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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.

Simultaneous acoustic (AE) and electromagnetic (EME) emissions during uniaxial compression of polycrystalline natural rocks which behave like brittle 'isotropic' piezoelectrics are studied experimentally. The plastic deformation of the brittle rocks is carried out mainly by microcracking which due to piezoelectric properties of the samples is accompanied by generation of electroacoustic waves. Such excitations can be interpreted as polarization currents and, consequently, as sources of electromagnetic fields that can be detected not only within the piezoelectric sample but also in its exterior. As a result, electromagnetic irradiation from deformed sample is accessible to be registered by an external antenna. The corresponding mechanism differs essentially from EME of moving dislocations in ionic crystals. We present the theoretical description of the electroacoustical irradiation during microcracking in isotropic piezoelectric plate. An agreement between theory and experiment has been discussed. ©2014 Elsevier Ltd. All rights reserved.

Simultaneous measurements of acoustic emissions (AE) and electromagnetic emissions (EME) during plastic deformation and destruction under uniaxial compression along 001 direction are made on LiF monocrystals after gamma irradiation by 60Co source. The irradiation doses are 1, 2, and 10 Mrad. The EME measurements in the radio-frequency range are carried out using two types of electromagnetic sensors: (i) a simple electrical stub antenna and (ii) a toroidal inductance coil. Two checking experiments on unirradiated crystals are performed as the starting point to discover the effect of gamma irradiation on acoustic and electromagnetic emissive ability of plastically deformed ionic crystals. Unirradiated LiF monocrystals demonstrate high-intensive EME at easy glide and work hardening stages, as well as at the fracture during destruction of the sample. At radiation doses more than ∼1 Mrad, in the active loading stage the EME of LiF monocrystals vanishes, except few individual electromagnetic pulses (only at 1 and 2 Mrad doses), which are time correlated with well-defined drop-jumps on the loading diagram and therefore can be associated with macroscopic crack openings. Moderate electromagnetic activity in irradiated crystals occurs only in the final stage of deformation at the complete fracture of the sample. Thus, after gamma irradiation the formation of polarization currents due to dynamic interaction between charged vacancies and moving dislocations is suppressed, and only EME connected with the redistribution of the free charge on the crack branches is observed. Acoustic emission diagrams of low-irradiated LiF are typical for the work hardening stage in crystals containing a great amount of strong point stoppers. At larger irradiation doses the AE diagram displays quite different behavior at low- and high-loading regions with a sharp boundary between them. The low-loading region shows poor AE activity, which changes sharply into high-active burst-like emission with an increase in loading. The boundary between two regions shifts to higher loadings with radiation dose. The higher is the radiation dose the lower is the relative intensity of AE in the high-stressed region. The physical mechanisms of EME and AE in gamma-irradiated ionic crystals are discussed. © 2011 American Institute of Physics.

In this paper, we performed experiments of uniaxial compression of granite samples and recorded time series of electromagnetic pulses during the evolution of the catastrophic fracturing process. The cumulative energy release of the electromagnetic emission (EME) up to the critical point at the moment of rupture was then calculated. It was shown, that the validity of the proposed hierarchy models for the catastrophic fracturing process of composite materials, in analogy to critical phenomena, can be experimentally established not only via acoustic emission data, but via electromagnetic emission data as well. The above conclusion could be a useful tool for the improvement of the earthquake prediction method, based on precursory electromagnetic signals. © Author(s) 2011.

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.

