Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 10 2 2010 10.5194/nhess-10-217-2010 http://www.nat-hazards-earth-syst-sci.net/10/217/2010/ http://www.nat-hazards-earth-syst-sci.net/10/217/2010/nhess-10-217-2010.html http://www.nat-hazards-earth-syst-sci.net/10/217/2010/nhess-10-217-2010.pdf 217 225 2010-02-09 Persistent pre-seismic signature detected by means of Na-K-Mg geothermometry records in a saline spring of Vrancea area (Romania) H. Mitrofan C. Marin constmarin@gmail.com D. Zugrăvescu F. Chitea M.-A. Anghelache L. Beşuţiu A. Tudorache "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 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>_b=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>_b=5.8 earthquake and consequently suggesting the existence of some kind of long-term precursory phenomenon. Bonjer, K.-P., Ionescu, C., Sokolov, V., Radulian, M., Grecu, B., Popa, M., and Popescu, E.: Source parameters and ground motion pattern of the 27 October 2004 intermediate depth Vrancea earthquake, in: EGU General Assembly, Vienna, Austria, 24–29 April 2005, EGU05-A-07538, 2005. Caporali, A., Aichhorn, C., Becker, M., Fejes, I., Gerhatova, L., Ghitau, D., Grenerczy, G., Hefty, J., Krauss, S., Medak, D., Milev, G., Mojzes, M., Mulic, M., Nardo, A., Pesec, P., Rus, T., Simek, J., Sledzinski, J., Solaric, M., Stangl, G., Vespe, F., Virag, G., Vodopivec, F., and Zablotskyi, F.: Geokinematics of Central Europe: New insights from the CERGOP-2/Environment Project, J. Geodyn., 45, 246–256, 2008. Chen, C.-T. A. and Millero, F. J.: Precise thermodynamic properties for natural waters covering only the limnological range, Limnol. Oceanogr., 31, 657-662, 1986. Cioni, R., Guidi, M., Pierotti, L., and Scozzari, A.: An automatic monitoring network installed in Tuscany (Italy) for studying possible geochemical precursory phenomena, Nat. Hazards Earth Syst. Sci., 7, 405–416, 2007. Clegg, S L. and Whitfield, M.: Activity coefficients in natural waters, in: Activity Coefficients in Electrolyte Solutions, edited by: Pitzer, K S., CRC Press, Inc., Boca Raton, Florida, Ch 6, 279–434, 1991. D\'iaz-González, L. and Santoyo, E.: A new precise calibration of the Na/K geothermometer using a world database of geothermal fluids and improved geochemometric techniques, Geochim. Cosmochim. Ac., 72, (12 Supplement 1), A215, 2008. D\'iaz-González, L., Santoyo, E., and Reyes-Reyes, J.: Three new improved Na/K geothermometers using computational and geochemiometrical tools: Application to the temperature prediction of geothermal systems (in Spanish), Rev. Mex. Cienc. Geol., 25, 465–482, 2008. Fournier, R O.: A revised equation for the Na/K geothermometer, Geothermal Resources Council Transactions, 3, 221–224, 1979. Fournier, R O. and Truesdell, A H.: Chemical indicators of subsurface temperature applied to hot spring waters of Yellowstone National Park, Wyoming, U.S.A, in: Proceedings of the U.N. Symposium on the Development and Utilization of Geothermal Resources, Pisa, Italy, 22 September–1 October 1970 (Geothermics, Special Issue 2), 2 (Part I), 529–535, 1970. Giggenbach, W F.: Graphical techniques for the evaluation of the of water/rock equilibration conditions by use of Na, K, Mg and Ca – contents of discharge waters, in: Proceedings of the 8th New Zealand Geothermal Workshop, Auckland, New Zealand, 37–42, November 1986. Giggenbach, W F.: Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators, Geochim. Cosmochim. Ac., 52, 2749–2765, 1988. Gvirtzman, Z.: Partial detachment of a lithospheric root under the southeast Carpathians: Toward a better definition of the detachment concept, Geology, 30, 51–54, 2002. Heidbach, O., Ledermann, P., Kurfeß, D., Peters, G., Buchmann, T., Matenco, L., Negut, M., Sperner, B., Müller, B., Nuckelt, A., and Schmitt, G.: Attached or not attached: slab dynamics beneath Vrancea, Romania, in: Proceedings of the International Symposium on Strong Vrancea Earthquakes and Risk Mitigation, Bucharest, Romania, 4–6 October 2007, 3–20, 2007. Idris, M.: Geothermal evaluation for thermal fluids of Helwan springs Egypt, in: Proceedings of the World Geothermal Congress 2000, Kyushu-Tohoku, Japan, 1295–1299, 28 May–10 June 2000. Mitrofan, H., Marin, C., Zugrăvescu, D., Tudorache, A., Beşu\ctiu, L., and Radu, M.: Transients of Giggenbach's 'Na-K-Mg-Ca Geoindicators' preceding the 27 October 2004, M$_w$ = 6.0 earthquake in Vrancea area (Romania), Terra Nova, 20, 87–94, 2008. Radulian, M., Bonjer, K.-P., Popa, M., and Popescu, E.: Seismicity patterns in SE Carpathians at crustal and subcrustal domains: tectonic and geodynamic implications, in: Proceedings of the International Symposium on Strong Vrancea Earthquakes and Risk Mitigation, Bucharest, Romania, 4–6~October 2007, 93–102, 2007. Radulian, M., Mândrescu, N., Panza, G F., Popescu, E., and Utale, A.: Characterization of seismogenic zones of Romania, Pure Appl. Geophys., 157, 55–77, 2000. Skelton, A., Claesson, L., Chakrapani, G., Mahanta, C., Routh, J., Mörth, M., and Khanna, P.: Coupling between seismic activity and hydrogeochemistry at the Shillong Plateau, Northeastern India, Pure Appl. Geophys., 165, 45–61, 2008. Sperner, B., Lorenz, F. P., Bonjer, K.-P., Hettel, S., Müller, B., and Wenzel, F.: Slab break-off – abrupt cut or gradual detachment? New insights from the Vrancea Region (SE Carpathians, Romania), Terra Nova, 13, 172–179, 2001. Truesdell, A H. and Lippmann, M J.: The lack of immediate effects from the 1979–80 Imperial and Victoria earthquakes on the exploited Cerro Prieto geothermal reservoir, Geothermal Resources Council Transactions, 10, 405–411, 1986. Valette-Silver, N J., Silver, P G., and Marquez, M R.: The effect of a tectonic disturbance on the Cerro Prieto geothermal field (B.C., Mexico), Geothermal Resources Council Transactions, 9(Part I), 515–520, 1985. Van Der~Hoeven, A. G A., Mocanu, V., Spakman, W., Nutto, M., Nuckelt, A., Matenco, L., Munteanu, L., Marcu, C., and Ambrosius, B. A C.: Observation of present-day tectonic motions in the Southeastern Carpathians: results of the ISES/CRC-461 GPS measurements, Earth Planet. Sc. Lett., 239, 177–184, 2005. Vaselli, O., Minissale, A., Tassi, F., Magro, G., Seghedi, I., Ioane, D., and Szakacs, A.: A geochemical traverse across the Eastern Carpathians (Romania): constraints on the origin and evolution of the mineral water and gas discharges, Chem. Geol., 182, 637–654, 2002