Konstantinos Eftaxias

*University of Athens, Faculty of Physics, Department of Solid State Section, Panepistimiopolis Zofrafos, Athens Greece* 

#### **1. Introduction**

216 Earthquake Research and Analysis – Statistical Studies, Observations and Planning

[13] Nur A. Dilatancy, pore fluids, and premonitory variations of ts/tp travel times. Bull

[14] Scholz C H, Sykes L R, Aggarwal Y P. Earthquake prediction: A physical basis. Science,

[15] Du X B, Yan Z D, Zou M W, et al. Process of source dynamics of the Jingtai earthquake

[16] Seismological Bureau of Sichuan Province. The Songpan Earthquake in 1976 (in

[17] Du X B, Zhang X J, Zhang H, et al. The spatial characteristics of the short-term and

[18] Ma J, Ma S L, Liu L Q. The stages of anomalies before an earthquake and the characteristics of their spatial distribution (in Chinese). Seismol Geol, 1995, 17: 363~371 [19] Zheng G. L., Du X. B., Chen J. Y., et al. Influence of active faults on erathquake-related

[20] Du X B, Tan D C. On the temporal and spatial clusters of one-year scale anomalies of

[21] Ye Q, Du X B, Chen J Y, *et al*. One-Year Prediction for the Dayao and Minle-Shandan

[22] А.P.Кrаеv., 1951.Geoelectrics Principle. Moscow State's technological and theoretical

[23] W.F. Brace., 1968. Electrical resistivity changes in saturated rocks during fracture and

[24] Chen D Y, Chen F, Wang L H, 1983. Study of rock resistivity under uniaxial press-Anisotropy of resistivity. Acta Geophysica Sinica, Vol. 26 (Supp.): 783~792 [25] Lu Y Q, Qian J D, Liu J Y. An experimental study on the precursory features of apparent

[26] Qian F Y, Zhao Y L, Huang Y N. Anisotropic parameters calculation of earth resistivity and seismic precursory examples.Acta Seismologica Sinica, 1996, 9(4): 617~627 [27] Du X B, Ye Q, Ma Z H, et al. The detection depth of symmetric four-electrode resistivity

[28] Guo Z J, Qin F Y. 1979. Physics of Earthquake Source (in Chinese). Beijing:

[29] О.М. Barsukov, 1979.A possible cause of electrical precursors to earthquake. Earth's

[30] Mei S R, Fen D Y, Zhang G M, 1993. Introduction of Earthquake Research in China.

[31] Crampin S, Evan R, Atkins B.K., 1984. Earthquake prediction: a new physical basis.

imminent anomalies of water radon before earthquake in the mainland of China.

anomalies of geo-electric resistivity (in Chinese, with an English abstract). Acta

earth-resistivity and the relation to seismicity (in Chinese, with an English

Earthquakes in 2003 (in Chinese, with an English abstract). Journal of seismological

resistivity and acoustic emission of large scale of granite specimen during the process of slowly dilatancy rupturing (in Chinese, with an English abstract).

observation in/near the epicentral region of strong earthquakes (in Chinese, with an English abstract). Chin J Geophys, 2008, 51: 1943–1949. http://www.agu.org/wps /cjg, Chinese J Geophys (in English), 2008, 51(6):

