Stress induced polarization currents and electromagnetic emission from rocks and ionic crystals, accompanying their deformation

International audienceA 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 1KHz 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 occurring in the earth during the earthquake preparatory stage

Electrical precursors of earthquakes in Aegean Sea during the last decade (1997?2007)

International audienceThe purpose of this study is to investigate some properties of the Seismic Electric Signals (SES) that preceded large earthquakes which occurred in the Aegean Sea (24?27)° E, (37?40)° N, during the last decade. Our main interest is focused on the important parameter of the lead time ?t, which is the time difference between the occurrence of the earthquake and the detection of the associated SES signal. Two groups of lead times, a short (i.e. ?t~ some weeks) and a long one (?t~ some months) have been observed. We examine whether this difference could be related to the regional tectonics. Furthermore the property of SES selectivity is discussed

Experimental evidence of the compatibility of the cumulative electromagnetic energy release data, with the hierarchical models for the catastrophic fracturing process

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

Evolving towards a critical point: A possible electromagnetic way in which the critical regime is reached as the rupture approaches

International audienceIn 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 radio-electromagnetic 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 electro-magnetic 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 electromagnetic critical behaviour of the failing system is

Possible relationship between Seismic Electric Signals (SES) lead time and earthquake stress drop

Stress drop values for fourteen large earthquakes with MW ≥ 5.4 which occurred in Greece during the period 1983–2007 are available. All these earthquakes were preceded by Seismic Electric Signals (SES). An attempt has been made to investigate possible correlation between their stress drop values and the corresponding SES lead times. For the stress drop, we considered the Brune stress drop, ΔσB, estimated from far field body wave displacement source spectra and ΔσSB derived from the strong motion acceleration response spectra. The results show a relation may exist between Brune stress drop, ΔσB, and lead time which implies that earthquakes with higher stress drop values are preceded by SES with shorter lead time

Analysis of Self‐Diffusion Data in V and Nb

Self‐diffusion studies have shown that V and Nb are characterized by a non‐linear Arrhenius behaviour. This behaviour is usually explained within the frame of the existence of two or three diffusion mechanisms and the relevant analysis requires the determination of a number of adjustable parameters. It is shown that the non‐linearity disappears when one plots In D vs. (BΩ)/(kT) instead of the usual In D vs. 1/T. Such an analysis does not need the introduction of any adjustable parameter and allows the determination of the activation entropy, enthalpy, and attempt frequency. Copyright © 1989 WILEY‐VCH Verlag GmbH & Co. KGa

Temperature and pressure variation of self-diffusion of Ge in relation to the bulk properties

A model has been suggested by Varotsos and Alexopoulos that connects the point defect parameters with bulk properties. The validity of this model is examined for the case of Ge. We find that it reproduces the temperature variation of the self-diffusion coefficient D in the temperature range 850-1176 K. The same model is found to give a correct estimation of the activation volume that results from the pressure variation of the self-diffusion coefficient. © 1991