Are There Pre-Seismic Electromagnetic Precursors? A Multidisciplinary Approach [chapter]

Konstantinos Eftaxias
2012 Earthquake Research and Analysis - Statistical Studies, Observations and Planning  
11 2 Will-be-set-by-IN-TECH spectrum ranging from kHz to MHz are produced by opening cracks, which can be considered as the so-called precursors of general fracture. The radiated EM precursors are detectable both at a laboratory [8] [9] [10] [11] [12] [13] [14] [15] [16] and geological scale . Data collection: Since 1994, a station has been installed and operated at a mountainous site of Zante island (37.76 o N − 20.76 o E) in the Ionian Sea (western Greece). The main aim of
more » ... his station is the detection of kHz-MHz EM precursors. Six loop antennas detect the three components (EW, NS, and vertical) of the variations of the magnetic field at 3 kHz and 10 kHz respectively; three vertical λ/2 electric dipoles detect the electric field variations at 41, 54 MHz, and 135 MHz respectively. These frequencies were selected in order to minimize the effects of the sources of man-made noise in the mountain area of the Zante Island. Moreover, two Short Thin Wire Antennas, oriented at EW and NS directions of length of 100 m, respectively, have been also installed. The aim of the last installation is the detection of ultra-low-frequency (< 1Hz) EM precursors rooted in a preseismic lithosphere-atmosphere-ionosphere-coupling. All the EM time series were sampled at 1 Hz. Such an experimental setup helps to specify not only whether or not a single EM anomaly is preseismic in itself, but also whether a sequence of EM disturbances at different frequencies, which are emerged one after the other in a short time period, could be characterized as preseismic one. Clear such EM precursors have been detected over periods ranging from approximately a week to a few hours prior to catastrophic EQs that occurred in Greece or Italy (e.g., [21, 22, [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] ). We emphasize that the detected precursors were associated with EQs: (i) occurred in land (or near coast-line); (ii) were strong, i.e., with magnitude 6 or larger; and (iii) were shallow. Recent results indicate that the recorded EM precursors contain information characteristic of an ensuing seismic event (e.g., [21, 22, [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] ). An important feature, observed both at laboratory and geophysical scale, is that it the MHz radiation precedes the kHz one [25, [27] [28] [29] 35, 36] . Studies on the small (laboratory) scale reveal that the kHz EM emission is launched in the tail of pre-fracture EM emission from 97% up to 100% of the corresponding failure strength [25 and references therein]. At the geophysical scale the kHz EM precursors are emerged from a few days up to a few hours before the EQ occurrence. The association of MHz, kHz EM precursors with the last stages of EQ generation is justified. The origin of EM emissions. The origin of EM emissions from fracture is not completely clear, and different attempts have been made in order to explain it [8, 32 and references therein]. A relevant attempt is related to the "capacitor model" [32] . In many materials, emission of photons, electrons, ions and neutral particles are observed during the formation of new surface. The rupture of inter-atomic (ionic) bonds also leads to intense charge separation, which is the origin of the electric charge between the micro-crack faces. On the faces of a newly created micro-crack the electric charges constitute an electric dipole or a more complicated system. The motion of a crack has been shown to be governed by a dynamical instability causing oscillations in its velocity and structure of the fractured surfaces. It is worth mentioning that laboratory experiments show that more intense fracto-emissions are observed during the unstable crack growth. Due to the crack strong wall vibration, in the stage of the micro-branching instability, it behaves as an efficient EM emitter [32] . Are there credible EM earthquake precursors? This is also a question debated in the science community. Despite fairly abundant circumstantial evidence, EM precursors have not been 218 Earthquake Research and Analysis -Statistical Studies, Observations and Planning Are There Pre-Seismic Electromagnetic Precursors? A Multidisciplinary Approach 3 adequately accepted as real physical quantities [1]. There may be legitimate reasons for the critical views. The degree to which we can predict a phenomenon is often measured by how well we understand it. However, many questions about fracture processes remain standing. Especially, many aspects of EQ generation still escape our full understanding. Kossobokov [38] states that "No scientific prediction is possible without exact definition of the anticipated phenomenon and the rules, which define clearly in advance of it whether the prediction is confirmed or not". We bear in mind that whether EM precursors to EQ exist is an important question not only for EQ prediction but also for understanding the physical processes of EQ generation. The comprehensive understanding of EM precursors in terms of physics is a path to achieve more sufficient knowledge of the last stages of the EQ preparation process and thus more sufficient short-term EQ prediction. A seismic shift in thinking towards basic science will lead to a renaissance of strict definitions and systematic experiments in the field of EQ prediction. A proposed strategy for the study of MHz and kHz EM precursors This chapter concentrates, in an appropriately critical spirit, on asking 3 crucial questions: (i) How can we recognize an EM observation as a pre-seismic one? (ii) How can we link an individual EM precursor with a distinctive stage of the earthquake preparation? (iii) How can we identify precursory symptoms in EM observations which signify that the occurrence of the prepared EQ is unavoidable? We shall attempt to approach the above mentioned questions in the simplest and most intuitive way, rather than emphasize mathematical rigor. In any case, the readers should be aware that this attempt refers to a snap-shot of a rapidly moving field. One wonders whether necessary and sufficient criteria, have yet been established, that permit the characterization of an EM anomaly as a real EM precursor. One of the main purposes of this contribution is to suggest a procedure for the designation of observed kHz / MHz EM anomalies as seismogenic ones. As it is said, an important feature, observed both at laboratory and geophysical scale, is that the MHz radiation precedes the kHz one [25, 28, 29 and references therein]. The remarkable asynchronous appearance of these precursors indicates that they refer to different stages of EQ preparation process. Moreover, it implies a different mechanism for their origin. Scientists ought to attempt to link the available various EM observations, which appear one after the other, to the consecutive processes occurring in Earth's crust. The following two stage model of EQ generation by means of pre-fracture EM activities has been proposed: The pre-seismic MHz EM emission is thought to be due to the fracture of the highly heterogeneous system that surrounds the family of large high-strength entities distributed along the fault sustaining the system, while the kHz EM radiation is due to the fracture of the aforementioned large high-strength entities themselves [e.g., [28] [29] [30] [32] [33] [34] [35] [36] 39] . In the frame of the above mentioned two stage model, the identification of MHz and kHz EM precursors requires different methods of analysis. 219 Are There Pre-Seismic Electromagnetic Precursors? A Multidisciplinary Approach
doi:10.5772/28069 fatcat:ypy256icibe67kggxuehnu7r64