ABSTRACT

We determined chloride and sulfate concentrations and δD_H2O and δ¹⁸O_H2O isotope ratios of interstitial fluids extracted from cores from two fault zones within the Taiwan Chelungpu fault. The fluids had higher concentrations of chloride and sulfate and higher δD_H2O and δ¹⁸O_H2O values than the local meteoric water. We concluded that the higher δD_H2O and δ¹⁸O_H2O values were the result of either entry of surface meteoric water at low altitude via fault zones, or isotopic fractionation by depletion at 50°C in a shallower fault zone, and at 125°C in the deeper fault zone. The high temperature in the deeper fault zone might have been earthquake-related frictional heating, which led to isotopic fractionation. However, neither of the above hypotheses explains the high concentrations of Cl^- and SO₄^2-. We concluded that the high concentrations of Cl^- and SO₄^2- might have resulted from mixing with seawater and dissolution of pyrite, respectively.