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A crucial question of the scientific community nowadays, concerns the existence of electric signals preceding earthquakes. In order to give a plausible answer to this question, we carried out two kinds of laboratory experiments of uniaxial deformation of ionic crystals and rock samples: a) In the first kind, stress induced polarization currents are detected and recorded. Our experimental results showed not only the existence of stress induced polarization currents before the fracture of the samples, but the possibility of the propagation of these signals, as well, through conductive channels, for distances much longer than the source dimensions. b) In the second, acoustic and electromagnetic signals are detected and recorded in the frequency range from 1 KHz to some MHz. The mechanism of generation of these signals is shown to be different for those emitted from piezoelectric and from non-piezoelectric materials. A plausible model is also suggested, on the compatibility of our laboratory results with the processes occuring in the earth during the earthquake preparatory stage. © European Geosciences Union 2004.

During the uniaxial compression of LiF ionic monocrystals, acoustic and electromagnetic emissions (EME) are detected. We observed that when the compression is performed in successive loading, unloading cycles and these emissions are being monitored, no new emissions will occur unless the maximum stress of the previous cycle is exceeded, meaning that the material presents memory characteristics. This is observed not only for the acoustic emission (AE), which is the well known Kaiser effect, but for the EME as well. In other words, the material appears to memorize and reveal the previously maximum stress it suffered while being deformed. The importance of an electromagnetic memory feature of a material can be related to various applications in material science, especially when the detection of AE is not feasible or gives false alert. Such cases may very well be earthquakes' predictive indications, monitoring of mines' stability, imminent landslides, etc. © 2008 American Institute of Physics.

It is known that during uniaxial compression of dielectric crystalline materials, acoustic and electromagnetic (EM) emission is detected. Such emissions are of particular interest, since they are related to the physical processes taking place during the preparation stage of an earthquake. In this letter, we report recent experimental results on the memory effects observed in the EM emission during successive cyclic mechanical loading-unloading of LiF crystal samples in a similar fashion, as it has been described by the so-called "Kaiser effect," i.e., the memory effect for the acoustic emission, and we discuss on a conceptual explanation. © 2005 IEEE.

The acoustic and electromagnetic emission detected during the uniaxial compression and before the catastrophic failure of dielectric crystalline materials is investigated in this letter. In particular, an appropriate experimental and measuring setup has been developed and used to simultaneously detect and monitor in both large and small scale the time series of acoustic and electromagnetic pulses emitted during deformation of a compressed LiF crystal sample, allowing a plausible mechanism of their origin to be suggested.

The electromagnetic (EM) activity observed before earthquakes has received a lot of attention of the scientific community. Various aspects have been proposed in order to interpret these observed EM phenomena, and the most plausible interpretation is based on the microfracturing electrification. The scope of this paper is the laboratory investigation of this aspect by carrying out laboratory experiments for the detection of EM emission from rock samples and other crystalline materials under uniaxial compression. This paper reveals physical processes that occur in the compressed material, in the microscopic scale, resulting in the observed macroscopic EM phenomena. It was experimentally verified that (a) an abrupt microcracking event generates temporally varying EM field, not only in piezoelectric materials but in non-piezoelectric ionic crystals as well. This implies that the EM generation mechanism is not necessarily of piezoelectric origin. (b) In the presence of a large number of microfractures, each one of them essentially acts as an elementary emission source and thus the resulting spectrum is correlated to the spectral content of each individual pulse and (c) the microcracking process might display criticality. © 2004 Elsevier Ltd. All rights reserved.