TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 41 (No. 2), pp. 97-102, 2007

Chemical and isotopic characteristics of interstitial fluids within the Taiwan Chelungpu fault

Tetsuro Hirono,1,2 Urumu Tsunogai,3 Kenji Maegawa,3 Tomohiro Toki,4 Masaharu Tanimizu,2 Wonn Soh,2 Weiren Lin,2 En-Chao Yeh,5 Sheng-Rong Song5 and Chien-Ying Wang6

1Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
2Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, Nankoku 783-8502, Japan
3Department of Natural History Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
4Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, Okinawa 903-0213, Japan
5Department of Geosciences, National Taiwan University, Taipei, Taiwan
6Institute of Geophysics, National Central University, Jhongli, Taiwan

(Received September 7, 2006; Accepted December 10, 2006)

Abstract: We determined chloride and sulfate concentrations and δDH2O and δ18OH2O 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 δDH2O and δ18OH2O values than the local meteoric water. We concluded that the higher δDH2O and δ18OH2O 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 SO42-. We concluded that the high concentrations of Cl- and SO42- might have resulted from mixing with seawater and dissolution of pyrite, respectively.
Key words: Chelungpu fault, Chi-Chi earthquake, pore fluid chemistry, isotopic fractionation, frictional heating

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