Coupling Between Seismic Activity and Hydrogeochemistry at the Shillong Plateau, Northeastern India

• Transient hydrogeochemical anomalies were detected in a granite-hosted aquifer, which islocated at a depth of 110 m, north of the Shillong Plateau, Assam, India, where groundwater chemistry is mainlybuffered by feldspar alteration to kaolinite. Their onsets preceded moderate earthquakes on December 9, 2004(MW = 5.3) and February 15, 2005 (MW = 5.0), respectively, 206 and 213 km from the aquifer. Theratios [Na+K]/Si, Na/K and [Na+K]/Ca, conductivity, alkalinity and chloride concentration began increasing3–5 weeks before the MW = 5.3 earthquake. By comparison with field, experimental and theoretical studies, weinterpret a transient switchover between source aquifers, which induced an influx of groundwater from a secondaquifer, 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. The ratio Ba/Sr began decreasing3–6 days before the MW = 5.0 earthquake. We interpret a transient switchover to anorthite dissolution causedby exposure of fresh plagioclase to groundwater interaction. This could have been induced by microfracturing,locally within the main aquifer. By comparison with experimental studies of feldspar dissolution, we interpretthat hydrogeochemical recovery was facilitated by groundwater interaction and clay mineralization, which couldhave been coupled with fracture sealing. The coincidence in timing of these two hydrogeochemical events withthe only two MW C 5 earthquakes in the study area argues in favor of cause-and-effect seismichydrogeochemicalcoupling. However, reasons for ambiguity include the lack of similar hydrogeochemicalanomalies coupled with smaller seismic events near the monitoring station, the >200 km length scale of inferredseismic-hydrogeochemical coupling, and the potential for far-field effects related to the Great Sumatra–Andaman Islands Earthquake of December 26, 2004.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Multidisciplinär geovetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geosciences, Multidisciplinary (hsv//eng)

Nyckelord

Hydrogeochemistry

seismic-hydrogeochemical coupling

water-rock interaction

Shillong Plateau

Endogenous earth sciences

Endogen geovetenskap

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