open access

https://doi.org/10.14943/gbhu.88.53

https://hdl.handle.net/2115/94328

Geodetic Observation around Mt. Usu by Leveling Survey

日本語

2025-03

北海道大学大学院理学研究院

Geophysical bulletin of Hokkaido University

88

53-74

It is widely known that Usu volcano experiences large-scale ground deformation with each eruption. The Geospatial Information Authority of Japan and Hokkaido University have conducted repeated leveling surveys around Usu volcano, capturing ground deformation during both eruptive and quiet periods. We analyzed past survey results to understand the spatial pattern of vertical deformation around Usu volcano and detect variations due to accumulation and intrusion of magma underground. During quiet periods between eruptive activities, signals of uplift patterns increasing westward along the coast of Uchiura Bay and subsidence over the entire mountain after the 2000 eruption were observed. In the early stage of the 2000 eruption, leveling surveys confirmed an overall uplift of the volcano. Using a tensile crack model and Markov Chain Monte Carlo (MCMC) analysis implemented in the pydeform software, we estimated an inflation source located approximately 2 km underground at the center of the mountain, tilted slightly southward in a sill- like shape. The location and inclination of this inflation source align well with the hypocenter distribution of pre-eruption seismic activity, suggesting that the overall uplift at the initial stage of the 2000 eruption was caused by this inflation source. To analyze deformation during quiet periods caused by magma accumulation to deep magma reservoirs, we estimated parameters for vertical displacement observed along the route passing through the southern foot of Usu volcano and along the Uchiura Bay coast between 1985-1986 and 1992, as well as between 2004 and 2010, assuming a spherical pressure source. When no constraints were applied to the parameter search, the optimal solution estimated the source to be located 20 km underground in the landward area north of the survey route, with a radius of 400 m. However, when constrained the depth of the source based on prior petrological studies, the estimated inflation source was at a depth of 10 km with a radius of 3-4km.

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