Geochemical features of gases and rocks along active faults

Abstract

Gases and rocks along several active faults in central Japan were analyzed to understand their chemical characteristics and to collect information on gases at depth which are useful for the geochemical earthquake prediction. The gases along fault zones possess chemically different characters according to their mode of occurrence. (1) Gases charged mainly within fault gouges (Type I) are characterized by high concentrations of H₂ and CO₂ and lack CH₄. CO₂ issues mainly through the intensely sheared zone of a fault where fracturing of rocks is most common. He/Ar ratio calculated to be high in the lithospheric airs at depth, however, is not always high in Type I gases. (2) Gases occurring as bubbles in flowing mineral waters from fault zones (Type II) show high concentrations of He and CH₄; they generally lack CO₂. The mineral waters examined are usually of high alkalinity and hence CO₂, if generated at depth, will be dissolved in the water during uprising. There exists a correlation between CH₄ content and N₂/Ar ratio in Type II gases collected over a wide area of the Ryoke metamorphic terrane consisting mainly of granitic rocks, suggesting some unique source of CH₄ and N₂ within the examined area. Hydrogen is rarely found in Type II gases but sometimes appears intermittently, suggesting that H₂ in Type II gases is not useful for the earthquake prediction, although the concentration in Type I gases might have a relation to fault activity. Fault gouges as well as brecciated rocks in fault zones extremely lose their calcium but retain ferrous iron. This variation trend of these elements during the pulverization under fault movements is different from that of the ordinary weathering. This feature can be attributed to some peculiar environment resulted from the chemical interaction among waters, gases and rocks in the fault zones. Acid fluids formed by CO₂ and water charged in the fault zones will break feldspars in parent rocks and dissolve out calcium in them, whereas H₂ will prevent iron from oxidation.