Item

Abstract

By analysing several years of discharge and electrical conductivity data from an artesian well in Kajaran, Armenia, we found a significant sensitivity of this well to distant large earthquakes. In general, the discharge increases co-seismically and the conductivity decreases post-seismically with a time delay of about 1 hr. The post-seismic trends of the conductivity reach a minimum after about 3 weeks and then need several months to recover to the pre-seismic level. For instance, the Mw= 7.6 Izmit earthquake in Turkey on 1999 August 17, at a distance of 1400 km, led to an increase of 25 per cent in discharge and a decrease of up to 6 per cent in conductivity. The discharge also shows tidal fluctuations of amplitude roughly 5 per cent (peak-to-peak) of the mean well production, whereas the tidal signal in the conductivity data is less significant and unstable. The maximum co-seismic static strain estimated for 11 earthquakes that induced an anomaly during the monitoring period is below 109, or at least one order smaller than the tidal strain. Therefore, the well-water anomalies related to the distant earthquakes are believed to be induced by seismic ground shaking rather than co-seismic deformation. We suggest mixing of groundwater as an explanation for the observations and present a model which considers specific conditions for this particular groundwater system: a confined aquifer with a high contrast in the hydrogeochemical composition between two different groundwaters and a macrofracture as the mixing zone which is hydraulically connected to the artesian well. The earthquake-related anomalies are believed to result from a local head increase near the artesian well, induced by the passage of seismic waves. Possible mechanisms are discussed, and the time histories of the anomalies are modelled.