Intermediate-Depth Earthquakes and Seismotectonics beneath southern Kyushu, Japan

利用鹿儿岛大学和九州大学微地震台网的高质量观测资料，研究了日本九州南部地区中深源地震的活动性、震源机制解和Wadati-Benioff带的构造。揭示了Wadati-Benioff带具有复杂的构造特征。最具特点的是地震带水平滑动，即樱岛火山NNE区地震带较SSW区向WNW方向移动约10 km。在130 km深度处，滑动沿沿着约50 km的范围逐渐发生，在70 km深度处，滑动沿约80 km的范围逐渐发生。值得注意的是，作为该地区特征之一的Kaimon火山下约160 km深处的极高地震活动发生在滑动区的边缘。地震带的剖面表现为曲率不断变化，可延伸至150- 200 km深度。具体地说，它具有波状特征，即在约65 km深度处向上凸起，在约50 km和100 km深度处向下凸起。震源机制解表明，在垂向上，张应力主轴平行于板片下降方向，压应力主轴垂直于板片下降方向。而在水平面上，挤压应力轴与大洋板块的汇聚方向一致。根据这些事实，可以认为，在几乎所有岛弧中所看到的下降过程所产生的力量中，有一些力量作用于九州南部下方的菲律宾海板块。这种力作用于复杂的地震带结构和中深层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.