Effectiveness of electromagnetic monitoring in studying earthquakes

Numerous researches conducted in connection with the study of earthquakes have shown that electromagnetic monitoring studies have led to some important results. From the Loma Prieta earthquake to the Guam earthquake, electromagnetic monitoring studies led to significant results. Since then, there have been numerous reports of possible electromagnetic precursors to earthquakes, some of which have involved frequencies covered by ELF/VLF (10—32 kHz) monitoring system Fraser—Smith et al. [1990]. Sometime later, they retrieved and started processing their ULF data. They had less reason to expect electromagnetic precursors in this latter data, because previous reports of precursory signals at frequencies below the ELF/VLF range have, with few exceptions, involved frequencies either below or predominantly below their ULF range (0.01—10 Hz) of operation. They found out that ELF/VLF data do not appear to show precursory activity, whereas ULF data contain a number of anomalous features that may prove to be earthquake precursors. The lack of observation of precursory ELF/VLF noise so close to the epicenters of several moderate to moderately-large earthquakes showed that ELF/VLF noise need not be a strong or obvious feature of every earthquake, as Fraser—Smith et al. [1990] reported in their paper. At present, numerous studies have been conducted in this area and researches are being improved. From my experience as a young researcher, it became clear that electromagnetic monitoring research is necessary, and that more important and significant results can be achieved if continuous research is conducted in a certain area. Thus, these studies may play a significant role in the detection of earthquake precursors.

Production Association monitored the Earth's naturally changing electromagnetic field for studying the changes in the cycle of electromagnetic fields and the acceleration of the propagation velocity of electromagnetic field variations as earthquake precursors were detected [Piriyev, 2017]. The territory of Azerbaijan is considered a tectonic territory, so it is very important to conduct electromagnetic monitoring there. The map presented in Fig.1 also clearly describes it, and the map points out that conducting electromagnetic monitoring research in the territory of Azerbaijan is one of the important issues. Because we know that electromagnetic monitoring studies are conducted in geodynamic active areas , and the main goal is aimed at detecting earthquake precursors.
Another example of that kind of researches is the Russian Trofimuk Institute of Petroleum Geology and Geophysics, Candidate of Tech-nical Sciences, senior researcher V.V. Potanov and Doctor of Geological and Mineralogical Sciences , leading researcher E. V. Pospeeva in their investigations saw differences in behavior of apparent resistivity curves and geoelectric models obtained during 2007 -2016 of instrumental observations. In their opinion those differences were quite signifficant and were probably related to changes in seismic activity of their studied territory [Potapov, Pospeeva, 2017]. Let's shortly analyze the electromagnetic monitoring studies conducted in Azerbaijan and Russia.
The study by [Kerimov, Agaguliyev, 2001, 2005Kerimov et al., 2006]. During 1997During -2002, under the leadership of the corresponding member of Azerbaijan National Academy of Sciences, professor Kerim Kerimov, «Geophysics and Engineering Geology» Production Union conducted monitoring of the Earth's natural changing electromagnetic field to stu- Azerbaijan and near territories in the years 4272016 as reported by [Yetirmishli et al., 2017, Fig. 1].
dy earthquake precursors, and as earthquake precursors, changes in the life cycle of electromagnetic fields and the speed of propagation of electromagnetic field variations were detected. One Azerbaijanian and two Eurasian patents were obtained confirming the existence of these two precursors. Agaguliyev [2001, 2005] had shown that from the moment characterizing the time of change of the magnetic field cycle, magnetic field intensity vectors were directed towards the future earthquake center. On the other hand, as a result of Kerimov et al. [2006], it was found that the propagation velocity of electromagnetic field (magnetic components) in the direction of the zone in which geodynamic tension occurred and which may subsequently result in an earthquake was rapidly changing , and the coherence of the electromagnetic waves observed before this process was disrupted. In my opinion, the result of a joint study of both processes showed above as an earthquake precursor may lead to the detection of seismic activity and in the future, this could create conditions for the location of earthquake centers related to geodynamic stress zones.
Let's take a deep look at the explanation of the problems mentioned above. It is known that the period of life of the waves, which is connected with the internal magnetic field and is irradiated from the geodynamic stress zones depends on the geodynamics, dimensions, depth, and power of an earthquake center that generates these waves [Piriyev, 2016]. On the other hand, we know that a skin effect was characterized by the period of waves and there is an effective propagation depth of waves corresponding to each period. From that point of view, it becomes clear that geodynamic stress zones will only irradiate waves of the same type and the same period within a certain frame of error.
In 2000-2003, Kerimov et al. [2006 examined the results of electromagnetic monitoring conducted in the Fatmayi polygon, and they came into the conclusion that the violation of the circulation of waves occurred as a result of the superposition of natural electromagnetic waves of cosmic nature, reflected from that depth by waves formed in connection with geodynamic stress zones and anisotrophy.
