Significant Temporal Changes in 13C in Dissolved Inorganic Carbon of Groundwater Related to Reservoir-triggered Seismicity

Abstract

Earthquake prediction in a time domain of a few weeks to months is the ultimate goal of earth scientists who have been attempting to evaluate seismic, geodetic, hydraulic, geochemical, isotopic, etc. precursors for this purpose. Two recent attempts toward earthquake forecasting in India's Koyna-Warna seismic area (Gupta et al. 2006, 2007) using earthquake nucleation phenomena were successful. Other precursors such as soil gas radon were recently attempted in a reservoir-triggered seismic area (Sukhija et al. 2007). Earlier studies (Gupta et al. 2000; Chadha et al. 2003, 2008) have demonstrated that pore pressure changes in such areas could be a valuable precursor; also promising were a large number of geochemical precursors studied in active seismic areas along geological faults (Virk et al. 1994; Igarashi et al. 1995; Tsungoai and Wakita 1995; King et al. 1995; Gupta et al. 2002; Das et al. 2006). In this paper we report for the first time significant temporal changes of δ13C values of dissolved inorganic carbon (DIC) in groundwater associated with earthquake processes in a reservoir-triggered seismic area. The δ13C of DIC of groundwater (Figure 1) is generally controlled by the reaction of CO2 in the topsoil with mainly marine carbonates in the unsaturated zone: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[ \mathrm{CaCO}\_{3}+\mathrm{CO}\_{2}+\mathrm{H}\_{2}\mathrm{O}{\rightarrow}2\mathrm{HCO}\_{3}^{-}+\mathrm{Ca}^{++}\] \end{document}(1) The concentration of carbon dioxide in the topsoil is about 100 times larger than in the atmosphere. The soil carbon dioxide is produced by root respiration and decay of dead plant material (Munnich 1957). The δ13C of marine carbonates ranges from –2 to +2‰ ( i.e., ±2 parts per 1,000; PDB, the standard established for carbon-13 measurements based on a Cretaceous marine fossil, Belemnitella americana , which was from the Pee Dee Formation in South Carolina) and that of organic material and the carbon dioxide in …