地震带的剖面随其曲率的变化一直延续到150-200公里深度。具体来说，它具有波浪状的特征，即在深度约65公里处向上凸起，在深度约50公里和100公里处向下凸起。震源机制解表明，在垂直面上，拉应力主轴平行于板的下降方向，压应力主轴垂直于板的下降方向。然而，在水平面上，压应力的轴线与大洋板块的会聚方向一致。根据这些事实，可以认为，一些力量作用于九州南部下方下降的菲律宾海板块，增加了几乎所有岛弧都可以看到的下降过程产生的力量。这种作用力作用于复杂的地震带结构和中深部地震的非下倾机制的产生。

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.