9:00 AM - 9:15 AM
[SVC33-01] On the 2022 thermal evolution of shallow zone beneath the Oana Crater, Azumayama Volcano: Implications from continuous observations of the total magnetic intensity
Keywords:volcano, Azumayama, geomagnetic field, continuous observation, hydrothermal system, inversion
Azumayama Volcano is one of the most active volcanoes in the Tohoku region. While no eruptive phenomena have been observed since the last phreatic explosion in 1977, the volcano has been notably active particularly since the increase of fumarolic activity at Oana crater in 2008. The Japan Meteorological Agency (JMA) started monitoring the activity with multiple item observations in 2001, detecting significant changes that suggested increased activity through several observations in 2014-15 and 2018-19 (Seki et al. 2021). In geomagnetic observations, following the start of the repeat measurement of the total magnetic force (F) in 2003, continuous F observations (six sites around the Oana Crater) have been in operation since November 2015, whereby some very rapid changes (>10 nT/year) were observed. The F data are considered to be one of the most useful information for imaging thermal changes in the shallow area below the crater.
In this presentation, we will report our attempt to estimate changes in the thermal conditions beneath the Oana Crater using a series of the continuous F observation data in 2022, with time resolution that could not be achieved with annual repeat surveys. At the beginning of the year, there was a phase change in the F data from magnetization (cooling) to demagnetization (heating), followed by further acceleration of the demagnetization trend in May. Taking advantage of the large rate of change in the F (thus associated with a relatively high signal-to-noise ratio) that characterizes the volcano's activity, we estimate time evolution of the thermal demagnetization source (as parametrised by the location of a uniformly magnetized sphere and its magnetic moment intensity) that explains the monthly difference of the F at the six observation site using MaGCAP-V (Earthquake and Volcano Research Department, Meteorological Research Institute, 2008). The results show that not only was the horizontal position of the heat source consistently identified slightly north of the Oana Crater over from late April to early October, but interestingly enough, its elevation increased gradually from around 1400 m to 1600 m (see Figure). No clear trend was found in the magnetic moment. Outside of the above period, the intensity of the F changes was not such as to allow a robust inversion for the location of the heat source. Whereas the analysis period is limited to about some four months, our results prove the usefulness of continuous F observations for monitoring of thermal state in a volcano.
The upward process of the heat source detected solely from the F observations may correspond to other observations around the Oana Crater (Volcanic Activity Explanatory Document for December 2022, Sendai Regional Headquaters). (1) In May, an increase in the SO2/H2S ratio was observed by the volcanic gas monitoring equipment northwest of the Oana Crater. (2) From around May to late August, crustal deformation suggesting expansion of the shallow part of the Oana crater was observed by a tiltmeter and GNSS. (3) From September to November, a slight expansion of the geothermal area was observed around the Oana Crater. Integrating these results with the results of this method, one may be able to make clearer the development of the thermal conditions in the shallow part of the Oana crater from May to November. In particular, the rate of increase in the elevation of the thermal source was greatest from mid-June to early August, when both (1) and (2) were at their peak, and a more detailed interpretation of the evolution of the hydrothermal system during the same period may be given. The F data can therefore contribute to a more detailed evaluation of activity than currently, where they are used to monitor the demagnetization and magnetization trends only.
In this report, we show the importance of keeping a close watch on the changes in the continuous observation of the total magnetic field around the Oana crater, where the possibility of further activation cannot be denied. Here, the heat source was simply estimated in a time-discrete manner, but future improvements could include the construction of a continuous time-evolving heat source model. It is expected that methods will be developed to contribute to more sophisticated activity assessment, such as sequentially updating the model as data accumulates.
In this presentation, we will report our attempt to estimate changes in the thermal conditions beneath the Oana Crater using a series of the continuous F observation data in 2022, with time resolution that could not be achieved with annual repeat surveys. At the beginning of the year, there was a phase change in the F data from magnetization (cooling) to demagnetization (heating), followed by further acceleration of the demagnetization trend in May. Taking advantage of the large rate of change in the F (thus associated with a relatively high signal-to-noise ratio) that characterizes the volcano's activity, we estimate time evolution of the thermal demagnetization source (as parametrised by the location of a uniformly magnetized sphere and its magnetic moment intensity) that explains the monthly difference of the F at the six observation site using MaGCAP-V (Earthquake and Volcano Research Department, Meteorological Research Institute, 2008). The results show that not only was the horizontal position of the heat source consistently identified slightly north of the Oana Crater over from late April to early October, but interestingly enough, its elevation increased gradually from around 1400 m to 1600 m (see Figure). No clear trend was found in the magnetic moment. Outside of the above period, the intensity of the F changes was not such as to allow a robust inversion for the location of the heat source. Whereas the analysis period is limited to about some four months, our results prove the usefulness of continuous F observations for monitoring of thermal state in a volcano.
The upward process of the heat source detected solely from the F observations may correspond to other observations around the Oana Crater (Volcanic Activity Explanatory Document for December 2022, Sendai Regional Headquaters). (1) In May, an increase in the SO2/H2S ratio was observed by the volcanic gas monitoring equipment northwest of the Oana Crater. (2) From around May to late August, crustal deformation suggesting expansion of the shallow part of the Oana crater was observed by a tiltmeter and GNSS. (3) From September to November, a slight expansion of the geothermal area was observed around the Oana Crater. Integrating these results with the results of this method, one may be able to make clearer the development of the thermal conditions in the shallow part of the Oana crater from May to November. In particular, the rate of increase in the elevation of the thermal source was greatest from mid-June to early August, when both (1) and (2) were at their peak, and a more detailed interpretation of the evolution of the hydrothermal system during the same period may be given. The F data can therefore contribute to a more detailed evaluation of activity than currently, where they are used to monitor the demagnetization and magnetization trends only.
In this report, we show the importance of keeping a close watch on the changes in the continuous observation of the total magnetic field around the Oana crater, where the possibility of further activation cannot be denied. Here, the heat source was simply estimated in a time-discrete manner, but future improvements could include the construction of a continuous time-evolving heat source model. It is expected that methods will be developed to contribute to more sophisticated activity assessment, such as sequentially updating the model as data accumulates.