11:00 〜 13:00
[SVC34-P11] Estimating the heat flux in Katanuma Crater Lake on Naruko Volcano, Miyagi, Japan
キーワード：鳴子火山、潟沼、熱流束測定
Katanuma is a crater lake on Naruko Volcano, which is famous for its highly acid (pH 2.2) lake water. The last eruption of Naruko Volcano was in 837CE, and the present Naruko Volcano is calm. However, the volcanic gas emission is continuous around and from the central bottom of Katanuma. Even calm volcanoes sometimes erupt without distinct precursors (e.g., 2014 Ontake and 2018 Motoshirane Volcanoes). Naruko Volcano is out of 24-hour monitored volcanoes by JMA, and it is important to evaluate the activity level from field surveys.
The heat flux can be an indicator of volcanic activity level. At Katanuma, the heat flux from the crater bottom was estimated several times in the past (Satake (1975); Sato (1995); Shikano et al. (2004)), based on the idea that vertical heat transfer is hindered by the thermocline during thermal stratification periods and water temperature in the lower layer (hypolimnion) could then increase by volcanic heat supply from the lake bottom. In the same manner, we estimated the heat flux at Katamuma in 2021, as a part of summer field seminar by the Division of Earth and Planetary Material Science, Faculty of Science, Tohoku University, and compared it with the past heat flux.
Heat flux (heat inflow per unit time and unit area; W/m2) is given by dividing the heat flow (total heat inflow per unit time; W) supplied from the lake bottom to hypolimnion by the top surface area of the hypolimnion. Heat flow is given as the product of volume, specific heat and rate of temperature increase of hypolimnion. In order to evaluate these parameters, temporal changes of vertical temperature and the bathymetric map are required for Katanuma. The temperature was recorded hourly at 0, 1, 3, 5, 6, 9, 14 and 19 m depths at the deepest point of Katanuma during June 6 and August 21, with supplementary measurements of 0.1 m and 1 m depth-intervals from the surface to the bottom on June 6 and August 21, respectively. The bathymetric map was made from the depth data measured on August 19, 20 and 21, 2021 at 231 points in total (Figure). The obtained bathymetric map was identical to that by Shikano et al. (2004).
The three previous studies detected the thermocline at 3-5 m depths, but somehow it was formed at much deeper, 9-13 m depths in 2021. As results, the volume and top surface area of hypolimnion were estimated to be 1.39×104 m3 and 5.04×103 m2, both of which were over one order smaller than those in the previous studies. In contrast, the rate of hypolimnetic temperature increase in 2021 (0.105 °C/day) was similar to or to about half of those in the previous studies (0.09~0.21 °C/day). These values gave the heat flow 7.06×104 W and the heat flux 14.0 W/m2. The heat flux was similar to or ~40% of those in the previous studies (15.6~35.8 W/m2), but the heat flow, corresponding to the total heat supply from the lake bottom, was over one order smaller than those in the past.
The decrease in heat flow may reflect the lowering in volcanic activity, but one-year observation is insufficient to conclude. The source of deepening the thermocline is also unclear. More additional observations are thus required to solve these questions.
The heat flux can be an indicator of volcanic activity level. At Katanuma, the heat flux from the crater bottom was estimated several times in the past (Satake (1975); Sato (1995); Shikano et al. (2004)), based on the idea that vertical heat transfer is hindered by the thermocline during thermal stratification periods and water temperature in the lower layer (hypolimnion) could then increase by volcanic heat supply from the lake bottom. In the same manner, we estimated the heat flux at Katamuma in 2021, as a part of summer field seminar by the Division of Earth and Planetary Material Science, Faculty of Science, Tohoku University, and compared it with the past heat flux.
Heat flux (heat inflow per unit time and unit area; W/m2) is given by dividing the heat flow (total heat inflow per unit time; W) supplied from the lake bottom to hypolimnion by the top surface area of the hypolimnion. Heat flow is given as the product of volume, specific heat and rate of temperature increase of hypolimnion. In order to evaluate these parameters, temporal changes of vertical temperature and the bathymetric map are required for Katanuma. The temperature was recorded hourly at 0, 1, 3, 5, 6, 9, 14 and 19 m depths at the deepest point of Katanuma during June 6 and August 21, with supplementary measurements of 0.1 m and 1 m depth-intervals from the surface to the bottom on June 6 and August 21, respectively. The bathymetric map was made from the depth data measured on August 19, 20 and 21, 2021 at 231 points in total (Figure). The obtained bathymetric map was identical to that by Shikano et al. (2004).
The three previous studies detected the thermocline at 3-5 m depths, but somehow it was formed at much deeper, 9-13 m depths in 2021. As results, the volume and top surface area of hypolimnion were estimated to be 1.39×104 m3 and 5.04×103 m2, both of which were over one order smaller than those in the previous studies. In contrast, the rate of hypolimnetic temperature increase in 2021 (0.105 °C/day) was similar to or to about half of those in the previous studies (0.09~0.21 °C/day). These values gave the heat flow 7.06×104 W and the heat flux 14.0 W/m2. The heat flux was similar to or ~40% of those in the previous studies (15.6~35.8 W/m2), but the heat flow, corresponding to the total heat supply from the lake bottom, was over one order smaller than those in the past.
The decrease in heat flow may reflect the lowering in volcanic activity, but one-year observation is insufficient to conclude. The source of deepening the thermocline is also unclear. More additional observations are thus required to solve these questions.