Abstract
In Japan, permafrost had been believed to be present on Mt. Fuji (3776 m asl.). However, available information on the permafrost on Mt. Fuji is mostly derived from studies undertaken in the 1970s and there has been no direct monitoring of ground temperature remaining at or below 0°C over one year. From 2008, we started monitoring ground temperature and related meteorological parameters near the summit. Surprisingly, our 3-m deep borehole showed the absence of permafrost even though the site is located in a cold climate with a mean annual air temperature below -5°C. Thus, we focused on the unknown mechanism that is keeping the ground warm on Mt. Fuji. In this study, we compare temporal variations of ground temperature with rainfall events during the ground-thawing period in 2009. A rapid increase of ground temperature immediately followed rainfall. Thawing of the ground was promoted by warm rainfall or cold rainfall getting heat from the warm surface soil when the water permeated the soil. Ground electrical resistivity indicates a dry surface above the seasonally frozen layer at the monitoring site in mid-summer. The high permeability of the volcanic debris usually keeps the ground dry and effectively allows heat transportation by rainfall infiltration. This study indicates that, in addition to heat conduction, heat transportation by rainfall infiltration has to be taken into account for ground thawing in the high mountains of Japan, because of warm humid conditions in summer.
