Possible precursors in groundwater ions and gases content in Kamchatka (Russia)

For many years, hydrogeochemicals have been collected with a mean sampling frequency of three days in the form of the most common ions and gases in the groundwater of five deep wells in the Southern area of the Kamchatka peninsula. In the last decade, five earthquakes with M>6.5 occurred at distances less than 250 km from these wells. In order to reveal possible precursors of these earthquakes we analysed the hydrogeochemicals collected. The quasi-periodic annual variation was filtered out, together with other slow trends, and then we smoothed out the high frequency fluctuations that arise from the errors in a single measurement. When examining the filtered and smoothed data, we labelled each signal with an amplitude greater than three times the standard deviation as an irregularity and we made a first attempt at defining an anomaly as an irregularity which occurs simultaneously in more than one type of ion or gas at each well. In a second definition we used the existence of an irregularity occurring simultaneously in each type of ion or gas at more than one well. Then we chose the minimum time interval between two successive earthquakes as the maximum temporal window between a possible anomaly and the subsequent earthquake. Concerning the ions content, we identified a total of 16 anomalies with 11 successes and 5 failures; for the gases content we identified a total of 25 anomalies with 9 successes and 16 failures. On the basis of the criteria chosen we find the possibility that the successes of ion contents as earthquake precursors is high; but in contrast there is a only small possibility that the successes in gases content are precursors. (C) 2000 Elsevier Science Ltd. All rights reserved.