Fluid-rock interaction in two seismically active areas : The Tjörnes Fracture Zone, northern Iceland and the Shillong Plateau, northeastern India

Abstract: Hydrogeochemical monitoring can improve our understanding of fluid-rock interaction, and may detect anomalies that are precursory to- or caused by- earthquakes. In this study, hydrogeochemical monitoring was carried out in two distinct tectonic settings, to study coupling between hydrogeochemical changes and seismic activity. The Tjörnes Fracture Zone, north Iceland (HU-01), and the Shillong Plateau, northeastern India (Silver Drop), are two seismically active areas with differing geology. Hydrogeochemical monitoring included regular groundwater sampling with subsequent major and trace element analysis and supporting stable isotope analysis at HU-01 only. Transition metal concentrations anomalies were detected at HU-01 prior to a MW 5.8 earthquake. Concentration increases of many major elements were detected after this earthquake. Hydrogeochemical recovery took place during the subsequent two years. The preseismic anomalies are interpreted as the result of increased fluid-rock interaction due to preseismic fracturing whereas the postseismic changes are attributed to source switching and/or mixing of fluids enabled by fault unsealing. The two-year recovery is probably related to the gradual sealing off of a fluid source. Pre- and post-seismic hydrogeochemical shifts (e.g. Na/Si, Ba/Sr) were detected at Silver Drop coinciding with two MW > 5 earthquakes. These shifts are likely due to changes in the type of feldspar weathering. A basalt dissolution experiment was carried out in order to understand the preseismic changes at HU-01. Rates and dissolution mechanisms interpreted from this experiment confirm that the metal anomalies at HU-01 could be induced by transient exposure of weakly altered basalt to groundwater interaction caused by microfracturing. Finally, from this study it can be concluded that a seismic event need not cause a hydrogeochemical shift, but a hydrogeochemical shift was always associated with a seismic event.