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

内陸地震震源域からの高Heガスの放出

梅田 浩司; 二ノ宮 淳; McCrank, G. F.*

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

活断層からCO, HO, H, Rn, He等の揮発性物質が放出されているという観測事例が多く報告されている。このうち、不活性ガスであるHeは、地殻中での化学反応を生じにくいことから、地下深部の化学的な情報を得るための重要な指標の一つである。これまでにも1984年長野県西部地震の前後に遊離ガス中のヘリウム同位体比(He/He比)が増加したことやSan Andreas断層ではマントル・ヘリウムが断層に沿って上昇していることが指摘されている。本研究では、新潟平野の活断層や最近の巨大内陸地震震源域において、温泉ガスのヘリウム同位体比の測定を行った。その結果、これらの地域では高いヘリウム同位体比が認められることから、遊離ガスや地下水に含まれるヘリウム同位体比は、変動地形学的根拠に乏しい活断層を検出するための有効な地球化学的指標であることが示唆される。

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.