NHESS Natural Hazards and Earth System Sciences NHESS Nat. Hazards Earth Syst. Sci. 1684-9981 Copernicus Publications Göttingen, Germany 10.5194/nhess-10-217-2010 Persistent pre-seismic signature detected by means of Na-K-Mg geothermometry records in a saline spring of Vrancea area (Romania) Mitrofan H. 1 Marin C. 2 Zugrăvescu D. 1 Chitea F. 3 1 Anghelache M.-A. 1 Beşuţiu L. 1 Tudorache A. 2 4 "Sabba Ştefănescu" Institute of Geodynamics of the Romanian Academy, J. L. Calderon str. 19-21, Bucharest, Romania "Emil Racoviţă" Institute of Speleology of the Romanian Academy, Calea 13 Septembrie 13, Bucharest, Romania Faculty of Geology and Geophysics, University of Bucharest, Traian Vuia str. 6, Bucharest, Romania Faculty of Chemistry, University of Bucharest, Regina Elisabeta str. 2-14, Bucharest, Romania 09 02 2010 10 2 217 225 Copyright: © 2010 H. Mitrofan et al. 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://nhess.copernicus.org/articles/10/217/2010/nhess-10-217-2010.html The full text article is available as a PDF file from https://nhess.copernicus.org/articles/10/217/2010/nhess-10-217-2010.pdf

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 (<i>m</i><sub>b</sub>=5.8) 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 <i>m</i><sub>b</sub>=5.8 earthquake and consequently suggesting the existence of some kind of long-term precursory phenomenon.

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