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Journal Articles

Relationship between $$^{3}$$He/$$^{4}$$He ratios and subduction of the Philippine Sea plate beneath southwest Japan

Umeda, Koji; Kusano, Tomohiro; Asamori, Koichi; McCrank, G. F.*

Journal of Geophysical Research; Solid Earth, 117(B10), p.B10204_1 - B10204_13, 2012/10

 Times Cited Count:3 Percentile:13.24(Geochemistry & Geophysics)

In order to elucidate the geographic distribution of $$^{3}$$He/$$^{4}$$He ratios in Southwest Japan, the data from a total of 924 sites were compiled and synthesized. There appears to be good correlation between variations in mantle helium and the geophysical evidence indicating the configuration of the subducting Philippine Sea plate. The helium isotopes observed on the Earth's surface may be efficient geochemical indicators of the configuration of a relatively younger, warm aseismic slab, especially useful in seismically inactive areas.

Journal Articles

High $$^{3}$$He emanations from the source regions of recent large earthquakes, Central Japan

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

Geochemistry, Geophysics, Geosystems (Internet), 9(12), p.Q12003_1 - Q12003_7, 2008/12

 Times Cited Count:15 Percentile:39.84(Geochemistry & Geophysics)

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 $$^{3}$$He/$$^{4}$$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 $$^{3}$$He/$$^{4}$$He ratios were observed in a zone along 37.5$$^{circ}$$N latitude including the source regions than in other regions in the Niigata sedimentary basin. Moreover, the $$^{3}$$He/$$^{4}$$He ratios are much higher than the 3.8 RA value, which is the expected $$^{3}$$He/$$^{4}$$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 $$^{3}$$He/$$^{4}$$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.

Journal Articles

Helium isotopes as geochemical indicators of a serpentinized fore-arc mantle wedge

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

Journal of Geophysical Research, 112(B10), p.B10206_1 - B10206_10, 2007/10

 Times Cited Count:20 Percentile:47.61(Geochemistry & Geophysics)

New helium isotope data from hot springs on the eastern Kyushu Island were obtained from a large number of sampling points in order to elucidate the geographical distribution of $$^{3}$$He/$$^{4}$$He ratios in the fore-arc regions of Southwest Japan. Systematic, regional variations in $$^{3}$$He/$$^{4}$$He ratios can be identified clearly on between Kyushu Island (0.093-0.79 R$$_{rm A}$$), Shikoku Island (0.21-2.5 R$$_{rm A}$$) and the Kii Peninsula (0.39-7.4 R$$_{rm A}$$). The overall distribution of relative mantle helium reveals a close association with the occurrence of non-volcanic deep low-frequency tremors (LFT), which would be related to the presence of upward aqueous fluids produced by slab dehydration, in Southwest Japan. A flux of $$^{3}$$He carried by aqueous fluids via the mantle wedge is very low in Kyushu Island where the LFT are not found, resulting in a $$^{3}$$He/$$^{4}$$He ratio lower than the atmospheric ratio.

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