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Abstract: Fracture induced physical fields allow a real-time monitoring of damage evolution in materials during mechanical loading. We investigate the preparation of the recently occurred L’Aquila earthquake in terms of a detected precursory electromagnetic anomaly. The precursor is well described by a recently introduced model for earthquake dynamics, which has been rooted in a nonextensive Tsallis framework starting from first principles. The analysis in terms of nonextensivity implies that the well established aspect of self-affine nature of faulting and fracture is hidden into the precursor, namely, the activation of the L’Aquila fault is a reduced self-affine image of the regional seismicity covering many geological faults. The Gutenberg–Richter magnitude–frequency relationship, the best known scaling relation for earthquakes, verifies the results based on nonextensivity. The latter suggests that the activation of the L’Aquila fault is a magnified image of the laboratory seismicity by means of acoustic and electromagnetic emissions. Finally, we present evidence for universality in magnetic storm, earthquake, and electromagnetic precursor occurrence by means of complexity and nonextensivity. [Copyright &y& Elsevier]

Abstract: MHz-to-kHz electromagnetic (EM) anomalies have been detected worldwide over time intervals ranging from a few days to a few hours prior to near-surface earthquakes (EQs) that have occurred on land with magnitude approximately six or larger. The MHz radiation systematically appears earlier than the kHz. Laboratory experiments on rock samples reveal a similar change in the frequency content during progressive deformation, i.e. the emissions exhibit a frequency shift from MHz to kHz just before failure of the samples. Herein, we attempt to associate these consecutively emerging MHz and kHz modes of the pre-fracture EM emission with successive distinct stages of the fracture preparation process and the associated fracture mechanisms. We base our approach on synergetic principles of physical mesomechanics. A universal principle of physical mesomechanics is that the plastic deformation of a solid is related to its loss of shear stability at the micro-, meso-, and macroscale levels. Global fracture is determined by the mechanism controlling the development of macroscale bands of localized deformation associated with meso- and microscale relaxation processes. The major result we obtain is that the shift from MHz to kHz EM activity may signal the transition of plastic flow localization from the mesoscale to the macroscale, culminating in global fracture. A complexity measure (Approximate Entropy) and a persistency–anti-persistency measure (Hurst exponent) also give evidence of a two stage-model of EQ preparation process: significant complexity decrease and accession of persistency can be confirmed in the strong kHz EM activity that emerges in the tail of the pre-seismic EM activity. Laboratory experiments on rock samples reveal similar symptoms of an imminent global instability. [Copyright &y& Elsevier]

We introduce a new model of the generation of pre-seismic electromagnetic emissions, which explains the observed phenomenology in terms of its geometry and fractal electrodynamics. Accumulated evidence indicates that an earthquake can be viewed as a critical phenomenon culminating in a large event that corresponds to a type of critical point. The principle feature of criticality is the fractal organization in both space and time. Earthquakes display a complex spatio-temporal behavior: in addition to the regularity in the rate of occurrence (e.g. Gutenberg-Richter law, Omori law), the spatial distribution of epicenters is fractal and earthquakes occur on a fractal structure of faults. Thus, the hypothesis that the fault develops as a fractal is reasonable. A mounting body of laboratory evidence suggests that micro-fracturing of rocks are associated with the appearance of spontaneous charge production and transient electromagnetic emissions (EME). The emitting, diffusing and recombination charge accompanying the micro-fracturing, can act as current generated during the crack opening. In this view, an active crack or rupture, can be simulated by a "radiating element." The idea is that a fractal geo-antenna (FGA) can be formed as an array of line elements having a fractal distribution on the ground surface as the critical point is approached. We test this idea in terms of fractal electrodynamics: we argue that the precursory VLF-VHF EM signals associated with recent earthquakes in Greece are governed by characteristics (e.g. scaling laws, temporal evolution of the spectrum content, broad band spectrum region and accelerating emission rate) predicted by fractal electrodynamics. [ABSTRACT FROM AUTHOR]

The ultra-low-frequency (ULF) magnetic variations recorded prior to the 11 March 2011 Tohoku earthquake (EQ) are analyzed using the method of critical fluctuations (MCF). The first application of this specific method to ULF magnetic variations is performed on the unprocessed H- (horizontal) and Z- (vertical) components, as well as on the unprocessed total intensity, F, of the recorded magnetic field. The motivation for the present study was given by recent research results reporting criticality features in the ULF magnetic fields prior to the 2011 Tohoku EQ. These results call for a further analysis in order to verify the existence of criticality embedded in the specific ULF recordings using an independent method, as well as to investigate the type of the embedded criticality. The application of MCF indeed verifies the existence of criticality in the ULF magnetic fields a few days to one week before the occurrence of the main shock. Specifically, clear signatures of intermittent criticality are found in the recordings of 4 March 2011, while indications of critical behavior are also found in 3 and 6 March recordings, although not of the “stability” of those of 4 March. Moreover, it is found that only the ULF data of the nearest, to the epicenter of the EQ, geomagnetic observatory presented criticality. This finding further corroborates the view that the presented signal is indeed a precursor to the specific EQ. [ABSTRACT FROM AUTHOR]

