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Alasdair Skelton, Stockholm University (Sweden)
Lillemor Claesson, Stockholm University (Sweden)
Govinda Chakrapani, Indian Institute of Technology (India)
Chandan Mahanta, Indian Institute of Technology (India)
Joyanto Routh, Stockholm University (Sweden)
Magnus Mörth, Stockholm University (Sweden)
Param Khanna, Wadia Institute of Himalayan Geology (India)
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We report the result of a hydrogeochemical monitoring program, which has been operational north of the Shillong Plateau, Assam, India from December 2003. The aim of this study is to test for coupling between groundwater chemistry in a granite-hosted aquifer, located at a depth of 110m, and seismic activity. Based on molar Na+/Ca2+ and HCO3-/SiO2 ratios after Garrels (1967), we interpret that groundwater chemistry was normally buffered by the alteration of feldspar (plagioclase) to kaolinite.
During the study, we monitored transient chemical changes which coincided temporally with a period of increased seismic activity. This included (1) MW = 5.3 and MW = 5.0 earthquakes which occurred on December 9, 2004 and February 15, 2005, south of the Shillong Plateau and 206 and 213 km from the sampling station, respectively, and (2) the Great Sumatra - Andaman Islands Earthquake of December 26, 2004. These are the only three MW > 5 earthquakes which have occurred during our study and for which our monitoring site is within their respective strain radii as given by Dobrovolsky et al. (1979).
The most dramatic chemical change was a ca. 2-fold increase of the ratios [Na+K]/Si, Na/K and [Na+K]/Ca. This was accompanied by significant increases of conductivity, alkalinity and chloride concentration. The onset of this chemical shift occurred 3-5 weeks before the first (MW = 5.3) earthquake. We interpret a transient switchover between source aquifers, which induced an influx of groundwater from a second and probably deeper aquifer, where groundwater chemistry was dominantly buffered by the alteration of feldspar to smectite. This could have occurred in response to fracturing of a hydrological barrier. We also recorded a rapid drop in the ratio Ba/Sr, which occurred 3-6 days before the final (MW = 5.0) earthquake. We interpret a transient switchover to anorthite dissolution caused by exposure of fresh plagioclase to groundwater interaction. This could have been induced by microfracturing, locally within the main aquifer. These changes were transient and "recovery" occurred over periods of 2-4 weeks. By comparison with experimental studies of feldspar dissolution, we suggest that hydrogeochemical recovery was facilitated by groundwater interaction and clay mineralization, perhaps coupled with fracture sealing.
The main argument in support of seismic-hydrogeochemical coupling is the coincidence in timing of two hydrogeochemical events with two MW ≥ 5 earthquakes. Reasons for ambiguity include the lack of similar hydrogeochemical anomalies temporally coupled with smaller seismic events which occurred much closer to the monitoring station, the >200 km length scale of inferred seismic-hydrogeochemical coupling, and the potential for far-field effects related to the Great Sumatra ? Andaman Islands Earthquake of December 26, 2004.
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