Fig. 6
Probability density function of explosion depth of a Strombolian explosion. The probability density function (PDF) of the explosion depth dexp is estimated from the PDFs of both ΔtsH–iH (Fig. 3b) and ccon (Fig. 5). The white dashed line at a depth of 300 m indicates the upper edge of the crack (1 km height), which is the source of the VLP events (Yamamoto et al. 1999). The total PDF in the region surrounded by a black line (PDF = 0.003) is over 0.5 (i.e., a high probability of occurrence). This diagram suggests that explosions mostly occurred in the region shallower than the crack top