A multi-spectral analysis of the kHz electromagnetic time series associated with Athens' earthquake (M = 5.9, 7 September 1999) is presented here, that results to the reliable discrimination of the fracto-electromagnetic emissions from the natural geo-electromagnetic field background. Five spectral analysis methods are utilized in order to resolve the statistically significant variability modes of the studied dynamical system out of a red noise background (the revised Multi-Taper Method, the Singular Spectrum Analysis, and the Wavelet Analysis among them). The performed analysis reveals the existence of three distinct epochs in the time series for the period before the earthquake, a “quiet”, a “transitional” and an “active” epoch. Towards the end of the active epoch, during a sub-period which is approximately starting two days before the earthquake, the dynamical system passes into a high activity state, where electromagnetic signal emissions become powerful and statistically significant almost in all time-scales. The temporal behavior of the studied system in each one of these epochs is further searched through mathematical reconstruction in the time domain of those spectral features that were found to be statistically significant. The transition of the system from the quiet to the active state proved to be detectable first in the long time-scales and afterwards in the short scales. Finally, a Hurst exponent analysis revealed persistent characteristics embedded in the two strong EM bursts observed during the “active” epoch. [ABSTRACT FROM AUTHOR]

Fracture-induced electromagnetic emissions (EME) have been observed from the laboratory to the geophysical scale permitting the monitoring of damage evolution. It has been shown that the first appearing MHz EME presents second-order phase transition characteristics and has been attributed to the fracture process of the heterogeneous medium surrounding the family of strong entities (asperities) distributed along the fault sustaining the system. The finally, abruptly, emerged strong avalanche-like kHz EME do not present any footprint of a second-order transition in equilibrium, while they have been attributed to the fracture of the family of the, relatively homogeneous, asperities themselves. In the present work we show that between these two stages of the fracture process, an intermediate stage exists. This is reflected to the tricritical behavior which is revealed for the kHz EME just before the emergence of the strong avalanche-like kHz emission. The identification of this tricritical behavior is performed using the method of critical fluctuations (MCF). The results obtained for the kHz time-series are compatible with the results obtained for an introduced model map which describes the tricritical crossover. A Hurst exponent analysis shows that this crossover indicates the limit of an antipersistence dynamics. It is finally shown, by the MCF, that first-order phase transition characterizes the final rupture of the asperities. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we investigate the dynamics of electromagnetic precursors, recorded prior to significant earthquakes in Greece. The analysis of these signals is performed using Fisher information, which is a powerful tool for investigating complex and non-stationary signals. Our results point to a decrease of the precursor complexity as the main tectonic event is approaching. The results are compatible to those derived using Tsallis entropy. The sensitivity of Fisher information and Tsallis entropy are compared on the basis of the features of the underlying fracture process which they reveal. [Copyright &y& Elsevier]

A wide variety of electromagnetic phenomena possibly related with earthquake (EQ) preparation processes have been reported in the literature during the last few decades. An interesting aspect in their study is the time series analysis of the related observables aiming at the investigation of any embedded dynamics. In this review article we focus on the study of fracto-electromagnetic emissions (fracto-EME) at the MHz band, the ultra-low frequency (ULF) magnetic field variations (<3 Hz) and the subionospheric very low frequency (VLF) propagation anomalies. We present recent analysis results for these electromagnetic signals using two independent methods which are known for their ability to uncover critical dynamics, the recently proposed method of critical fluctuations (MCF) and the natural time (NT) analysis method. Our results show that all three considered electromagnetic signals present critical characteristics from a few weeks up to a few days before the main shock occurrence. On the other hand, signatures for the departure from the critical (highly symmetrical) state towards a low symmetry state, a state during which there is high localization of the EQ preparatory process, have been identified in specific cases for the MHz fracto-EME as well as for the ULF magnetic field variations. Based on a multidisciplinary analysis, a four-stage model of EQ dynamics by means of fracto-EME in the MHz and kHz bands has recently been proposed. The hypothesis that the precursors considered in this article emerge during the spatially extensive phase of EQ preparation, which corresponds to the first stage of the abovementioned four-stage model, as well as their relation with the foreshock seismic activity are discussed. [ABSTRACT FROM AUTHOR]

