Permalink : http://hdl.handle.net/2115/32757

JaLCDOI : 10.14943/gbhu.71.19

Spatial distribution of volcanic blocks, as well as landing distance from a crater, landing angle and diameter of the blocks has been well investigated on and around the crater in cases of the 1938 Asama and the 1977 Usu eruptions by some researchers. Then, this paper reproduces detailed kinematic aspects of the eruptions so as to interpret the above-mentioned field data, on the basis of a generalized ballistic equation of ejected volcanic blocks proposed by the present authors. Calculations are made for combination of various parameters, such as air resistance to the blocks, direction of the wind, orientation dependence of the initial velocity and so on. The observed wind velocities are referred in the calculation. Comparison between the calculated and the observed results leads to the following conclusion: (1) In the 1938 Asama eruption, the distribution of volcanic blocks shows that the direction of the explosion principal axis is due east (N90°E) and its inclination angle is 30°. The angle dependence of the initial velocity (M = 1.5) is almost the same as that in the Sakurajima volcano. (2) In the 1977 Usu eruption, direction of the explosion principal axis is N170°E and its inclination angle is 20°, and the angle dependence of the initial velocity is large as much as M = 5.0. (3) The air resistance to the blocks is estimated as about Kd = 0.5 in both the eruptions. This value is valid in other volcanic eruptions because the air resistance seems to be same everywhere.