伊豆大島における火山活動の解明と活動評価への適用に向けた合同重力観測

Microgravity measurements have been conducted in Izu-Oshima volcano by Earthquake Research Institute (ERI), the University of Tokyo and Meteorological Research Institute (MRI), Japan Meteorological Agency in order to investigate subsurface volcanic processes. In 2018, ERI and MRI launched a cooperative observational study to better understand these processes and to apply the data to evaluation of volcanic activity. In the cooperative observation, hybrid absolute and relative gravity measurements were conducted to obtain the absolute gravity changes for the whole island. Further, it was intended for the data to be utilized for constraining instrumental properties such as the scale factor of relative gravimeters. The reading-dependent scale factor for the CG-5#033 relative gravimeter obtained in a previous study was checked using gravity values from the two absolute gravity sites at the volcano as well as additional calibration measurement data from sites of the Japan Gravity Standardization Net 2016. We verified that the scale factor correction worked effectively, so that apparent gravity changes of uncorrected data are sufficiently suppressed, and that the corrected gravity differences coincide well with the reference gravity differences obtained at the absolute gravity sites. The hybrid absolute and relative gravity measurements conducted in November every year from 2018 to 2021 revealed the absolute gravity changes for the whole island. The observed yearly gravity changes reached a maximum of 39 μGals. Relative gravity changes obtained by the CG-5#033 since 2008 show large gravity disturbances of about 80 μGals, particularly between sites with larger altitude differences. Such observed yearly absolute gravity changes and large relative gravity disturbances cannot be explained solely by the free-air effects predicted by GNSS ellipsoidal height changes and －0.3086 mGal/m of vertical gradient of the normal gravity, and are probably caused by infiltration of meteoric water into the volcanic edifice. At the central cone, Mt. Mihara, where continuous subsidence is observed by GNSS, a continuous gravity increase has also been observed. The rate of the observed gravity increase is 6.6-11.1 μGal/year, which is higher than 3.5-3.6 μGal/year, the value predicted by free-air effects. Currently, the cause of such large residual gravity changes is uncertain. Possible causes include gravitational effects by a continuous contractive deformation and mass relocation due to frequent collapses of the crater wall, which occurred at Mt. Mihara, as well as mass addition at depth: specifically, magma intrusion into subsurface void spaces as often reported in basaltic volcanoes.

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書誌情報

ja : 東京大学地震研究所彙報
en : Bulletin of Earthquake Research Institute, the University of Tokyo

巻 97, p. 13-32, 発行日 2023-03

ISSN

収録物識別子タイプ

PISSN

収録物識別子

00408972

書誌レコードID

収録物識別子タイプ

NCID

収録物識別子

AN00162258

出版者

東京大学地震研究所

言語

出版者

Earthquake Research Institute, The University of Tokyo

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2023-02-22 08:49:27.956120

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