RECONSTRUCTION OF COLUMN HEIGHT CHANGE DURING THE 1707 ERUPTION OF FUJI VOLCANO, JAPAN

The 1707 eruption of Fuji volcano, known as the Hoei eruption, caused the worst ash fall disaster in the Japanese history. Despite the availability of a large number of historical documents on this eruption, the detailed eruption sequence was obscure since the correlation between the chronological and geological sequences remained robust in previous studies. We reconstructed the sequential change in column height from the detailed correlation between the timelines reestablished from historical documents and geological units.

The eruptive deposit was subdivided into 17 units on the basis of their facies, and isopach maps were drawn for each unit. The mass of each unit was obtained using the method proposed by Fierstein and Nathenson (1992). Since eruption column height is a function of magma discharge rate (Woods, 1988), the column height of each unit was obtained from the mass and duration of the eruption. The duration was inferred from historical documents. The correlations of each unit to the timeline were based on the colour of tephra, grain size, pattern of ash fall, unconformity assumed to be formed by rainfall, and ash fall distribution.

While reestablishing the timeline of the unit boundaries, we found that not all unit boundaries represent the hiatus or relenting phase of ash fall. We detected only six obvious quiet intervals from historical documents. Many of the boundaries may represent hiatus or relent phases that are too subtle to detect from the historical documents. We defined an eruptive pulse as the period of continuous tephra fall divided by the obvious quiet interval. We then divided the course of the Hoei eruption into 3 stages on the basis of the pattern of eruptive pulses. The characteristics of the three stages are described as follows.

Stage I is characterized by quick firing of two energetic eruptive pulses (pulses 1 and 2; up to 25 km high column), with each of them showing intense outburst initially, followed by a decrease in intensity. Stage II consists of discrete firing, resulting in the formation of a relatively low eruption column (≤15 km). Stage III is principally characterized by sustained column activity without a clear repose time. In stage III, the column height appears to be always above 13 km and at least three distinct active periods are recognized in which the column height is presumed to exceed 16 km.