CO- AND POSTSEISMIC CONDUCTIVITY ANOMALIES MONITORED AT AN ARTESIAN WELL IN SOUTHERN ARMENIA

Co- and postseismic anomalies of the specific electrical conductivity were repeatedly observed at an artesian well in southern Armenia related to earthquakes with epicentral distances up to 1400 km. Typically, the conductivity sharply drops 60-70 minutes after the event by 3-5%. The measurement accuracy is ±0.5%. The delay time is due to the fact that the borehole volume has to be replaced before any changes in the water quality are detectable at the surface. Thus, we call this drop co-seismic to distinguish it from a nearly exponential post-seismic decay during the succeeding 3-4 weeks which reduces the conductivity of the water by another 3-5%. It takes several months for the conductivity to recover back to the pre-event level. The water discharge of the free flowing well increased up to 25 % immediately after an earthquake. The co- and postseismic strains calculated for the monitoring site are in the order of 10^(-10) and 10^(-11) for the strongest events, they are below 10^(-11) for all other earthquakes between December 1995 and December 1999. How can strain events below 10^(-10) reduce the specific electrical conductivity of a mineral water by 5 to 10 %? The answer is related to the special site conditions. We developed a model that explains the observations in terms of a changed mixing ratio between two fluid systems as a result of a pressure disturbance triggered by the seismic waves passing the observation site.