Seismol Soc Amer, 1972, 62: 1217~1222

Acta Seismologic Sinica, 1996, 9: 461~470

Seismol Sinic, 2011, 33: 187~197

research, 2005, 28(13): 226~230

liber press of Soviet Union, 10~50

Northwestern Seismol J, 1990, 12: 35~41

1220~1228

Seismological Press, 100~170

Beijing: Seismological press, 302~307

Geophys. J.R.astr.soc, Vol.76, 147~156

Physics, Vol. 8, 85~90

(M=6.2). Acta Seismologic Sinica, 1994, 7: 379~388

Chinese). Beijing: Seismological Press, 1979. 4~5, 86~91, 103

abstract). Earthquake Research in China, 2000, 6: 283~292

fractional sliding, J. Geophys. Res., Vol. 73, 1433~1444

1973, 181: 803~810

In recent years, the wind prevailing in the scientific community does not appear to be favourable for earthquake (EQ) prediction research, in particular for the research of short term prediction [1]. Sometimes the arguments were extended to the extreme claim that any precursory activity is impossible [2]. Considering the difficulties associated with such factors as the highly complex nature, rarity of large EQs and subtleties of possible preseismic signatures, the present negative views are not groundless. It is difficult to prove associations between any two events (possible precursor and EQ) separated in time. To a certain extent, the aforementioned negative views were due to the fact that in the last decades the study of seismic precursors was expected to lead in a relatively short period of time to EQ prediction. However, the EQs are nothing but physical phenomena, and science should have some predictive power on their future behaviour of any physical system. In spite of this scepticism of the scientific community, the research towards the possible prediction of EQs in the future continues. This is attempted now with a more critical view taking into account new ideas and performing detailed theoretical, laboratory, field, and numerical investigations. Significant progress has been made in the research of precursory pattern changes of seismicity (e. g., Wyss and Martirosyan,[3]; Huang et al. [4]; Huang [5]) and the intermediate-term prediction of large EQs world-wide is already in the statistically proven stage (e g., Kossobokov et al. [6]). More recently, even the efforts to shorten the lead time to the "short-term" range are being made (e. g., Keilis-Borok et al.[7]). Some significant new waves have been rising in EQ science!

An EQ is a sudden mechanical failure in the Earth's crust, which has heterogeneous structures. The use of basic principles of fracture mechanics is a challenging field for understanding the EQ preparation process. A key fundamental question in strength considerations of materials is: *when does it fail?* Thus, a vital problem in material science and in geophysics is the identification of precursors of macroscopic defects or shocks. It is reasonable to expect that EQ's preparatory process has various facets which may be observed before the final catastrophe. *The science of EQ prediction should, from the start, be multidisciplinary!*

The present contribution focuses on fracture induced electromagnetic (EM) fields, which allow a real-time monitoring of damage evolution in materials during mechanical loading. Crack propagation is the basic mechanism of material failure. EM emissions in a wide frequency

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

Are There Pre-Seismic Electromagnetic Precursors? A Multidisciplinary Approach 219

**2. A proposed strategy for the study of MHz and kHz EM precursors**

(i) How can we recognize an EM observation as a pre-seismic one?

aware that this attempt refers to a *snap-shot* of a rapidly moving field.

occurrence of the prepared EQ is unavoidable?

other, to the consecutive processes occurring in Earth's crust.

This chapter concentrates, in an appropriately critical spirit, on asking 3 crucial questions:

(ii) How can we link an individual EM precursor with a distinctive stage of the earthquake

(iii) How can we identify precursory symptoms in EM observations which signify that the

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

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

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

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.,28-30,32-36,39]. In the frame of the above mentioned two stage model, the identification of MHz and kHz EM precursors

prediction.

preparation?

anomalies as seismogenic ones.

requires different methods of analysis.

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 [8-16] and geological scale [17-37].

*Data collection*: Since 1994, a station has been installed and operated at a mountainous site of Zante island (37.76*oN* <sup>−</sup> 20.76*oE*) in the Ionian Sea (western Greece). The main aim of this 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 (< 1*Hz*) 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., [21,22,25-37]). 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., [21,22,25-37]).

An important feature, observed both at laboratory and geophysical scale, is that it the MHz radiation precedes the kHz one [25,27-29,35,36]. 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 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

*Data collection*: Since 1994, a station has been installed and operated at a mountainous site of Zante island (37.76*oN* <sup>−</sup> 20.76*oE*) in the Ionian Sea (western Greece). The main aim of this 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 (< 1*Hz*) 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., [21,22,25-37]). 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

An important feature, observed both at laboratory and geophysical scale, is that it the MHz radiation precedes the kHz one [25,27-29,35,36]. 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

*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

*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

at a laboratory [8-16] and geological scale [17-37].

characteristic of an ensuing seismic event (e.g., [21,22,25-37]).

micro-branching instability, it behaves as an efficient EM emitter [32].

is justified.

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.
