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The motivation

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Fracture-induced electromagnetic emissions (EME), also known as electromagnetic radiation (EMR), in a wide frequency spectrum ranging from the kHz to the MHz bands are produced by opening cracks, which can be considered as the so-called precursors of general fracture.

Improvements in the MHz-kHz EME technique (or EMR technique) have permitted a real-time monitoring of the fracture process at the laboratory scale. However, the MHz–kHz EM precursors are detectable not only at the laboratory but also at the geological scale, forming a separate section of the research field known as seismo-electromagnetics. The idea that the fracture-induced MHz-kHz EM fields should also permit the monitoring of the gradual damage of stressed materials in the Earth’s crust, as it happens in the laboratory experiments, in real-time and step-by-step, in principle cannot be excluded.

We consider earthquakes (EQs) as large-scale fracture phenomena. One cannot ignore the profound analogies between failure precursors at the laboratory and at geophysical scales. Thus, it has early been suggested that “the mechanism of EQs is apparently some sort of laboratory fracture process”.

On the other hand, it has been emphasized that it is often difficult to study the kinetics of brittle rocks’ fracture at the laboratory, due to the rapid unstable fracture growth in the last and more interesting stages. At the laboratory scale the fault growth process normally occurs violently in a fraction of a second. Therefore, crucial information probably is lost. A major difference between laboratory and natural processes is the order-of-magnitude differences in scale (in space and time), allowing the possibility of experimental observation at the geophysical scale, for a range of physical processes, which are not observable at the laboratory scale. Thus, the idea that field observations by means of EM anomalies will probably reveal features of the last crucial stages of EQ generation, which are not clearly observable at the laboratory scale, in principle, it cannot be excluded.

This has been the motivation of our reasearch over the past ~30 years.

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The network

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Based on this idea, during the time period 1992-1995, the first pilot network comprising of 4 remote telemetric stations for the recording of electromagnetic emissions in the MHz and kHz bands was deployed on the Island of Crete, south Greece by Prof. C. Nomicos, while since 1994, an exemplary telemetric station has been operating on Zakynthos (Zante) Island (Greece) under the guidance of Prof. C. Nomicos and Prof. K. Eftaxias, mainly aiming at the detection of kHz-MHz EM precursors.

The Zakynthos exemplary telemetric station has been installed in a carefully selected mountainous site in the southwest part of the island (37.76^o N–20.76^o E) providing low EM background noise. The measurement system is mainly comprised of (i) six loop antennas detecting the three components (EW, NS, and vertical) of the variations of the magnetic field at 3 kHz and 10 kHz, respectively, and (ii) three vertical ^λ/2 electric dipole antennas detecting the electric field variations at 41, 54 and 135 MHz, respectively. Moreover, two Short Thin Wire Antennas (STWA), oriented at EW and NS directions of length of 100 m each, have been installed. The last installation aims for the detection of a different type precursor, namely, ultra-low-frequency (<1 Hz) EM precursor rooted in a pre-seismic lithosphere-atmosphere-ionosphere (LAI) coupling. All the time-series are sampled at 1 Hz.

From 1995 to 1998 a reduced version of the above-mentioned system, recording only MHz and kHz EME (specifically: 3 kHz NS, 3 kHz EW, 10 kHz NS, 10 kHz EW, 41 MHz and 46 MHz) was used for the deployment of more telemetric stations which gradually formed a telemetric network spanning all over Greece, the ELSEM-Net (hELlenic Seismo-ElectroMagnetics Network), that is in full unattended operation until now. All stations, excluding the Zakynthos station, of ELSEM-Net use the already existing field facilities (seismic stations) of the National Seismological Network of the Institute of Geodynamics, National Observatory of Athens (NOA/IG). The data from all stations are collected at the NOA/IG Center, and then transmitted to the Electronics and Computer Technologies Lab (ECTLab) of EEE/UniWA for storage and processing. The interactive map shows all the currently operating telemetric stations.

More details on the instrumentation of the telemetric network can be found here.

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Our research results

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Our research, in an appropriately critical spirit, focuses on finding answers to the following crucial questions:

1. How can one recognize a MHz or kHz EM anomaly as an EQ-related one?
2. How can one link an individual MHz and kHz EM precursor with a distinctive stage of the EQ preparation?
3. How can one identify precursory symptoms in EM observations which signify that the occurrence of the prepared EQ is unavoidable?
4. Are the MHz-kHz EM precursors consistent with other precursors?
5. Are the systematically observed preseismic EME characteristics, which are commonly considered as “puzzling features”, really “puzzling” ones or are they crucial precursory features of the EQ preparation process? Specifically:
   (i) EM silence in all frequency bands appears before the main seismic shock occurrence.
   (ii) Although strain changes are largest at the time of EQ, there are not co-seismic EME.
   (iii) EM silence is also observed during the aftershock period.
   (iv) Are the fracture-induced EME, if they really exist, detectable by ground-based observatories?

Our up to now research has led to the proposal of a four-stage model for earthquake generation (FSMEG). We think that this model provides answers to all the crucial questions raised in the previous section. The proposed model is summarized as follows:

A fault (blue lines) is embedded in a heterogeneous environment. The MHz EME are emitted during the fracture of a disordered medium surrounding the major fault over a critical circle (yellow). The kHz EME are emitted during the fracture of the asperities (green highlighted area).

• First stage: The initially observed MHz EM anomaly is due to the fracture of the highly heterogeneous system that surrounds the formation of strong brittle and high-strength entities (asperities) distributed along the rough surfaces of the main fault sustaining the system. The MHz EME can be described by means of a second-order phase transition in equilibrium.

• Second stage: The appearance of tri-critical behavior in the final stage of MHz EME, or in the initial stage of kHz EME, or in both, signalizes a next, distinct, state of the EQ preparation process.

• Third stage: The finally abruptly emerging strong sequence of kHz EM avalanches originates in the stage of stick-slip-like plastic flow, namely, the fracture of asperities themselves. The burst-like kHz EME does not present any footprint of a second-order transition in equilibrium.

• Fourth stage: Finally, the systematically observed EM silence in all frequency bands before the time of the EQ occurrence is sourced in the process of preparation of the dynamical slip which results in the fast, even supershear, mode that surpasses the shear wave speed and corresponds to the observed EQ tremor.

Recently, we specifically focus on the extraction of as much as possible information that can result from the analysis of the MHz EME, such as the study of spontaneous symmetry breaking (SSB) and post-SSB power laws in analogy to thermal systems, which provides information on whether a strong main EQ is expected and whether a second main EQ is expected or not after the occurrence of a strong main EQ. Moreover, we are particularly interested in the analysis of seismicity and its relation to fracture-induced EME as well as other seismo-electromagnetic precursors, while we are also interested in machine learning / deep learning applications to seismic risk assessment.

This is an ongoing research, continuously testing the above-mentioned hypothesis of FSMEG and examining new aspects of the involved processes that could enhance our understanding of EQ preparation as reflected to the observed fracture-induced EME, as well as other pre-EQ observables (seismo-electromagnetic or not). Our main tools come from the field of complex systems time-series analysis.

The full list of our relevant publications can be found in the corresponding section of this site.