Abstract
The great eruption of Asama volcano in 1783 Ieft a remarkable deposit of air-fall pyroclastic materials mainly on the east-southeastern foot of the volcano. We have collected descriptions of the pyroclastic fall events of this eruption from about 190 old documents which were recently compiled and published by Hagiwara (1985-1995). The data base created from these documents amounted to ca. 980 records from which thickness data of the deposits were compiled. In this study, we compared three types of thickness data: 1) data obtained in the field, 2) directly measured data as described in old documents, 3) recalculated data from the volume of the deposit as described in old documents. Thickness data obtained in the field, 160 years (Minakami, 1942) and 210 years (Yasui et al., 1997) after the eruption coincide with each other within the limits of error. Two types of thickness data from old records show a well-established trend of sharp decrease with the distance from the vent, but the scatter of data points is so large that it is not possible to draw isopach lines over the entire deposit. Directly-measured thicknesses are about 1.7 times as large as the thicknesses calculated from the volume of the deposit, thus suggesting exaggerations in the former. Thicknesses calculated from the volume of the deposit appear about 2 times as much as the Minakami’s (1942) data suggesting possible post-depositional compaction. However, due to the lack of reliable data, both in old documents and in the measurements on historic deposits of other volcanoes, it was not possible to conclude that the post-depositional compaction plays an important role, suggesting a strong need for further research.
