[S-VC32] 火山噴火のダイナミクスと素過程

[SVC32-P04] 火道形状が伊豆大島1986年サブプリニー式噴火のダイナミクスに与える効果

*小園 誠史¹、石橋 秀巳²、奥村 聡³、三輪 学央⁴ (1.東北大学大学院理学研究科地球物理学専攻、2.静岡大学理学部地球科学科、3.東北大学大学院理学研究科地学専攻、4.防災科学技術研究所)

キーワード：玄武岩質マグマ噴火、伊豆大島、火道流モデル、火道形状

Izu-Oshima volcano in Izu Arc is one of the active volcanoes in Japan, and its eruption history for the past 200 years is characterized by repetitive moderate eruptions by basaltic or basaltic andesite magma in 1876-77, 1912-14, 1950-51, and 1986-87. In the 1986 eruption, not only lava flows and fire fountains but also a sub-Plinian eruption occurred, and these activities have been deeply investigated by geological, petrological, and geophysical observations. Because these observations provide valuable information about magmatic and geological parameters, the 1986 sub-Plinian eruption is a preferable event for investigating the dynamics of an explosive eruption in mafic magmatism. This study investigated the conduit flow dynamics during the 1986 sub-Plinian eruption of Izu-Oshima volcano, using a 1-D steady conduit flow model. Following the observations that the magma plumbing system beneath Izu-Oshima is characterized by a feeding of dyke and the sub-Plinian eruption occurred as a fissure eruption, we considered a dyke-like conduit geometry by applying a pseudo-dyke conduit in which its horizontal cross-section is ellipsoidal. Under proper parameter settings constrained from geological, petrological, and geophysical observations, we identified conduit geometry conditions for the steady solution of conduit flow corresponding to the sub-Plinian eruption to exist. In contrast to a purely cylindrical conduit, the dyke-like conduit geometry allows us to widen the ranges of geometric parameters for the solution to exist. We also found that the distribution of magma overpressure in the conduit strongly depends on the conduit geometry. If the aspect ratio of the horizontal cross-section is constant throughout the conduit (e.g., the cylindrical conduit), the overpressure is not generated in the conduit. On the other hand, when the conduit geometry is composed of deeper and shallower dykes with large and small aspect ratios, respectively, a localized over-pressurization occurs in the region before magma fragmentation. This over-pressurization in the dyke-like conduit may induce a characteristic crustal deformation similar to that caused by a vertical tensile fault. It is crucial to consider the effects of the conduit flow with the dyke-like geometry on the deformation for precise monitoring of eruption sequence based on geodetic signals in the future eruptions at Izu-Oshima volcano.