Radon Anomalies in Ground Water before Earthquakes in Tokyo

Abstract

Radon contents in ground waters in Tokyo have been measured since 1976. The correlation between earthquake and radon anomaly will be evaluated easily, when both earthquakes and radon anomalies are a few. In addition, the high reliability of the correlation will be obtained, if an earthquake and an anomaly occur at almost same time. The six earthquakes occurred in 1976'1990 were chosen based on the magnitude (>6.0) and the epicentral distance (<100 km). Radon anomalies shortly before the six earthquakes were investigated at the stations where few anomalies have been detected. Anomalies which can be considered to relate with earthquakes appeared near around the dates of the Ibaraki-Chiba (1985) and the Yamanashi-Kanagawa (1983) earthquakes. The anomalies appeared in 6 d before-4 d after the earthquakes, and no other anomalies had appeared in over 600 d before the earthquakes. Then it is presumed that these anomalies would be earthquake precursors. The anomalies were found at the stations distributed in 50-70 km epicentral distances and on the compress quadrants of the earthquake mechanism.