Abstract
The Usu Volcano erupted and the destructive pyroclastic flow and surge struck the Abuta settlement in Hokkaido, northern Japan on March 23, 1822. Currently the degree of burns suffered by the victims from the pyroclastic flow and its connection to the distance between disaster-stricken sites and the crator have not been determined.
The purpose of this study is to estimate the degree of burns of the Wajin （ancient Japanese people） and Ainu people suffered from the eruption of Usu Volcano in 1822. To estimate the degree of burns, historical records based on the direct observations of the burnt bodies were used, with the aid of the minute reports on the casualties by Unzen-Fugen-Dake Volcano eruption in Kyushu, south-western Japan in 1991.
Six Wajin and 72 Ainu perished due to the pyroclastic flow and surge from Usu Volcano. The six Wajin died within 48 hours and the 72 Ainu people were regarded to have died within 24 hours after their being burned.
In the case of the six perished Wajin, and 44 perished Ainu people, at least 80% of their body’s total surface area were regarded to have been covered by serious burns （deep burns and/or dermal burns）. Their Prognostic Burn Index （P.B.I.） were regarded to be at least 100 respectively. Even today, in spite of the modern medical treatment available, the surviaval rate of people suffering from such burns is under 30%.
On the other hand, in the case of the 28 perished Ainu people whose bodies were not found, at least 80% of their body’s total surface area were covered by the most serious burns （deep burns）. Their P.B.I. were regarded to be at least 120 respectively. The degree of burns of these 28 perished people almost corresponds to the P.B.I. level where the survival rate is “very low”, even with the modern medical treatment offered by hospital burn wards.
Many of these six perished Wajin and 72 perished Ainu people had severe facial burns. So they were regarded to have suffered serious inhalation injuries. The degree of burns suffered by victims who were closer to the Usu Volcano crator was more severe than those victims who were farther from the crator. And the degree of burns suffered by the Ainu living in more flammable thatched houses was more severe than the Wajin in less flammable wooden houses on the coast.
Two people, one Wajin and one Ainu, were forced to enter into the sea by the pyroclastic flow and surge and survived. They received burns on the tops of their heads, necks, and upper parts of their backs and shoulders. In the case of these two people, 5～15% of their total body surface area was regarded to have been covered by burns （epidermal burns and/or superficial dermal burns） without any inhalation injuries. The degree of burns of these two people corresponds to the lowest level of burn that patients can be treated for without entering a hospital.
The degree of burns suffered by the victims had the tendency of “distance decay”, in which the extent of the burns was less severe the further the victims were from the crator.