Experiments on the plastic deformation of LiF ionic monocrystals under uniaxial compression are performed with simultaneous recording of acoustic (AE) and electromagnetic (EME) emissions. A strong correlation between AE and EME events has been found, which clearly demonstrates that the observed EME is caused by a dynamical interaction between moving dislocations and charged vacancies in the ionic lattice during work hardening. The mechanism proposed to explain EME is based on the assumption that gliding edge dislocations sweep up the vacancies of a preferable sign. As a result, when a dislocation pileup is formed, a certain nonequilibrium charge density is accumulated at its head, resulting in electric polarization of the deformed crystal. As the external loading increases, a locked dislocation pileup bursts through the stoppers and quickly loses its bounded charge. The relaxation of this charge produces an intrinsic polarization current generating an electric pulse. It is assumed that the relaxation current can be described as an athermic viscous motion of vacancies under the kinetic friction force ∼Bv (B is the friction coefficient and v is the vacancy velocity) in a self-consistent electric field determined by the distribution of the total charge density. A nonlinear integro-differential equation of motion for the nonequilibrium charge density is derived. For a special form of the initial charge density distribution, an automodel solution of this equation describing the polarization current has been built. The electrical signal generated by an acting slip system has been calculated. By comparing the calculated and experimentally measured electric signal patterns, the friction coefficient for the linear chain of vacancies (the analog of an edge dislocation extra plane) in LiF has been estimated to be B 0.9× 10-5 g cm-1 s-1. This value is in accordance with the corresponding coefficient for dislocations in ionic lattices. © 2007 The American Physical Society.

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.

Strong electromagnetic (EM) anomalies have been detected, from MHz to kHz, prior to the three destructive earthquakes occurred during 1995-1999 in Greece. The observed sequence of EM anomalies before the Kozani-Grevena earthquake (K-G) of Ms = 6.6 on May 13, 1995 showed some important characteristics: (i) an increasing electromagnetic emission rate; (ii) an emergence of lower kHz frequencies with large amplitudes at the tail of the MHz electromagnetic anomaly; (iii) an electromagnetic quiescence approximately a few hours before the earthquake and (iv) a total absence of EM anomalies during the aftershock period. All these features are compatible with those reported by other authors. This sequence of the field observed EM signals revealed, in terms of emission pattern, similarities to the laboratory acoustic (AE) emissions during different stages of failure preparation process in rocks. Copyright 2002 by the American Geophysical Union.

Two electromagnetic (EM) anomalies have been detected in the VLF frequency band before the Athens earthquake (EQ) (Mw = 5.9, Sept. 7, 1999) with the following characteristics: (i) The first and second anomaly lasted for 12 and 17 hours respectively with a cessation of 12 hours; (ii) The second anomaly ceased at about 9 hours before the EQ; (iii) The larger anomaly, the second one, contains approximately 80% of the total EM energy received; (iv) No EM disturbance has been recorded in the VHF frequency band unlike with other cases, e.g., the Kozani Grevena and Egion-Eratini earthquakes. The fault modeling of the Athens EQ, based on information obtained by radar interferometry, predicts two faults. The main fault segment is responsible for 80% of the total energy released, while the secondary fault segment for the remaining 20%. Moreover, a recent seismic data analysis supports the hypothesis that a two-event solution for the Athens EQ, is more likely than a single event solution. In addition, the absence of surface rupture explains the absence of EM detection in the VHF frequency band. The present analysis reveals that the properties of the preseismic electromagnetic anomalies might be considered as signatures of a pending earthquake.

A Special Issue (Geophys. Res. Lett. 23, 1996) was focused on the question whether the VAN predictions outperform random chance. The majority of the participants of this Debate was selected to be critics against VAN, but Varotsos and co-workers accepted to participate and responded to the critical comments. VAN critics, including Geller (1996), share the same 'requirements', which decrease the success rate by 50% and the alarm rate by a factor 4-5. Furthermore, these 'requirements', when applied to an ideally perfect earthquake prediction method (which successfully predicts all earthquakes above a certain threshold, and does not issue any false alarm), lead to the following paradoxes: (a) the success rate is below 100%, (b) the alarm rate is 23% only, and (c) the ideal precursors are 'postseismic' signals. Recent statistical treatments by Hamada (1996) and by Aceves et al. (1996) coincide to the conclusion that VAN predictions cannot be ascribed to chance.

