ABSTRACT

The Iwate-Miyagi Nairiku Earthquake in 2008 (M7.2) occurred on June 14th 2008 in the northeastern Japan, Iwate Prefecture. We collected water and gas samples in hot springs from seven localities around the epicenter of the earthquake on June 21st and 22nd, one week after the earthquake, and measured the ³He/⁴He ratio and He and Ne concentrations of the dissolved gases. We found 10-85% increases in ³He/⁴He ratios in five hot springs, while we found 11-35% decrease in two others. The positive anomalies were interpreted as upwelling of aqueous fluid containing mantle ³He beneath the earthquake region. It is possible that ascending fluids triggered the earthquake.