Short-term Earthquake Prediction Based on Seismic Precursory Electric  Signals Recorded on Ground Surface.

7. OVERALL CONCLUSIONS

   The study of strong earthquakes is not the same, as it could be, when studying other physical phenomena. Actually, if a seismological experiment is conducted in real time, it is not guaranteed that within the time span of the experiment some strong earthquakes will take place, serving thus the purpose of this research. On the other hand, it is completely different to fracture rock specimens, under laboratory conditions, from studying an earthquake under natural conditions. The character of the rare occurrence of strong earthquakes poses a real problem to the study of their prediction.
   As a first approach, the chaotic, in time, behavior of their occurrence led the researchers to apply statistical methods. Some "return times" of occurrence, at specific, seismogenic areas, were found through the used, statistical methodologies. This was especially facilitated, where long seismic history was recorded in the past, thus enabling statistical methods to be applied.
   Consequently, seismological research, aiming into earthquake prediction, developed with, mainly, statistical methods (and use of earthquake catalogs) in countries with low seismicity, while in other highly seismogenic countries the earthquake prediction topic was additionally dealt with other methodologies, too.
   An important parameter, generally, in any “in situ" earthquake prediction research project, is the country, where this project will be performed. It is evident, that not all countries are suitable for such research activities. What is actually required is a high, seismicity country, which will provide in a reasonable time span (as long as the research project lasts) the adequate, seismic events, so that any postulated theoretical scheme can be successfully tested.
   Greece, a country with large seismicity, due to its location on a major, seismic belt, is the country where this research took place. This facilitated, a lot, the development of the methodology, already presented, by providing strong enough, seismic events data in a rather short period of time.
   In the course of the development of the earthquake prediction methodology, the basic principles, as in any research field, were followed. In brief, these are as follows:

     - Observations of some physical parameters are made in nature.

     - A theoretical model is postulated, which justifies the generation of the observations.

     - Following the theoretical model, theoretical observations are made.

     - Comparison is made between the theoretical and the real observations in nature.

   In case, a satisfactory agreement is found between the theoretical observations and the real ones, then the model is adopted as a valid one that represents what actually happens in nature, regarding this physical phenomenon. In a different case, the postulated model is disregarded and a new one is being searched.
   A problem the seismologists, who study the topic of earthquake prediction, face is that there is no physical model mechanism that accounts for the generation of the earthquakes. Consequently, there is no way to analyze the problem, in physical, mathematical terms, and hence, to devise a corresponding earthquake, prediction scheme. On the other hand, the earthquake prediction problem is referred as a multidisciplinary one, quite often. Actually, what is suggested by the term "multidisciplinary" is that geologists propose geological methods to be involved, geochemists suggest geochemistry, engineering geologists suggest rock mechanics, geophysicists suggest geophysical methods and so on.
   In the course of the presentation of the methodology for earthquake prediction based on electrical signals, recorded, on ground surface, three different physical models were postulated. The first one is the oscillating, lithospheric plate, due to Moon - Sun interaction, which generates the tidal waves. The second one is the homogeneous ground Earth. This facilitates the epicentral area determination. The third one is the lithospheric, seismic energy flow model, which facilitates the magnitude calculation. Instead of one physical model, required, as a prerequisite, three different models were combined in order to provide an acceptable solution for all predictive earthquake parameters (time, location, magnitude).
   Consequently, the term "multidisciplinary" is defined now, in the field of Physics, as stress-strain maxima of a seismogenic area, due to lithospheric oscillation, seismic energy release through an "open lithospheric physical system" and homogeneous, electrical properties of Earth, due to long wavelengths of the used electrical signals.
   The corresponding theory, which was used for each model, was presented, following basic principles of Physics and Geophysical theories. Each model was tested against actual observations that comply with the theoretical ones. Therefore, the postulated models represent, to a very large degree, the seismological reality of an under study seismogenic region.
   Largely innovative ideas were introduced, in order to overcome the various difficulties that were met in the course towards earthquake prediction. The most severe problem, met, is the noise contamination of the preseismic signals. This was treated with the newly, introduced, "noise injection" technique.
   Moreover, the presence of various, different, electrical signal-generating mechanisms in the focal area was treated by the adoption of the "apparent point current source". The term "apparent" is quite often used in Applied Geophysical methods. Finally, the energy conservation law of Physics was introduced for the analytical calculation of the magnitude of a future earthquake.
   The three prognostic (location, time, magnitude) parameters of a future strong earthquake were calculated analytically, using the latter three physical models. Following the basic principle of scientific research, these methodologies were validated by real strong earthquake data. Consequently, the proposed methodology that complies with the seismological observations made by seismological methods is a valid one. In contrast to statistical methodologies, it is a, more or less, deterministic methodology, which is characterized as "short-term prediction" or even more as “immediate prediction”, since the time window which is achieved by this methodology, is of the order of hours - days.
   As a by-product of the method which is followed for the determination of the “magnitude” parameter of an earthquake, are the compiled "Seismic, Potential Maps". These maps can be considered as prognostic tools for an intermediate-term prediction.
T   he obvious question is: are all strong earthquakes predictable? The answer is no!! Actually, predictable are the earthquakes that are capable to generate electrical signals and consequently, the presented methodology can be applied to. These earthquakes are the ones, which occur in the crystalline part (crust) of the lithosphere. The main idea is that, preseismic electrical signals are generated, due to crystalline deformation, by any valid strain inducing physical mechanism. The earthquakes, located, in the part of the Earth’s outer-shell, where plasticity exists (upper mantle, lower lithosphere), are not capable to produce preseismic, electrical signals. Therefore, in this case this methodology cannot be used.
   It is anticipated that, this methodology can be improved further more. A topic that should be studied in detail is the application of a weighting factor in the calculated azimuths, according to the used signal amplitude. In the present methodology, the real azimuthal direction is determined as the average value of all individual azimuths, calculated from all data points, regardless of the corresponding signal amplitude. Instead, a threshold was selected, so that the preseismic signal would be kept above a certain level from ambient noise.
   Another topic that is very interesting and should be addressed to is the preseismic signals azimuthal directions, in relation to the regional - local stress field, observed, at each seismogenic area.
   As far as it concerns the practical application of the methodology, a wide, monitoring network is required, in order to cover, in an even density, a largely, extended, seismogenic country.
   In the particular case of the Balkan Peninsula which presents high seismicity, is required a network which will cover up the neighboring countries of Greece (Albania, FYROM, Bulgaria, and Turkey).
   Finally, it must be understood that this presentation does not exclude the existence (in the future) of any other methodologies, with even better accuracy than the one, which has been achieved with the present work. I hope that some other eager and younger researcher will either improve it or invent a better one, in the near future.
   What is only required, is to believe that a problem shouldn’t be considered unsolvable, just because it was not solved for a long period of time, in the past.