Forest damage by surge Pyroclastic surges accompanied strong wind and blew down a large quantity of trees in the depositional area. Zones of seared trees produced by hot ash clouds are distributed around the surge deposition area. Fine ash-fall deposits associated with pyroclastic flows are dispersed around the volcano. The deposits covered hillslopes and caused debris flows owing to low infiltration rates at the early stage of the eruption. Beginning in 1991, debris flows were generated by rainstorms and damaged more than a thousand houses at the foot of the volcano.
Pyroclastic flows can be one of the most lethal volcanic phenomena and severely damage our life.
We have to consider the generation of pyroclastic flows for volcanic hazard assessment at Unzen.