During the uniaxial compression of LiF crystals, transient polarization electric currents are recorded, with density of the order of some nA/cm2 which are attributed to the movement of segments of charged dislocations relative to their Debye-Huckel cloud, consisting of point defects. On the other hand, during the microfracturing regime of the deformation process, near field electromagnetic disturbances in the frequency band of some KHz to some MHz are detected, which are attributed to the electrical phenomena accompanying the microcrack opening. Furthermore, experiments with γ-irradiated LiF crystals were carried out. A careful consideration of the aforementioned experimental results could support the more plausible interpretation concerning the origin of the electrical phenomena preceding fracture.

In our preceding Reply, we indicated that Mulargia et al. [1996] made (beyond their obvious error that they checked their predictive ‘’rule” only for its ‘’learning period”) a number of mistakes; we also showed that their ‘’rule” does not correspond to a meaningful algorithm. Mulargia et al.’s Re-Rebuttal admits that Mulargia er al. [1996] actually made a number of mistakes due to a ‘’bug in the [Mulargia et al.’s, 1996] code”, which not only omitted from their list two (non-”predicted” by their ‘’rule”) ‘’large” earthquakes (EQs), but also scored two missed (”large”) EQs as successfully ‘’predicted”. Furthermore, they now admit that Mulargia et al.’s [1996] rule ‘’is certainly not an efficient predictor”, in contrast to their earlier claims. The main issue of our present Reply is to point out that Mulargia et al., in their Re-Rebuttal, now make a very serious error, when constructing the errors diagram: they confuse predictions of main shocks with those of the aftershocks, and hence incorrectly conclude that one can ‘’build very simple, zero-cost predictive tools superior to VAN”. We show that their erroneous procedure leads to the following paradox: when a ‘’rule” (which fails to predict all main shocks) correctly ‘’predicts” a number of aftershocks, one can (incorrectly) claim that he found a predictive ‘’rule” superior to the ideal prediction method; the latter (i.e., the ideal one), in spite of the fact that it predicts all main shocks, is (incorrectly) obtained to correspond to ‘’random predictions”.

Mulargia et al. [1996] claim that earthquakes (EQs) can be ‘’predicted” (in retrospective) ‘’much more efficiently than VAN” using a ‘’rule”, they obtained from PDE catalogue. We show that this claim is undoubtedly wrong. Their ‘’rule” issues a great number of false alarms, which exceeds that of the ‘’predicted” EQs (mainly aftershocks) by a factor larger than 10. The errors diagram recommended by Keilis-Borok [1996], reveals that Mulargia et al.’s [1996] ‘’rule” corresponds to a non-meaningful algorithm; on the other hand, this diagram reflects that VAN is meaningful.

Our earlier publications show that VAN method does not fail requirements (1) and (2) suggested by Jackson [1996]. No subjective ex-post facto decission was necessary for the evaluation of the success because, for the large majority of VAN predictions, the values of Delta M, Delta r and Delta t were published before. the period 1987-1989 under discussion; in a few cases only (three out of 29), related with the observation of the new phenomenon of the SES electrical activity, the value of Delta t was determined in 1988. Furthermore, a careful inspection -from physical point of view- shows that the three plausibility criteria, suggested by Jackson (to be obeyed by a candidate prediction technique), are actually met by VAN-method.

