Izu-Oshima volcano, one of the most active volcanoes in Japan, started a series of eruptions on Nov. 15, 1986. One of the characters of the eruptions is the small-scale explosions with flashing arcs, which were the visualized shock waves as a phase change of H2O in the air, occurred at the summit lava-lake. The flashing arcs, seen on the movie, show several characteristics; 1) they spread and propagate semi-spherically, and 2) the launching of semi-spherical ballistics follows the flashing arcs. Analysis of the movie indicates that the velocities of flashing arcs were 300-440 m/s and their sources were located almost on the surface of the lava-lake. We paid attention to the ballistics in the light of scaling laws, which were established from field explosion experiments, so that we were able to estimate explosion energy as about 8x10^9 J. Additionally, assuming that the flashing arcs were produced by an explosion, such as a bursting of pressurized-bubbles, we could get information of their condition, a relationship between inner-pressure and the size.

Key words:
Izu-Oshima 1986 eruption, flashing arc, bubble bursting, explosion energy