地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
総説
地殻変動からマグマ溜まりを推論する
山崎 雅
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ジャーナル フリー

127 巻 (2018) 2 号 p. 111-138

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 Ground displacement in a volcanic region constrained by precise geodetic measurements has motivated us to infer magmatic activity at depth, e.g., inflation/deflation of a magmatic deformation source. The inference was initially made by predicting the elastic response to a pressurized magma chamber. A prediction with an unrealistically high chamber pressure has then introduced other rheological models, including elastoplastic and viscoelastic rheologies. Viscoelasticity predicts transient behavior of deformation even though the source mechanism is constant with time, depending on the viscosity structure of the crust and the presence of an elastic layer in the uppermost crust, which requires volcano deformation to be considered a more complex phenomenon, but provides an excellent opportunity for inferring a magma chamber not only as a deformation source but also as a zone that has rheologically less strength. Indeed, geodetically measured rates of volcano deformation, often having constrained crustal viscosities in the range of ∼1017-1019 Pa s, are higher than those of postseismic deformation in regions where there is less volcanic activity. This suggests that crustal viscosities are lowered by higher geothermal gradients due to the presence of magma beneath volcanic regions. Understanding volcano deformation in terms of the interaction between magmatic activity and rheological properties of the crust also has the potential to offer a dynamical aspect to petrological and geophysical images of a magma chamber. The volcano deformation model is now required to introduce the magma plumbing system into the response of a viscoelastic crust, in which a magma chamber has excess pressure, which varies with time during conservation of the mass of magma, making it possible to predict throughout volcano dynamics from accumulation of magma to eruption in more self-consistent way.

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© 2018 公益社団法人 東京地学協会
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