Tsukasa Ohba

Effects of explosion energy and depth to the formation of blast wave and crater: Field Explosion Experiment for the understanding of volcanic explosion

We made field explosion experiments as an analogue of volcanic explosion to understand the relationship between the explosion condition and the resultant surface phenomena. The main parameters we employed were explosion depth and explosion energy. Through the experiments we confirmed that scaled depth, which is the depth divided by cube root of energy, is the main parameter determining the properties of explosive volcanism. The energy assigned to blast wave decreased exponentially against the scaled depth. The scaled crater diameter became maximum when the scaled depth was about 4 × 10−3 m/J¹/³. Scaled crater diameters by nuclear, chemical subsurface and some volcanic explosions were almost the same. From the scaling law, the overpressure at crater rim was estimated to be several MPa, which corresponded to typical rock strength. Probably the ground-forming materials were broken inside the area where overpressure exceeded their strength.

Effect of explosion energy and depth on the nature of explosion cloud: A field experimental study

Field explosion experiments using dynamite were carried out to investigate the effect of explosion energy and depth on the nature of explosion cloud. The scaling law of explosion was established among the energy, depth, and the nature of explosion cloud; shape, height and duration of flow out. The shape of explosion cloud changed systematically from funnel type via elongated to short types with increasing the scaled depth of explosion. At the shallow scaled depth, both scaled height and scaled duration increased with increasing scaled depth. Both values reached the maximum at 0.003–0.004 m/J1/3 of cube-root scaled depth, and then decreased with increasing scaled depth. If the cube-root scaled depth was over about 0.01 m/J1/3, we could not observe any explosion cloud on the ground surface. These experimental results should be applicable to the understanding of volcanic explosions if the buoyant effect of internal heat in the explosion cloud was negligible and ground medium is identical. We applied these results to the explosion clouds of Usu phreatic explosion at 14:53 h on 17 April 2000, and estimated that the explosion energy and the depth were 4×109 J and 6 m, respectively. Errors caused by uncertainty of scaling factors are less than one order of magnitude for the energy and depth.