9:15 AM - 9:30 AM
[SVC33-02] Thermal demagnetization in March 2021 and recent volcanic activity at Tarumae Volcano
Keywords:magnetic changes, thermal demagnetization, crustal tilt change, multi-parametric observation, volcanic activity monitoring
Introduction
Tarumae Volcano, located in the southwestern part of Hokkaido, is an active volcano that began its volcanic activity approximately 9,000 years ago. Since the last magmatic eruption in 1909, small phreatic eruptions took place repeatedly until 1981, and after that it has been generally calm. However, the summit lava dome continues to be thermally active from 1999.
Since 1998, the Sapporo Regional Headquarters has conducted geomagnetic total field repeat surveys at Tarumae Volcano, approximately once a year. From 2011 and onwards, Hokkaido University added survey points and has continued surveying with JMA. Furthermore, JMA started continuous observation at six sites on the volcano in 2015.
In this presentation, we discuss the thermal demagnetization event from March 2021, which has detected by these monitoring efforts.
Magnetic changes detected by geomagnetic continuous observation and repeat survey
Prolonged magnetic field changes suggesting cooling re-magnetization due to the thermomagnetic effect have been observed since 2010. Subsequently, some of our continuous stations detected a sharp trend reversal implying a turnaround to demagnetization around March 2021 (Japan Meteorological Agency, 2022). After that, the demagnetizing trend slowed down, but was followed by a re-acceleration in the spring of 2022. It slowed down again by the fall of 2022, and were almost flattened out by early 2023.
A demagnetization source assuming a single magnetic dipole was estimated at a depth of about 200 m beneath the lava dome in our analysis. It is even shallower than that of Meteorological Research Institute (2008). Note that the misfit between the theoretical and observed values are large at some survey points on the southeastern side of the dome. These outliers may reflect the geomagnetic changes related to the localized thermal activities at Crater A and Fissure H that distribute near the relevant survey points.
Recent volcanic activities since March 2021
There was a temporary B-type earthquakes swarm beneath the dome on the 9th March, and a small volcanic tremor on the 18th March, 2021. During the subsequent period of thermal demagnetization, pulsating tilt changes towards the direction of the dome have been repeatedly recorded by the bubble-type borehole tiltmeter at about 500 m northwest from the dome. In addition, the slowdown of the thermal demagnetization since late 2021 was accompanied by an increase in fumarolic emission from some vents. Furthermore, the ground temperature at the north of Crater A temporarily was elevated up to around 100ºC from June to October 2021. Meanwhile, constantly-occurring B-type microseismicity in the shallow part beneath the dome calmed down during the period of above-mentioned demagnetization and tilt events. Interestingly, the seismicity restarted when the demagnetization and tilting events resumed.
For future volcano monitoring
The repeat survey has a relatively high spatial resolution, while the continuous observation has excellent temporal resolution. The thermal demagnetization event since 2021 at Tarumae Volcano is a good example of the combination use of the repeat survey and the continuous observation to reveal such small-scale magnetic field changes.
In order to better understand the physical processes related to the sequential demagnetization events, it would be necessary to compare them with other kinds of monitoring data, as well as the similar events of other volcanoes.
Tarumae Volcano, located in the southwestern part of Hokkaido, is an active volcano that began its volcanic activity approximately 9,000 years ago. Since the last magmatic eruption in 1909, small phreatic eruptions took place repeatedly until 1981, and after that it has been generally calm. However, the summit lava dome continues to be thermally active from 1999.
Since 1998, the Sapporo Regional Headquarters has conducted geomagnetic total field repeat surveys at Tarumae Volcano, approximately once a year. From 2011 and onwards, Hokkaido University added survey points and has continued surveying with JMA. Furthermore, JMA started continuous observation at six sites on the volcano in 2015.
In this presentation, we discuss the thermal demagnetization event from March 2021, which has detected by these monitoring efforts.
Magnetic changes detected by geomagnetic continuous observation and repeat survey
Prolonged magnetic field changes suggesting cooling re-magnetization due to the thermomagnetic effect have been observed since 2010. Subsequently, some of our continuous stations detected a sharp trend reversal implying a turnaround to demagnetization around March 2021 (Japan Meteorological Agency, 2022). After that, the demagnetizing trend slowed down, but was followed by a re-acceleration in the spring of 2022. It slowed down again by the fall of 2022, and were almost flattened out by early 2023.
A demagnetization source assuming a single magnetic dipole was estimated at a depth of about 200 m beneath the lava dome in our analysis. It is even shallower than that of Meteorological Research Institute (2008). Note that the misfit between the theoretical and observed values are large at some survey points on the southeastern side of the dome. These outliers may reflect the geomagnetic changes related to the localized thermal activities at Crater A and Fissure H that distribute near the relevant survey points.
Recent volcanic activities since March 2021
There was a temporary B-type earthquakes swarm beneath the dome on the 9th March, and a small volcanic tremor on the 18th March, 2021. During the subsequent period of thermal demagnetization, pulsating tilt changes towards the direction of the dome have been repeatedly recorded by the bubble-type borehole tiltmeter at about 500 m northwest from the dome. In addition, the slowdown of the thermal demagnetization since late 2021 was accompanied by an increase in fumarolic emission from some vents. Furthermore, the ground temperature at the north of Crater A temporarily was elevated up to around 100ºC from June to October 2021. Meanwhile, constantly-occurring B-type microseismicity in the shallow part beneath the dome calmed down during the period of above-mentioned demagnetization and tilt events. Interestingly, the seismicity restarted when the demagnetization and tilting events resumed.
For future volcano monitoring
The repeat survey has a relatively high spatial resolution, while the continuous observation has excellent temporal resolution. The thermal demagnetization event since 2021 at Tarumae Volcano is a good example of the combination use of the repeat survey and the continuous observation to reveal such small-scale magnetic field changes.
In order to better understand the physical processes related to the sequential demagnetization events, it would be necessary to compare them with other kinds of monitoring data, as well as the similar events of other volcanoes.