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Mitrofan H.,Geodynamics of the Romanian Academy | Marin C.,Romanian Academy of Sciences | Zugraavescu D.,Geodynamics of the Romanian Academy | Chitea F.,Geodynamics of the Romanian Academy | And 5 more authors.
Natural Hazards and Earth System Science

A six year-long hydrochemical monitoring operation was conducted in Vrancea seismic zone (Romania), addressing a saline spring that proved to be suitable for Na-K-Mg geothermometry diagnosis. During the considered time-interval (2003-2009), only one important earthquake (mbCombining double low line5.8) occurred in Vrancea region, this circumstance providing an unambiguous reference-moment between pre-seismic and post-seismic periods. On occurrence of that earthquake, an anomalous fluctuation of the Na-K temperature was detected - a result largely similar to previous ones recorded worldwide (California, southwest Egypt, northeast India). Yet such fluctuations may not necessarily be induced by earthquake-associated processes: they can occur also 'routinely', possibly reflecting some environmental, meteorologically-induced 'noise'. It was therefore important to examine whether the variations observed in the data values could be plausibly related to a seismogenesis process. By additionally investigating (in a 'scattterplot' diagram) the correlation between the Na-K temperatures and the values of a so-called 'maturity index', a specific pattern emerged, with pre-seismic data-points plotting in a distinct domain of the diagram; moreover, those data-points appeared to describe a 'drift away' pathway with respect to the remaining data-points 'cluster', recorded during the subsequent 4 years of post-seismic monitoring. The 'drift away' pattern persistently evolved for at least 18 months, ending just before the m b=5.8 earthquake and consequently suggesting the existence of some kind of long-term precursory phenomenon. Source

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