Increased radon and thoron in the Verkhne–Paratunka hydrothermal system, southern Kamchatka prior to the catastrophic Japanese earthquake of March 11, 2011

This paper reports results from the first-ever simultaneous measurements of the volumetric activities of radon and thoron in subsoil gas and in the near-surface air. These measurements were in progress during the precursory period and occurrence of the March 11, 2011 disastrous earthquake in Japan at a distance of 2000 km from the observation site. The earthquake was preceded by an anomalous increases in radon and thoron activity in the subsoil gas, and afterwards in the air, 44.7 days prior to the earthquake. The anomalous values persisted for 24.9 days. We discuss the features and causes of these increases.