中部九州地域の火山周辺の地震活動とそのテクトニクス  [in Japanese] Seismic activity and tectonic significance near the volcanic areas in central Kyushu, Japan  [in Japanese]

九州地域の地震活動域は, 震源の深さによって著しい分布の違いがある。震源の深さが20 kmまでのほとんどの地震は、中部九州にほぼ東西に並ぶ複数の火山, 鶴見岳、九重山、万年山、阿蘇カルデラ、島原半島などの周辺の限定された地域で密集して発生している。これら地域の地震活動の中で, とくに阿蘇カルデラ西部の地震活動は, 阿蘇中岳の噴火活動の開始直前か初期に活発化し, 火山活動が最盛期では低調になり, 火山活動と相補的な関係にあるようにみえる。また, 発震機構は, 中部九州地域では横ずれ成分を含んだ正断層的で伸張軸が南北水平でほぼ同一の解となる。従って, 中部九州地域の地殻上部のせいぜい10 km程度までは, 一様な広域的な応力場で規定され, そのテクトニクスは共通の基盤上にあるといえる。

On a broader scale, the central Kyushu district lies midway in the zone of contemporary tectonic activity that runs westward from Beppu Bay, through the Kuju and Aso volcanoes, to the Unzen volcano. This tectonic active zone is characterized by frequent shallow earthquakes. But the distribution of earthquakes is not uniformly spread over the tectonic zone. The active seismic regions are restricted to some parts near the volcanic areas. At the volcanic regions, several earthquakes warms have occurred as short bursts of spatially clustered activitiy in one or two days. Most of the earthquakes are located at the western sides of volcanic areas. Although many earthquakes have occurred at the northern side of the Oita Kumamoto tectonic line, few earthquakes have been recognized at the southern side. This remarkable contrast between the seismic activity in the northern and southern side of this tectonic line should be emphasized. Also, it is worth noting that at the eastern of the Aso and Kuju volcanoes the seismic activity is very low inspite of existence of many faults and lineaments. These results seem to be closely related to the tectonic line and the Aso and Kuju volcanoes. On the other hand, the central Kyushu district is situated in the tectonic structure at which the strike slip type and normal type fault system produces the active seismicity. The fault plane solutions indicate that the faults are directed from northeast to southwest, and that the mechanisms are the combination of both types, normal and srike slip with the right lateral slip. Tension axes are invariably in a horizontal north-south orientation. These evidences indicate that the stress-field is characterized by remarkable north-south extension and are in common throughout central Kyushu.