著者

馬越 孝道 清水 洋 松尾 のり道

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.39, no.5, pp.223-235, 1994-11-20 (Released:2017-03-20)

参考文献数

22

被引用文献数

1

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Fugendake, the main peak of Unzen Volcano of Kyushu Island in southwest Japan, started to erupt on November 17, 1990, after 198 years of dormancy, and lava extrusion has continued over three years since May 1991. Hypocenters of earthquakes which occurred before and during that eruption were precisely determined using P-wave arrival time data from five selected seismic stations near the focal region. The hypocenters in Chijiwa Bay are distributed in Chijiwa Caldera. Two linear arrangements of epicenters directed nearly from west to east emerged clearly in the western part of the Shimabara Peninsula, whose hypocentral depths became shallower toward the summit of Fugendake. The distribution of hypocenters is restricted by the fault systems which have been formed by the crustal movements of Unzen Graben. The stress which generates these earthquakes is dominated mainly by the north-south extension ; consistent with the regional tectonic stress. The directions of pressure axes are controlled by the magmatic pressure beneath the focal region. It is inferred from the hypocentral distribution and the orientations of pressure axes that the magma involved in the 1990-94 eruption is situated below an inclined boundary between seismic and aseismic regions in the western part of the Shimabara Peninsula. The magma ascent path is located at 13±2 km in depth beneath the western shore of the Shimabara Peninsula, becoming shollower eastward with an angle of elevation of 40〜50°.

著者

馬越 孝道 清水 洋 松尾 のり道

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.39, no.5, pp.223-235, 1994-11-20

被引用文献数

9

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Fugendake, the main peak of Unzen Volcano of Kyushu Island in southwest Japan, started to erupt on November 17, 1990, after 198 years of dormancy, and lava extrusion has continued over three years since May 1991. Hypocenters of earthquakes which occurred before and during that eruption were precisely determined using P-wave arrival time data from five selected seismic stations near the focal region. The hypocenters in Chijiwa Bay are distributed in Chijiwa Caldera. Two linear arrangements of epicenters directed nearly from west to east emerged clearly in the western part of the Shimabara Peninsula, whose hypocentral depths became shallower toward the summit of Fugendake. The distribution of hypocenters is restricted by the fault systems which have been formed by the crustal movements of Unzen Graben. The stress which generates these earthquakes is dominated mainly by the north-south extension ; consistent with the regional tectonic stress. The directions of pressure axes are controlled by the magmatic pressure beneath the focal region. It is inferred from the hypocentral distribution and the orientations of pressure axes that the magma involved in the 1990-94 eruption is situated below an inclined boundary between seismic and aseismic regions in the western part of the Shimabara Peninsula. The magma ascent path is located at 13±2 km in depth beneath the western shore of the Shimabara Peninsula, becoming shollower eastward with an angle of elevation of 40〜50°.

著者

清水 洋 馬越 孝道 松尾 ★道

出版者

日本地質学会

雑誌

地質学論集 (ISSN:03858545)

巻号頁・発行日

no.41, pp.13-18, 1993-06-25

被引用文献数

5

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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。