MODELLING GROUND WATER CONDUCTIVITY ANOMALIES INDUCED BY DISTANT EARTHQUAKES

In the analysis of the multiparameter data monitored since more than 4 years at an artesian well in Kajaran, Armenia, we found that the specific electrical conductivity of the well water responses significantly to distant moderate- and major-scale earthquakes. The sensitivity of the system was shown again by the anomalies induced by the Izmit earthquake, August 17, 1999, whose epicenter was at a distance of 1400 km. Both co- and post-seismic conductivity anomalies are uniformly of negative sign and satisfy a power-law attenuation with the epicenter distance. The post-seismic curve is relatively well characterized and reproducible. In particular, the conductivity parameter is more sensitive to earthquakes than the water discharge which is monitored since September 1998, too. In the conductivity parameter the maximum earthquake-induced offset is hundreds to thousands times larger than the amplitude of earth tides, and in the water discharge such signal ratio is several tens. The quasi-static strain of all observed earthquakes has been estimated to be limited by 10^(-9) which is at least one order lower than the tidal strain. It is therefore difficult to explain the phenomena with the poroelastic response of the hydrological system. We propose the degassing of oversaturated ground water triggered by seismic waves as a reasonable mechanism. Using a simple model, we give a preliminary quantitative explanation of the observed hydrological changes induced by distant earthquakes.