Criticality of complex systems reveals itself in various ways. One way to monitor a system at critical state is to analyze its observable manifestations using the recently introduced method of natural time. Pre-fracture electromagnetic (EM) emissions, in agreement to laboratory experiments, have been consistently detected in the MHz band prior to significant earthquakes. It has been proposed that these emissions stem from the fracture of the heterogeneous materials surrounding the strong entities (asperities) distributed along the fault, preventing the relative slipping. It has also been proposed that the fracture of heterogeneous material could be described in analogy to the critical phase transitions in statistical physics. In this work, the natural time analysis is for the first time applied to the pre-fracture MHz EM signals revealing their critical nature. Seismicity and pre-fracture EM emissions should be two sides of the same coin concerning the earthquake generation process. Therefore, we also examine the corresponding foreshock seismic activity, as another manifestation of the same complex system at critical state. We conclude that the foreshock seismicity data present criticality features as well. [ABSTRACT FROM AUTHOR]

Abstract: Laboratory experiments have shown that, during a fracture, the breaking of a bond launches a propagating stress wave which may trigger the breaking of other bonds. We examine here the possibility that the same holds on a geophysical scale. Based on a nonextensive approach, we examine whether the transient stresses of seismic waves from a major earthquake (EQ) can trigger a considerably distant significant EQ. We use three different analytical approaches: (i) a recently introduced fragment-asperity interaction model for EQ dynamics based on nonextensive Tsallis statistics; (ii) the Hurst exponent; (iii) organization in terms of Fisher information. We find that the triggered seismicity displays higher nonextensivity, increased persistent behavior, and a higher level of organization. Using the same approaches, we further elucidate the link between the associated precursory kHz electromagnetic (EM) activity and the last stage of the impending EQ generation. We examine whether the statistics of regional seismicity could be a macroscopic reflection of physical processes in the EQ source, as would be expected by the fractal nature of fracture and faulting. [Copyright &y& Elsevier]

Abstract: Earthquakes (EQs) are large-scale fracture phenomena in the Earth’s heterogeneous crust. Fracture-induced physical fields allow a real-time monitoring of damage evolution in materials during mechanical loading. Electromagnetic (EM) emissions in a wide frequency spectrum ranging from kHz to MHz are produced by opening cracks, this can be considered as the so-called precursors of general fracture. We emphasize that the MHz radiation appears earlier than the kHz on both laboratory and geophysical scales. An important challenge in this field of research is to distinguish characteristic epochs in the evolution of precursory EM activity and identify them with the equivalent last stages in the EQ preparation process. Recently, we proposed the following two-stage model. (i) The first epoch, which includes the initial emergent MHz EM emission, is thought to be due to the fracture of a highly heterogeneous system that surrounds a family of large high-strength asperities distributed along the activated fault sustaining the system. (ii) The second epoch, which includes the emergent strong impulsive kHz EM radiation, is due to the fracture of the asperities themselves. A catastrophic EQ of magnitude Mw=6.3 occurred on 6 April, 2009 (06/04/09) in central Italy. The majority of the damage occurred in the city of L’Aquila. Clear kHz–MHz EM anomalies had been detected prior to the L’Aquila EQ. Here, we investigate the seismogenic origin of the MHz part of the anomalies. The analysis, which is in terms of intermittent dynamics of critical fluctuations, reveals that the candidate EM precursor (i) can be described as analogous to a thermal continuous phase transition and (ii) has anti-persistent behavior. These features suggest that this candidate precursor was triggered by microfractures in the highly disordered system that surrounded the backbone of asperities of the activated fault. A criterion for underlying strong critical behavior is introduced. In this field of research, reproducibility of results is desirable; and is best done by analyzing a number of precursory MHz EM emissions. We refer to previous studies of precursory MHz EM activities associated with nine significant EQs that have occurred in Greece in recent years. We conclude that all the MHz EM precursors studied, including the present one, can be described as analogous to a continuous second-order phase transition having strong criticality and anti-persistent behavior. [Copyright &y& Elsevier]

Abstract: We investigate, within the framework of nonextensive Tsallis entropy, particular aspects of the relation between signal analysis and signals originating in a very special nonlinear system: the focal area of an impending earthquake. The Tsallis-like time-dependent entropy is shown to be a rather powerful tool for providing a novel quantitative strategy for monitoring the focal area states just before the earthquake occurrence. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]