Unraveling the hazards of 1707 eruption of Fuji volcano using paleomagnetism and historical documents

The 1707 eruption of Fuji volcano is the latest and largest explosive eruption through its eruption history. It caused an unprecedented natural disaster and great damage to the people's lives in the Hoei era of Edo period, so it is called the Hoei eruption in Japan. By using paleomagnetic methods in addition to the interpretation of historical documents and the tephrostratigraphy, we found some evidence that pyroclastic fall deposits of the early phase of eruption kept high temperature after the emplacement. The Hoei eruption continued sixteen days from about 10:00 am 16th December 1707 to 1st January 1708. According to the historical documents and geological data of distal facies (e.g. Miyaji et al., 2011), the Hoei eruption discharged a large amounts of fall out tephra of 1.8 km3 in total, and some of villages on the eastern foot of the volcano were buried in the early stage of eruption. Those tephra are divided into four units, Ho-I, II, III, and IV, demonstrating upward compositional change from dacite to basalt. Although the previous studies on distal facies of the fall out tephra are successful to understand the eruption sequence and its scale, the proximal facies of the Hoei eruption have rarely been investigated after Tsuya (1955). Therefore, we obtained paleomagnetic directions of thermal demagnetization from the proximal facies of the Hoei eruption, and clarified the NRM of the lithics have high and low temperature components bounded at 340 or higher in the erosional valley of Goten-niwa, which is a part of pyroclastic cone with relative height over 20 m at the southeastern end of the Hoei crater rim. The volcanic blocks of essential material show a gradual upward compositional change from andesite to the basalt, so it is certain that the lithics were reheated in the units Ho-II. The units Ho-II represents the fall out tephra of the eruptions between the evening of 16th to 17th December, when were most violent explosive eruptions. The old painting illustrating the evening of 16th December shows the red heat volcanic ejecta from the vent is traveling to the southeastern direction. And other illustration in Motohiro-koki, daybooks of chief retainer in the Tokugawa shogunate, shows the transformed landscape after Hoei eruption on February 1708, when the pyroclastic cone of Goten-niwa did not accumulated snow in spite of winter.