Abstract
Recently, two shallow MW 6.6 inland earthquakes, the 2004 mid‐Niigata prefecture earthquake and the 2007 Niigataken Chuetsu‐oki earthquake, occurred in the Niigata sedimentary basin near the boundary between the Eurasian and the North American Plates. In order to elucidate the geographic distribution of the 3He/4He ratios in and around the seismic source regions, new helium isotope data from hot spring gases and water samples from much denser sampling than in previous studies were determined. Although temporal changes in the 3He/4He ratios obtained from each well were unknown before and after the earthquakes, higher 3He/4He ratios were observed in the earthquake source regions compared to other regions. Geochemical evidence for the presence of hot spring gases and natural gases with anomalously high 3He/4He ratios (>3.8 RA) indicates infiltration of mantle fluids derived from the subcrustal lithosphere. A plausible source of mantle helium can be attributed to upward mobilization of aqueous fluids generated by dehydration of the subducting Pacific Plate slab, resulting in the acquisition of primordial helium from a mantle wedge as the fluids into the crust. In addition, aqueous fluids may raise the Earth's surface along permeable conduits leading to the reactivation of high‐angle inverted normal faults and the triggering of recent earthquakes.
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