Measurements of the dielectric properties of limestone under pressure and their importance for seismic electric signals I. Sakellis a,b, ⁎, A.N. Papathanassiou a , J. Grammatikakis a a University of Athens, Physics Department, Section of Solid State Physics, Panepistimiopolis, GR 15684 Zografos, Athens, Greece b National Center for Scientific Research ‘Demokritos’, Institute of Materials Science, GR 15310 Agia Paraskevi Attikis, Athens, Greece abstract article info Article history: Received 9 October 2013 Accepted 25 December 2013 Available online 18 January 2014 Keywords: Dielectric relaxation Conductivity Activation volume Rocks Earthquake forecast Pressure The pressure and temperature evolution of an intense dielectric relaxation mechanism in as-received and wetted limestone (from Ioannina region (Greece)) are studied experimentally through complex impedance spectrosco- py. The relaxation time decreases on compression, yielding a negative value for the corresponding activation volume. Furthermore, the role of water incorporated in the mineral was examined, revealing the remarkable fact that water not only enhances the absolute value of negative derivative of Gibbs energy with respect to pressure but also lowers significantly the energy barrier of the corresponding physical processes. Both findings are crucial for the explanation of the sensitivity of an electrotelluric station at Ioannina region in detecting Seismic Electric Signals emitted prior to earthquakes. © 2013 Published by Elsevier B.V. 1. Introduction Negative activation volume is – by definition – related with a pro- cess, the characteristic activation energy of which is reduced as pressure is increased, or in other words by increasing pressure the corresponding process (that may comprise various relaxation mechanisms) becomes energetically more favorable to occur and the corresponding relaxation time is reduced. In general, there is no restriction as far as the sign of activation volume is concerned. The experiments show that negative activation volumes are not commonly found (Fontanella et al., 1982, 1996; Papathanassiou et al., 2010, 2011, 2012; Sakellis et al., 2012). However, the negative activation volumes are of key importance for the validity of the mechanisms proposed for the generation mechanism of seismic electric signals (SES). These are low frequency electric signals that are observed (Varotsos and Alexopoulos, 1984a, 1984b; Varotsos et al., 2002, 2003a, 2003b, 2005, 2006a, 2006b) before earthquakes. Varotsos et al. (1982), (see also Varotsos and Alexopoulos, 1986) treat- ed the earth's crust as a solid rich in ‘dipoles’ in a polarizing field (i.e., the mechanical stress field), and assumed that negative activation volumes exist in rocks. They asserted that the gradually increasing stress in the focal area before an earthquake reduces the relaxation time of these ‘dipoles’. Thus, at a certain critical stress (pressure), smaller than that at which rupture occurs, the relaxation time of ‘dipoles’ becomes short and the ‘dipoles’ undergo a transition from random-orientation state to an oriented one. This yields the emission of a transient polarization current before the rapture, which constitutes, an SES. SES are observed at stations situated on certain localities in the Earth's crust termed sensitive. In these localities, the electric field variations are considered to be significantly enhanced due to both large-scale and small-scale het- erogeneous properties of the earth's crust (Varotsos and Lazaridou, 1991). It is the basic scope of the present paper to investigate the existence of negative activation volume in limestone samples which were collect- ed from Ioannina region (in Northwestern Greece) where a sensitive SES recording station is located. In addition, the critical role of water incorporated within these samples was also studied in detail for the following reasons: The time evolution of the stress field prior to an earthquake, results in the dynamic compression of water-filled rocks (Morgan and Nur, 1986; Morgan et al., 1989). Specifically in rocks, strong structural and compositional heterogeneity as well as porosity give rise to several polarization mechanisms such as, interfacial polari- zation, double layer polarization, space charge polarization, and defect dipole polarization (Correia, 1997; Varotsos, 2005). Furthermore, when pores are filled – partially or fully – with water, dielectric proper- ties are enhanced on the one hand and on the other hand water induces dilatancy when rock is pressurized. The latter occurs in the hypocenter area of an earthquake which is surrounded by water saturated porous Journal of Applied Geophysics 102 (2014) 77–80 ⁎ Corresponding author at: University of Athens, Physics Department, Section of Solid State Physics, Panepistimiopolis, GR 15684 Zografos, Athens, Greece. Tel.: +30 2107276729; fax: +30 2107661707. E-mail address: e_sakel@phys.uoa.gr (I. Sakellis). 0926-9851/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jappgeo.2013.12.013 Contents lists available at ScienceDirect Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo