Aeromagnetic constraints on the subsurface structure of Usu Volcano, Hokkaido, Japan
Shigeo Okuma 1 2 Tadashi Nakatsuka 1 Yoshihiro Ishizuka 11 Geological Survey of Japan, AIST, AIST Tsukuba Central 7, Higashi 1-1-1, Tsukuba 305-8567, Japan.
2 Corresponding author. Email: s.okuma@aist.go.jp
Exploration Geophysics 45(1) 24-36 https://doi.org/10.1071/EG13041
Submitted: 30 April 2013 Accepted: 26 September 2013 Published: 19 November 2013
Abstract
Usu Volcano, Hokkaido, Japan consists mainly of dacitic volcanic rocks underlain by basaltic somma lava and Pliocene–Pleistocene andesitic volcanic rocks, and erupts every 20–30 years. The most recent eruption, in 2000, was the first since 1978. We conducted a helicopter-borne high-resolution aeromagnetic survey almost three months after the start of this eruption. We calculated magnetic anomalies on a smoothed observation surface using a reduction method, assuming equivalent anomalies below the actual observation surface. We conducted three-dimensional (3D) imaging of magnetic anomalies to constrain the subsurface structure. Our model indicates that there are magnetisation highs in the main edifice of Usu Volcano, which may reflect the subsurface distribution of the Usu somma lava. Meanwhile, magnetisation lows lie north-west of the Nishi-Yama Craters Area and on Higashi-Maruyama Cryptodome, where nearby Pliocene and Pleistocene volcanic rocks, respectively, are found. The reverse magnetisation observed at outcrops close to these sites could explain the magnetisation lows.
Although the survey improved our understanding of the surface and subsurface distribution of volcanic rocks in the edifice and basement of Usu Volcano, some limitations remain. No information about the magmas intruded during the recent eruptions in 1977–1978 and 2000 was obtained by the survey, though some of these intrusions were revealed by other geophysical data. The small magnetic contrast between the intruded magmas and their host rocks is the most probable reason. Perhaps the intruded magmas (in particular, those of the most recent eruption) had not cooled enough to become strongly magnetised by the time the survey was conducted.
Key words: aeromagnetic survey, lava, magnetic anomaly, Usu Volcano, volcanic rocks.
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