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.