Several research groups have made efforts to accumulate reliable data on various geochemical and hydrological parameters for periods longer than 10 years. The data accumulation has enabled them to characterize background fluctuations in the data and consequently to examine their relation to earthquake occurrences without prejudice. As a result, a number of earthquake-related anomalies have consistently been identified in various geochemical and hydrological observations, suggesting that they are sensitive indicators of earthquake occurrence. Comprehensive monitoring based on multicomponent and multi-station observations has begun to result in the detection of more reliable precursory phenomena which appeared simultaneously in two or different kinds of observations at several stations.
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