High ³He emanations from the source regions of recent large earthquakes, central Japan

Abstract

[1] Recently, two shallow M W 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 3 He/ 4 He 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 3 He/ 4 He ratios obtained from each well were unknown before and after the earthquakes, higher 3 He/ 4 He 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 3 He/ 4 He 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.

Hydrogeochemical Characteristics and Genesis of Geothermal Water from the Ganzi Geothermal Field, Eastern Tibetan Plateau

“…Noble gases can be used as a basic indication of gas origins due to their chemically inert nature. Helium is an excellent tracer for demonstrating the influence of crustal and mantle volatiles [63][64][65]. 3 He is derived from the mantle, and 4 He is mainly produced by the radioactive decay of U and Th in the crust [66,67].…”

Helium Isotopic Signature of a Plate Boundary Suture in an Active Arc–Continent Collision

“…the Pyrenees; the Urals) and deep structures (e.g. the Yarlung-Zangbo suture; Japan; Italy). ,− For example, the source of 3 He close to the Yarlung-Zangbo suture is likely either asthenosphere accessed by faults and shear zones that cut through subducting Indian lithospheric mantle or incipient melt of the Asian lithospheric mantle at the Moho north of the northern edge of underthrust India (the “mantle suture”) which must therefore lie close to the Yarlung-Zangbo suture . This fluid transfer may also be influenced by earthquakes, where changes in volumetric strain drive fluid migration and affect the helium isotope ratios (e.g.…”

A large intraplate earthquake triggered by latent magmatism

“…Helium, because it is characterized by high mobility, a monoatomic structure, and low solubility in water, is an excellent natural tracer for elucidating mantle‐crust interactions in different geotectonic provinces. Because 3 He is essentially primordial and retained in the Earth's interior, whereas 4 He is mainly produced in the crust by the decay of U and Th, helium isotopic variations are considered to provide not only potentially useful information for indicating the addition of mantle volatiles but also provide a way to determine migration pathways from the subcrustal lithosphere [e.g., Kennedy et al , 1997; Güleç et al , 2002; Umeda et al , 2008]. …”