Measurements of the dielectric properties of limestone under pressure and their importance for seismic electric signals

•Complex impendence experiment at various T and P for as-received & wetted limestone•Negative activation volume in samples from a sensitive SES station is traced.•Findings support a model suggested for the SES generation before an earthquake.•Water incorporated in the samples enhances the value of negative activation volume.•Water lowers the energetic threshold of the process to be activated, more than 80%.

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 spectroscopy. 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.