[S-GD01] 地殻変動

[SGD01-01] アイスランド南西部Krisuvik火山における粘弾性地殻変動に関する予備研究

*山崎 雅¹、Freysteinn Sigmundsson²、Sigrún Hreinsdottir³、Halldór Geirsson² (1.産業技術総合研究所地質調査総合センター、2.アイスランド大学ノルディック火山センター、3.GNSサイエンス)

Crustal viscoelasticity plays an important role in crustal deformation, particularly at active volcanoes beneath which an effective crustal viscosity is likely to be lowered by the presence of magma. However, volcano deformation has usually been examined in terms of crustal elasticity, in which the crust instantaneously responds to the incremental/decremental volume change of a deformation source. Thus, crustal viscoelasticity should be examined for more realistic interpretation of geodetic data precisely measuring volcano deformation. This preliminary study evaluates geodetic data from the Krisuvik volcano, southwest Iceland, to explore the applicability of crustal viscoelasticity. We use available GNSS time series of displacements from two sites in a period of 2007-2019: at Krisuvik (KRIV) and Mohalsadalur (MOHA). The data show surface uplift followed by continuous subsidence, similar to that reported in some other volcanoes, e.g., Kutcharo Caldera and Yellowstone Caldera. Simple viscoelastic model with a sill-like deformation source can be used to fit observed post-inflation subsidence, where the inflation source is at ~0.5 km west of and ~0.5 km south of MOHA and at ~4 km north of KRIV, at a depth of ~5 km. Under the assumptions made in the model, the thickness change of the sill at the centre at the end of the inflation period is ~1-2m, and the effective crustal viscosity is significantly lower than that regionally estimated from glacio-isostatic deformation in Iceland (in response to thinning ice caps). The overall first-order characteristic of the post-inflation surface displacement is sufficiently satisfied by adjusting the viscoelastic model parameters. However, it is found that the present model can not explain some features of the observations. In particular, the post-inflation subsidence appears to increase linearly with time, rather than exponentially, and later in the period the surface has subsided to a lower elevation than that before the surface inflation initiated. These clearly indicate a necessity of additional subsidence mechanism, possibly including depressurised deformation source due to degassing and/or regional plate spreading. This study shows the potential of crustal viscoelasticity to explain volcano deformation as a first-order approximation, but a higher ordered better fitting to the data requires other additional deformation mechanisms.