九州中・西部の地震活動  [in Japanese] Seismic activity in middle and western Kyushu  [in Japanese]

九州中・西部の地殻内地震活動およびそれらの発震機構から推定される地殻応力場の特徴を調べた。その結果, 地震活動については, 別府-島原地溝帯や日奈久断層系など当該地域の顕著な地質構造に対応した帯状または線状の分布が明かになった。また, 天草灘では, 地溝の両側の縁に沿って2重の地震活動帯が明瞭に認められた。発震機構については横ずれ型が卓越している。張力軸はおおよそ南北であり, 地殻変動から推定される広域地殻応力場と矛盾しないが, 起震応力としては東西圧縮力が無視できない。さらに, 深発地震を用いて最上部マントルのS波偏光異方性を調べた結果, 先行S波の偏光は, 別府-島原地溝帯を境にその北側と南側でそれぞれ北西-南東方向と北東-南西方向に卓越する可能性が示された。

Seismic activity and tectonics tress were investigated in the crust of middle and western Kyushu. Several seismic zones were found in the studied area. Most of them are trending in the ENE-WSW or NE-SW direction, which coincides with the geological features. The Beppu-Shimabara-Amakusanada graben corresponds to the most seismically-active zone. The Hinagu fault system is also active. It is notable that a double seismic zone can be clearly recognized along the Amakusanada graben. The strike-slip type solution is dominant in composite focal mechanisms obtained in this study. The tension axes of the solutions strike to the N-S or NW-SE direction, while the compression axes are in the E-W or NE-SW direction. The seismogenic-stress distribution inferred from the focal mechanisms is grossly consistent with the tectonic stress revealed by geodetic measurements. Shear wave anisotropy in the uppermost mantle was investigated using deep earthquakes, in order to check a ridge-transform model proposed by Tada (1985). Our result may suggest that the spreading direction of the uppermost mantle is NW and SW in the north and the south of the Beppu-Shimabara graben, respectively.