Radon, Cl− and SO42− anomalies in hot spring water associated with the 1995 earthquake swarm off the east coast of the Izu Peninsula, Central Japan

Radon concentrations were continuously monitored in hot spring water in a 200-m-deep well in the Yugano hot spring area, Izu Peninsula, Japan from July to December 1995. Concentrations of Cl− and SO42− were measured in the hot spring water about once a month from May to December 1995. The Rn concentrations in the hot spring water increased significantly in September and October 1995, when the 1995 earthquake swarm off the E coast of the Izu Peninsula occurred at a distance of about 30 km from the observation well. The 1995 earthquake swarm began on 11 September and became most active from the end of September to the beginning of October. The Rn concentration rose gradually from 8 September, 3 days before the onset of the swarm activity, increasing by about 50% by 17 September. It remained high in October but had returned to normal by the end of November. However, Cl− and SO42− concentrations doubled suddenly from 22 to 23 September and remained high until the end of November. A good correlation between Cl− and SO42− concentrations suggests the same mechanism for their anomalous increases, probably mixing of water with high Cl− and SO42− concentrations caused by crustal deformation related to the seismic swarm activity. However, the anomalous increase in Rn concentration, which began 15 days before these anion increases, cannot be explained by the same water mixing mechanism. A possible mechanism for the anomalous Rn increase is the formation of microcracks caused by compressional stress, which preceded the onset of the earthquake swarm.