No abstract

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.

Abstract. In analogy to the study of critical phase transitions in statistical physics, it has been argued recently that the fracture of heterogeneous materials could be viewed as a critical phenomenon, either at laboratory or at geophysical scales. If the picture of the development of the fracture is correct one may guess that the precursors may reveal the critical approach of the main-shock. When a heterogeneous material is stretched, its evolution towards breaking is characterized by the appearance of microcracks before the final break-up. Microcracks produce both acoustic and electromagnetic (EM) emission in the frequency range from VLF to VHF. The microcracks and the associated acoustic and EM activities constitute the so-called precursors of general fracture. These precursors are detectable not only at laboratory but also at geophysical scales. VLF and VHF acoustic and EM emissions have been reported resulting from volcanic and seismic activities in various geologically distinct regions of the world. In the present work we attempt to establish the hypothesis that the evolution of the Earth's crust towards the critical point takes place not only in a mechanical but also in an electromagnetic sense. In other words, we focus on the possible electromagnetic criticality, which is reached while the catastrophic rupture in the Earth's crust approaches. Our main tool is the monitoring of micro-fractures that occur before the final breakup, by recording their radioelectromagnetic emissions. We show that the spectral power law analysis of the electromagnetic precursors reveals distinguishing signatures of underlying critical dynamics, such as: (i) the emergence of memory effects; (ii) the decrease with time of the anti-persistence behaviour; (iii) the presence of persistence properties in the tail of the sequence of the precursors; and (iv) the acceleration of the precursory electromagnetic energy release. Moreover, the statistical analysis of the amplitudes of the electromagnetic fluctuations reveals the breaking of the symmetry as the theory predicts. Finally, we try to answer the question: how universal the observed Correspondence to: K. A. Eftaxias (ceftax@phys.uoa.gr) electromagnetic critical behaviour of the failing system is?

Abstract:Electromagnetic (EM) anomalies were detected in a wide frequency band prior to the two strongest earthquakes that occurred in Greece during 1995, i.e., at Kozani-Grevena and Egio-Eratini areas. The analysis of these EM anomalies indicates that these EM anomalies should be pre-seismic ones. 'Pvo basic characteristics emerged: (i) The lead time of each disturbance was systematically smaller in the case of Kozani-Grevena earthquake than that of the Egio-Eratini earthquake. (ii) An almost simultaneous cessation of the EM anomalies, although they had different onset times and were detected in a wide frequency range. As for the seismic activity that occurred in Greece during the last year, pre-seismic EM anomalies were detected before (i) the September 7, 1999, Athens earthquake and (ii) other smaller EQs. The possibility to predict the parameters of an impending EQ from the EM anomalies is also investigated.

The low-temperature thermal depolarization spectrum of natural single-crystal calcite exhibits two relaxation mechanisms: the lower temperature ͑LT͒ one located at 188 K and the medium temperature ͑MT͒ one located at 230 K. Extensive thermally stimulated depolarization current ͑TSDC͒ experiments proved that the LT band is related to the rotation of defect dipoles, while the MT one to the interfacial polarization. A theoretical quantitative model, which involves the SiO 2 inclusions in the matrix, is developed and successfully interprets the appearance and the behavior of the MT band.