Keywords:Kusatsu Shirane, Phreatic eruption, eruption energy
Based on the analysis of broadcasted news movies, explosion energy of the Moto-Shirane eruption on 23rd January was estimated using the method by Ohba et al. (2003). The initial explosion at the summit of the Kagami-Ike North Cone captured by NHK and FUJI TV was funnel-shaped, indicating that the explosion source was very shallow (< 10 m). Compared with the geomorphology captured in the movie, the explosion cloud was estimated as c. 200 m in height, and accordingly, the explosion energy was estimated as 1.5 x 1011 J. According to the experiments by Goto et al. (2002), an explosion with this energy leaves a crater with a diameter of 30 m. A Taiwanese skier captured the successive explosions from another crater in the western base of the cone. The explosions spouted vertically elongated jets which correspond to a scaled explosion depth of 0.004 m/J1/3. The movies provided the information of the crater location, distance from the photographer, and the heights of the clouds (60 - 100 m). The energy of individual explosion ranged from 108 to 109 J in order. Although the exact total number of explosions cannot be estimated from the movie, the number of explosions can be assumed as in order of 102, because the explosions occurred roughly once a second in the movie and the duration of the eruption was less than five minutes. As a result, the total energy discharged from this crater is in order of 1011 J supposedly. The first explosion at the summit was exceptionally big, and the following explosions were two to three orders smaller in energy. The total energy of the following successive small explosions was close to that of the initial explosion (order of 1011 J). Assumed the total energy of entire eruption was 1012 J, mass of water that has equivalent thermodynamic energy of vaporization is 5 x 105 kg (assumed the enthalpy of vaporization is 2000 kJ/kg). The water volume is as small as a sphere with a diameter of 10 m. Assumed high-temperature volcanic gas (assumed as high-T steam here) mixed with cold groundwater at 0.5 MPa (c. 50m in hydrostatic depth) to create high-T steam to explode, the thermodynamic energy of 1012 J is obtained by mixing between 300 – 380 tons of cold water and a larger amount of volcanic gas. The estimated volume of volcanic gas ranges from 800 to 10000 tons, depending on the initial temperatures of volcanic gas and the resultant steam.