Abstract
Gas bubbles separating from ground water has been monitored continually by a gas chromatograph and an α scintillation Rn detector at a well of Nagashima spa, a southern suburb of Nagoya, central Japan since October 1997. The Nagashima well is 1500m deep and located at a distance of 5km from the Yoro active fault. Concentrations of He, H2, Ar, N2 and CH4 and Rn in the gas bubbles are analyzed. H2/Ar anomalies have been recorded in relation to some earthquakes (M<3.1) occurred in and near the northern part of Ise Bay between June and August 1999. The accumulated data support the idea that H2 is generated through the mechano-chemical reaction between groundwater and fresh silicate surfaces of slip planes presumably induced by preseismic slips. The H2/Ar anomalies can be classified into “spike-like change” and “ramp function-like change” based on the variation patterns of the observed H2/Ar anomalies. The two types of anomalous changes of H2/Ar ratio can be interpreted by using two models for the preseismic deformation on a slip plane as groundwater conduit. One is the aseismic stick-slip model for local contact areas on the slip planes, and the other is the crack growth model by stress corrosion. If mechano-chemical reaction of fresh silicate surfaces created by the stick-slip or the crack growth with water can produce H2 gas, the observed temporal variation of H2/Ar ratio possibly corresponds to the rate of stick-slip or crack growth and increase of newly produced rock surfaces. The ramp function-like increases of H2/Ar suggest increasing rates of stick-slip or crack growth for a prolonged period preceding earthquakes along the active fault zone.
