High He emanations from the source regions of recent large earthquakes, Central Japan

Umeda, Koji; Ninomiya, Atsushi; McCrank, G. F.*

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 Plate. In order to elucidate the geographic distribution of He/He ratios in and around the seismic source regions, new helium isotope data from hot spring gases and water samples with a much denser sampling were determined. Higher He/He ratios were observed in a zone along 37.5N latitude including the source regions than in other regions in the Niigata sedimentary basin. Moreover, the He/He ratios are much higher than the 3.8 RA value, which is the expected He/He ratio at the present time for volcaniclastics of Middle Miocene age. Previous geophysical models of the mechanism for initiation of the 2004 mid-Niigata Prefecture Earthquake proposed that enhanced pore-fluid pressure around the fault system, due to infiltration of aqueous fluids derived from the subducting Pacific Plate slab into the lower crust, reduces frictional strength and triggered the main shock rupture. Geochemical evidence for emergence of hot spring and natural gases with anomalously high He/He ratios also indicates that upward migration of aqueous fluids with mantle volatiles from the subcrustal lithosphere to the Earth's surface could be attributed to the hydraulic gradient along the high permeability of the rupture zone.