Based on the study of pre-seismic very high frequency (VHF) and very low frequency (VLF) electromagnetic signals, we attempt to establish a set of necessary conditions referring to the underlying critical stage of the earthquake preparation process. This study combines concepts from spectral analysis associated with critical point hypothesis, results from laboratory experiments of rupture and seismological arguments. These conditions are fully satisfied in the case of the VLF-VHF pre-seismic signals associated with the Kozani-Grevena earthquake in Greece.

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Abstract. An important question in geophysics is whether earthquakes (EQs) can be anticipated prior to their occurrence. Pre-seismic electromagnetic (EM) emissions provide a promising window through which the dynamics of EQ preparation can be investigated. However, the existence of precursory features in pre-seismic EM emissions is still debatable: in principle, it is difficult to prove associations between events separated in time, such as EQs and their EM precursors. The scope of this paper is the investigation of the pre-seismic EM activity in terms of complexity. A basic reason for our interest in complexity is the striking similarity in behavior close to irreversible phase transitions among systems that are otherwise quite different in nature. Interestingly, theoretical studies (Hopfield, 1994;Herz and Hopfield 1995;Rundle et al., 1995;Corral et al., 1997) suggest that the EQ dynamics at the final stage and neural seizure dynamics should have many similar features and can be analyzed within similar mathematical frameworks. Motivated by this hypothesis, we evaluate the capability of linear and non-linear techniques to extract common features from brain electrical activities and pre-seismic EM emissions predictive of epileptic seizures and EQs respectively. The results suggest that a unified theory may exist for the ways in which firing neurons and opening cracks organize themselves to produce a large crisis, while the preparation of an epileptic shock or a large EQ can be studied in terms of "Intermittent Criticality".

The pressure derivative v of the Gibbs energy g for "unbound" and "bound" defects (i.e. vacancies or interstitials attracted by aliovalent impurities) is studied. It is shown that for the formation and migration of defects the ratio v/g depends solely on well-known macroscopic properties; applications are presented for self-diffusion in f.c.c. and b.c.c. metals and for ionic conductivity of silver halides. In the case of ionic solids containing "bound" defects, a gradual variation of pressure results in the appearance of a transient polarization or depolarization current that maximizes a t a certain pressure Per. The way of analysing this piezo-stimulated current (PSC) spectrum is discussed. The fact that the value of PCr does not have to coincide with the pressure a t which fracture occurs provides a useful tool for applying PSC to geophysics.Die Druckableitung v der Gibbsschen Energie g fur ,,ungebundene" und ,,gebundene" Defekte (d. h. Vakanzen oder Zwischengitterstorstellen, die durch aliovalente Storstellen angezogen werden) wird untersucht. Es wird gezeigt, daB fur die Bildung und Wanderung der Defekte das Verhaltnis v/g allein von gut bekannten makroskopischen Eigenschaften abhiingt, Anwendungen werden fur die Selbstdiffusion in k.f.z.-und k.r.2.-Metallen und fur die Ionenleitung in Silberhalogeniden angegeben. Fiir den Fall von Ionenfestkorpern, die ,,gebundene" Defekte enthalten, fuhrt eine graduelle h d e r u n g des Druckes zum Auftreten eines nichtstationiiren Polarisations-oder Depolarisationsstromes, der ein Maximum bei einem kritischen Druck P,, annimmt. Die Mdglichkeit einer Analyse dieses piezostimulierten Strom(PSC)-Spektrums wird diskutiert. Die Tatsache, dall der Wert von P,, nicht mit dem Druck ubereinstimmen muB, bei dem der Bruch auftritt. liefert eine nutzliche Methode fur die Anwendung des PSC in der Geophysik.

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