The study by [Potapov, Pospeeva, 2017]. I briefly describe the way how they carried out electromagnetic monitoring. The recent report of Vladimir V. Potapov and Elena V. Pospeeva [2017] would like to provide relevant news in the search of earthquake precursors by method of MTS. Here I will discuss their significant results starting from the scientific literature before their study.
Possible views on earthquake precursors. Goldin et al. [2004] studied the area before and after Chuyskoe earthquake (or Altai earthquake) and came into the conclusion that seismic activity observed in the considered zone in 2002, according to the spatial distribution of the epicenters , differs significantly from the structure of the aftershock processes. Modern ideas about the geodynamic processes occurring in the interior of the Earth during the preparation of an earthquake give reason to believe that these processes are reflected in the data of electromagnetic soundings with artificial and natural sources. Among the methods that allow obtaining information about the deep structure of the Earth , magnetotelluric sounding (MTS) occupies one of the leading places, the main result of the interpretation of which is to reveal the spatial distribution of deep electrical conductivity, reflecting the thermodynamic conditions in the Earth's crust and upper mantle. At present , quite a lot of experience has been accumulated in prognostic regime magnetotelluric observations for studying the geodynamic processes occurring in the Earth's tectonosphere before and during an earthquake [Moroz et al., 2006;Rybin et al., 2009;Matyukov et al., 2010;Batalev et al., 2013]. When studying geodynamic processes using the electromagnetic field of the Earth, there are two approaches: identifying changes in the electrical properties of the geological environment and the structure of variations in the electric and magnetic fields. Electromagnetic monitoring using the MTS method in the Altai Mountains has been carried out for 10 years and based on the implementation of the first approach. The observations were carried out at four points located in the epicenter zone of the Chuiskoe earthquake. MTS works were carried out by fifthgeneration equipment of the Canadian company «Phoenix Geophysics LTD» in the range of periods 0.003-10000 s. MTU-5 measuring modules were used , which made it possible to register five components (Ex, Ey , Hx, Hy , Hz) of the magnetotelluric field (MT field). A cross-shaped installation with an electric dipole length of 100 m was used. The components of the MT field were recorded for 22-24 hours. «Phoenix Geophysics LTD» software was used to process field data; 1D and 2D inversion of experimental data was carried out using the «WinGLink» software package [Pospeeva, 2006].
The surveys were mainly conducted at points 1P1 and 1P2. From the conducted analysis it follows that the lowest deviation (2 %) from the average level is characterized by curved , obtained in the sounding point 1P1 (Fig. 2, a). It increases up to 5 % in the interval S1 due to significant variations in the level of curved resistance (from 12 to 30 Ohm · m). The  [Potapov, Pospeeva, 2017]. The layers 15 are coloured in order to show differences in apparent resistivity for the period 20072016. positive results with the correct approach. This shows that continuous research can lead to more positive results. The significant studies to investigate the relationship between VLF signals and earthquakes by different researchers. The following table ( Table 2 ) summarizes the electromagnetic monitoring studies conducted in different years. A lot of researches have been done on the relationship between earthquakes and the variations in very-low-frequency (VLF) signals that occur in the lower layer of the ionosphere before the earthquake.
Conclusions. In the world practice of electromagnetic monitoring research , there are many practical examples of positive results of magnetotelluric research for monitoring geodynamic processes in seismically active regions. The most promising results were obtained in the ULF / VLF range. One of the main problems is that MT signal observed on the surface of the earth is extremely weak, that makes magnetotelluric sounding susceptible to cultural noises. As one of the most common cultural noises during the acquisition of MT data , power line noise has plagued geophysicists for decades [ Butler , Russell , 1993;Cohen et al., 2010]. To suppress power line noise, there are trap circuits designed in most of the acquisition instruments, however, the fundamental frequency of the power line noise is changeable with the fluctuate of the load current, hence the MT data are still seriously disturbed by the power line noise. deviation of curves from the average in point 1P2 is 5 % in total, except for curves registered in 2010 and 2012, where it is more than 20 % (Fig. 2, b).
Significant variations in the level of the apparent resistivity curves cause differences in the determined parameters of the geoelectric section (resistivity and power). For example, in point 1P1, significant variations in resistivity values are noted in the interval S1, while in the interval S2, h1 and h2 are within the confidence interval (Fig. 3). Potapov and Pospeeva [2017] found out that according to the electromagnetic monitoring by method of MTS in the epicentral zone of Chuiskoe earthquake (or Altai earthquake) this method allows you to see the differences in the behavior of the apparent resistance curves and geoelectric models obtained in different years of instrumental observations. These differences are quite significant and are probably associated with a change in the seismic activity of the studied territory. A technique has been developed for analyzing the behavior of apparent resistance curves and parameters of a geoelectric section, based on which further studies will be carried out.