1986 年 39 巻 1 号 p. 47-55
Weekly monitoring of radon concentration in soil gas in some active fault areas has been carried out using cellulose nitrate film as a part of earthquake prediction research. As the results of observation, we found increase of track density (tracks/cm2day) along the Median Tectonic Line (distance 100km) and the Matsushiro fault (_??_100km) since summer of 1982 and along the Ateta fault (_??_25km) since summer of 1983. We considered it as a precursor of the Western Nagano Prefecture Earthquake, 1984, by the reason that this increase just fit with the empirical formulas which relate the distance and apperance time of the precursory anomaly against the magnitude of an earthquake.
At the same time, we found an annual variation of track density higher in summer and lower in winter. Supposing this annual variation being due to the seasonal variation of soil temperature, using uranium ore as source of random. As the result of experiment, it was clarified that track density increased with increased exposure temperature.