A direct comparison of the predicted magnitude values (M(pred)) to the actual magnitude values (M(EQ)) of the earthquakes (EQs) is allowed only when both values, i.e., M(pred) and MEQ, refer to the same scale. In view of the fact that the Seismological Institute of the National Observatory of Athens (SI-NOA) publicly announces as MEQ the M(L)+0.5 value (Where ML the local magnitude), VAN made it clear long ago, that the predicted values M(pred) (after a proper calibration) referred to M(L)+0.5. Therefore, a self-consistent evaluation of VAN-predictions should consist of a direct comparison of M,red with the actual M(L)+0.5. Unfortunately, Wyss [1996] confuses the discussion by proceeding to a direct comparison of M(pred) With M(s)(PDE); this is not allowed because the values of M(L)+0.5 exceed, on the average, M(s)(PDE) by 1.0 unit. An additional confusion arises from the fact that the relation suggested by Hamada [1993], i.e., M(L)+0.5=m(b)+0.3, is misinterpreted by Wyss as saying M(s)(PDE)=m(b)+0.3. These two alterations by Wyss reveal that his Figures 1 and 2 are erroneous. Wyss [1996] also criticizes VAN, because (in an early publication) Varotsos et al. [1981b] used the Preliminary Bulletin of SI-NOA, instead of the final one. First of all, the final bulletin could not be used by VAN at that time, because it appeared (more than one year) after the publication of the paper by Varotsos et al. [1981b]. Secondly, the correlation between SESs and EQs is evident,when we use consistently, either the preliminary, or the final bulletin of SI-NOA. On the other hand, Wyss [1996] claims that he could not find any correlation between EQs and SESs; we show that this is due to the fact that Wyss included, in his study, small EQs that occurred several hundreds km away from the measuring VAN station (i.e., in Albania, western Turkey, etc.), but he simultaneously deleted the small magnitude EQs that occurred very close to that station. Wyss’s procedure is, of course, not acceptable and hence his Appendix B is wrong. Furthermore, Wyss’s claim that VAN added 25% of events to the list, is shown to be untrue. Beyond the unusual fact that Wyss quotes ‘’VAN’s statements” that have never been published by VAN, the following is also noted: although Wyss [1996] uses quotation marks (in order to indicate that he reproduced exactly what VAN said), he adds critical wording to VAN statements and hence their true meaning is drastically changed. For example, Wyss states: ‘’Varotros et al. [l981a] had first formulated that SESs ‘’occurred a few minutes before each earthquake [related to that SES]” (Varotsos et al. [1981a]).” Thus, Wyss leads the reader to the wrong conclusion that VAN initially claimed that SES have a lead time of a few minutes, and that VAN changed it later. However, we show that this lead time (published by VAN) referred to another the of precursor, and not to SES, but the words in brackets (which are added by Wyss) alter the true meaning of our statement.

Transient variations of the electric field are detected, prior to the failure of a rock sample which is subjected to uniaxial compression at a variable rate. These precursory electric signals are attributed to the stress induced polarization of the sample and seem to have a form similar to the so called Seismic Electric Signals (SES), which are detected in Greece by the VAN network, prior to earthquakes. The emitted electric signals seem to follow in form the variations of the first time derivative of the externally applied stress. A tentative model for the origin of these signals is also discussed. Copyright 1994 by the American Geophysical Union.

In order to understand the role of electrical properties in the vicinity of VAN stations, that have already been found to be appropriate for the collection of seismic electric signals (SES), a magnetotelluric (MT) investigation was carried out over of the entire VAN network. We present preliminary MT results for two stations of the VAN telemetric network in continental Greece. A procedure was developed by which a "real time" subtraction of the telluric inductive component from the electric measurements can be made. The resulting real time recording of the residual electric field allows a better identification of the SES. Such an example is presented for an SES that preceded a 5.5 magnitude earthquake in western Greece. © 1993.

In the 573-2098 K temperature region the diffusion coefficient of N in V varies by 10 orders of magnitude. In the present paper we show that this variation can be accounted for by the anharmonic properties of the bulk material, i.e. the temperature variations of the isothermal bulk modulus and the lattice parameter. © 1991.

The classical Jost model is applied to the calculation of defect parameters in CaF2, SrF2 and BaF2. The calculation is carried out for both processes i.e. migration and formation and the results are compared with experimental data. © 1990.

It has been argued recently that for a given process in solids involving defects (for example, defect formation, diffusion, solution of impurities, etc.) the corresponding enthalpy hi and entropy si have to be interconnected through a relation. Where β denotes the thermal volume expansion coefficient and B the isothermal bulk modulus of the host material. Thus the microscopic parameters characterizing a certain process are connected to a macroscopic quantity F characterizing the material itself. More specifically, the quantity si/hi depends solely on the bulk properties of the host crystal and it is independent of the various elements which participate in the process. In the present note the relation has been checked for the solubility of various foreign atoms in silicon.