Abstract
On 31 March 2000, the Usu volcano located in Hokkaido, Japan began erupting after 22 years of dormancy. After the first phreato‐magmatic explosion, there were many small‐scale phreatic explosions at the Nishiyama and Kompirayama Craters. In April and May, observations were made using a piezo blast‐sensor and a visible video recorder to get a better understanding of volcanic explosions. The data thus obtained were analyzed using the scaling‐laws that relate surface phenomena to both explosion energy and depth. The analysis using the shape of the explosion‐cloud and the peak over‐pressure of the volcanic blast‐wave indicated that the average explosion energy and depth of the phreatic explosions of mid‐April ranged from 1010 to 1012 J and originated from between just beneath the ground surface to a depth of a few tens of meters.
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