A pyroclastic flow accompanies a pyroclastic surge. A pyroclastic surge killed many people who were taking photographs on a far]off hilltop when a large pyroclastic flow occurred at Mount Unzen in 1991. Volcano hazard maps must include information on pyroclastic surges as well as debris flow, lava flow, and pyroclastic flow. Currently, in studying volcanic hazard maps the area one kilometer around an area prone to pyroclastic flow is designated as the area where a pyroclastic surge may reach. This seems too large. In this paper, data on the pyroclastic surge that occurred at Mount Unzen were analyzed. It was found that the distance the pyroclastic surge travels is proportional to the velocity of the pyroclastic flow. An empirical equation was obtained. This equation will be applied when making a volcano hazard map. It was also found that a pyroclastic surge leaves the pyroclastic flow's main body and flows separately when the pyroclastic flow bends at an angle larger than 20 degrees. These results may be applied in analyzing pyroclastic surges associated with the lava dome collapse type pyroclastic flow. More research is needed on other types of pyroclastic flow.


Key wordsFpyroclastic surge, hazard map, volcano, Mount Unzen