Anomaly in atmospheric radon concentration: a possible precursor of the 1995 Kobe, Japan, earthquake.

Clear seismic-related anomalies in the radon (222Rn) concentration of the atmosphere were observed prior to the Kobe earthquake (magnitude 7.2) on 17 January 1995. The radon anomalies were observed at a monitoring station in Kobe, which is located about 20 km away from the epicenter. The means of radon concentration in the atmosphere for each day were calculated using the data observed between April 1984 and March 1994, in order to obtain the normal radon variation. The difference between the smoothed radon concentration and the smoothed mean radon concentration is the residual value. Using the weekly averages of residual values in the historical period, the weekly residual value in the validation period were predicted. The historical period was from April 1984 to March 1994. The validation period was from April 1994 to January 1996. The seismic-related radon anomaly higher than the 99% confidence limit of the residual value of radon concentration in the atmosphere was observed beginning about 2 mo before the earthquake.