Experimental evidence on formation of imminent and short-term hydrochemical precursors for earthquakes
Introduction
Geochemical signals can often bear witness to seismic activity (Roeloffs, 1988, Kingsley et al., 2001, Favara et al., 2001, Belin et al., 2002, Du et al., 2006). Variations of ion concentrations in groundwater, gas compositions and isotopic ratios that have been found in the epicenter areas and far from the epicenter before, during and after major earthquakes are considered to be due to the action of crustal stress related to earthquake generation and seismic wave propagation (Rikitake, 1976, Zhang et al., 1988, King et al., 1994, Léonardi et al., 1999, Du and Kang, 2000, Wu and Cai, 2001, Luo et al., 2002, Claesson et al., 2004). Hydrochemical monitoring studies conducted over more than 20 a have shown typical anomalies of Na, Ca, Mg, Cl, F, SO4, HCO3, trace element concentrations and concentrations of gaseous species in wells in China usually occur less than one day to several hundred days before some MS > 5.0 earthquakes (Wu and Zhao, 1995, Geng et al., 1998, Tong et al., 2000, Wu and Cai, 2001, Ye et al., 2004, Huang et al., 2004, Yang et al., 2006, Skelton et al., 2008). The duration of precursory hydrochemical anomalies related to major earthquakes in the world ranges from less than one day to 500 days. A precursory time is usually <40 days within an epicentral distance <100 km (Hartmann and Levy, 2005). Co-seismic geochemical anomalies have been observed at a distance in excess of hundreds of kilometers. For example, the time series of Rn concentrations in some wells, at an epicentral distance of <550 km, in Southeastern China, showed co-seismic anomalies in response to the 21 September 1999 Chi–Chi M ⩾ 7.3 earthquake in Taiwan (Huang et al., 2004). The observed hot springs and artesian springs in Taiwan clearly show chemical anomalies correlated with earthquakes. The factors controlling the chemical anomalies in subsurface waters are the types, the depths and the size of a reservoir as well as the ion species of the water body (Song et al., 2005, Song et al., 2006, Yang et al., 2006). Most hydrochemical variations are explained by the mixture of waters from different aquifers caused by aquifuge breakage and migration of deep fluid along a active faults, and meteoric water addition to an aquifer (Geng et al., 1998, Kingsley et al., 2001, Favara et al., 2001, Claesson et al., 2004, Van der Hoven et al., 2005). As Koretsky (2000) summarized, in hypothesising on the existence of a stress transmission process, modifications of the hydrogeochemistry of water in a well may be caused by: (1) the flow of waters with different chemistries into the well, and (2) the circulation of the groundwater into new zones, as a consequence of the intensification of the micro-fracturing processes and/or changes in existing cracks produced by induced stress. Experiments conducted by mixing NaCl solution with water in a hydrodynamic channel indicated that the variation amplitudes of ion concentrations attenuated obviously and the anomaly patterns varied with increasing migration distance of the solution, which was also related to the stress state (Yu et al., 2000). However, the ductile aquifuges at the hydrochemical observation sites were usually not broken, and no fault displacements were found before an earthquake. In this case, hydrochemical variations occurring before earthquakes can not be explained by the mixing model (Toutain et al., 1997).
The hydrochemical compositions of an aquifer would not vary significantly over a short time if equilibrium between groundwater and rock was approached. However, departure from equilibrium can result from fracturing of the aquifer due to tectonic stress. This in turn results in hydrochemical anomalies. With increasing tectonic stress before the occurrence of an earthquake, formation of micro-fractures in rock masses results in increasing the surface area of rocks and therefore increasing Rn concentrations in the groundwater (Igarashi et al., 1995, Teng, 1980). Skelton et al. (2008) coupled the seismic activity with the chemistry of groundwater related to micro-fracturing within a granite aquifer. This suggests that water–rock reactions in brittle aquifers create some hydrochemical anomalies related to earthquakes. However, the rates of reactions between minerals and groundwaters in response to fracturing of aquifers are undetermined.
The causes of seismic hydrochemical precursors remain a puzzle. It is difficult to identify the imminent and short-term precursors from the observed hydrochemical anomalies even though some hydrological anomalies have been used as earthquake precursors in the last decades. Furthermore, to what extent hydrochemical variations are caused by water–rock reactions in a micro-fractured aquifer in a short time, and which elements or ions are more sensitive has not been clarified. Therefore, the emphasis of this paper is put into quantitatively investigating the rates of mineral dissolution in response to fracturing within a brittle aquifer by experimentally simulating interactions between micro-fractured rock and waters in a short time in order to highlight coupling between seismic activity and groundwater chemistry.
Section snippets
Samples
Gray coarse-grained granodiorite was collected from Fangshan, Beijing. This rock consists mainly of plagioclase, hornblende, mica and quartz. Brown trachyandesite was collected from Tengchong, Yunnan province, southwestern China. The rock is composed of matrix and plagioclase phenocrysts. The matrix is brown glass with pores. The chemical compositions of the rocks have been determined (Table 1, BGMRBM, 1991, Fan et al., 1999). The rock samples were mechanically crushed and separated into seven
Results
The chemical compositions of the experimental solutions are shown in Fig. 1, Fig. 2, Fig. 3. Analytical results for the solutions for soaking granodiorite with deionized water indicate that concentrations of HCO3 Ca, Na and K increase with soaking time, but those of Cl and SO4 show no evident variation (Fig. 1A). The ion concentrations decrease with increasing grain size (Fig. 2A). Ion concentrations in the solutions after soaking granodiorite with deionized water for 12 h show, from high to low
Temporal response of ion concentrations in solutions
Ion chromatographic analyses show that water–rock reactions at room temperature for 6 h can result in measurable variations of ion concentrations (Fig. 1). After soaking granodiorite grains of 180 and 2000 μm with deionized water from 6 to 168 h, Ca in the experimental solutions increased from 9.04 to 22.0 mg/L and 6.27 to 16.3 mg/L, and HCO3 from 29.7 to 65.8 mg/L and 11.76 to 50.70 mg/L, respectively. However, Na, K and Mg concentrations varied slightly from 6 to 168 h; Cl and SO4 show even values
Conclusions
Based on the experimental data, the following conclusions can be drawn:
- (1)
Reactions between water and rock grains for 6 h to one week can result in measurable hydrochemical variations. This indicates water–rock interaction in brittle aquifers caused by micro-fracturing in response to crustal stress may be one of the genetic mechanisms of imminent hydrochemical precursors of earthquakes.
- (2)
The extent of hydrochemical variation is generally directly proportional to the extent of rock fracturing and
Acknowledgements
The authors from China thank Prof. F. Takemura for inviting them to work in the Institute for Geothermal Science, Kyoto University. The authors are graceful to the anonymous reviewer for the comments that were helpful for improving the paper. This work was supported by Kyoto University, Japanese COE, Earthquake Science Foundation (Rank No. B07002) and Ministry of Science and Technology, China (Rank No. 2005DFA20980).
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