Determination of the correlation between the energy-density changes in geomagnetic field and seismic events is a challenging scientific topic that allows the study of local tectonics using magnetometers. The magnetised properties of the crustal field of the Earth change due to natural sources and/or human activities that affect the environment. The latter can be avoided by setting up observatories in “geomagnetically-quiet” locations, while the natural sources, which describe the combined effects due to changes in the core, lithosphere, external or electromagnetically induced field, cannot be easily eliminated. This research focused on the investigation of local changes in the geomagnetic field in relation to evidently significant local tectonics in the vicinity of the PIA (Piran, Slovenia) geomagnetic observatory. It is obvious that geomagnetic measurements from PIA contain much higher levels of noise compared to the surrounding magnetometers in Italy and Croatia. According to previous geodynamic studies, the position of the PIA observatory is specific, since it is located at an Adriatic microplate that collides under the Eurasian plate. At this point it can be assumed that the reason for high-level noise in geomagnetic observations is due to the still ongoing Adria-Eurasia collisional process. Furthermore, the study of the earthquake on 1 November 2015 with a magnitude of 4.2 and its epicentre 150 km from PIA showed the correlation between higher energy density of the Earth’s magnetic field and the earthquake occurrence. From the results acquired by the computational strategy described in the research, it is obvious that, as expected, a few days prior and after the earthquake higher Earth’s magnetic field indicated some significant changes in the local geomagnetic field that might occur due to the increased stress in the Earth’s crust in the north-eastern part of the Adria-Eurasia collisional zone.
Understanding the factors that influence the quality of the measurements of energy-density changes in the geomagnetic field is a difficult scientific topic for each individual magnetometer at a specific Earth location. The specificity comes from the fact that the crustal field has a non-unique induced and remnant-magnetised part. The remnant-magnetised part depends on the magnetic properties of the sub-surface rock and the history of the core field, which is specific for each part of the Earth. While performing geomagnetic measurements at a specific location it is important to know the geological structure of the territory. Moreover, in geomagnetic measurement performance several unpredictable events can occur, which can be due to human activities that affect the environment as well as due to unpredictable natural sources.
This research focused on the impact of natural events on the territory of Slovenia that could be the most significant reason for the noise in measurements of the local geomagnetic field acquired at the Slovenian PIA geomagnetic observatory. Shortly after efforts to set up the geomagnetic observatory, magnetograms of daily measurements showed evidently unexpected high-level noise in measurements [
Obviously, the location of the PIA geomagnetic observatory is close to the convergent boundary of two lithospheric plates, namely the Adriatic microplate and Eurasian lithospheric plate (
Several authors have proven the correlation between geomagnetism and tectonics activities of the Earth’s crust [
Within the context of this short review of the current state of the research field, the driving motivation for the research presented in this paper was to accomplish some further experiments and analyse the results in order to better understand some of the main factors that influence the quality of the geomagnetic measurements from the Slovenian PIA geomagnetic observatory. The authors’ basic research questions were, “What is the reason for such noise in the geomagnetic measurements at PIA?” and “Can the PIA observatory detect earthquakes?”. The results of the research are based on two case studies, namely “a set of quiet days” and “a set of days within the earthquake”. Based on the results, a discussion of the analysis and the authors’ interpretation follows each part of the research.
The outline of the paper is as follows. In Section 2, basic theoretical explanations and the theory of an earthquake detection using magnetometers by energy density changes of the geomagnetic field is given. In Section 3 theoretical aspect of geomagnetic data processing is described. The main characteristics of the observatory PIA and measurements are given in Section 4, followed by the case study of 1 November 2015 earthquake detection by the use of geomagnetic measurements. Discussion of the study is given in Section 5, followed by conclusions and future plans in Section 6.
Due to the increased stress in the Earth’s crust, a separation of the electric charges occurs, which create electric fields, and as a result of the motion of electric charges, a magnetic field is created. There are not only electric and magnetic phenomena in the area with the separation and then re-joining of electric charges, but also electromagnetic phenomena [
The most prevalent method of determining a tectonic stress in the Earth’s strata is using measurements of their electric resistance. Namely, the electric resistance of stones changes in accordance with changes in their structure, water content and mineralisation. When the geological structure changes due to the influence of external forces, the size of the stones’ pores change, which causes further changes in the electric resistance of the stones. The capillary action starts—that is, water flow into the pores is what causes the movement and separation of electric charges [
The variety of models can be used to obtain a more precise explanation of the electromagnetic signals that arise due to changes in tectonic stress in the Earth’s crust. However, none of them could explain all the forms of electric, magnetic and electromagnetic signals caused by changes in tectonic stress [
At the location of an earthquake, seismic activity changes in accordance with long-term (secular) or short-term (seasonal) geomagnetic field variations. During magnetically calm days, (solar quiet-days Sq) random changes in the horizontal component of the geomagnetic field can occur [
The state in the universe and near our planet directly influence changes in the Earth’s magnetic field. Striking “breaking waves” during the solar wind and the sudden increases in the speed of solar wind cause geomagnetic storms. Therefore, when the influence of changes in tectonic stress on the geomagnetic field measurements is assessed, it is also necessary to consider the influence of the Sun on the Earth’s magnetic field [
The magnetic field does not propagate in an empty space in the same manner as do the electrostatic and gravitational fields. The direction of a vector is different in every point of the magnetic field. Moreover, the strength of the magnetic field decreases more quickly than that of the electrostatic or gravitational field [
External sources of electric power maintain the electric current in the coil. The energy W is stored in the magnetic field of this coil [
W = ∫ 0 j u i d t = ∫ 0 j L d i d t i d t = 1 2 L i 2 = 1 2 μ 0 B 2 S l . (1)
In Equation (1), coil with number of loops N, cross section S, and length of magnetic line of force l, are:
- magnetic flux density B = μ 0 N i ;
- coil inductance L = μ 0 N 2 S / l and;
- volume V = S l .
The energy density contained in the magnetic field W / V is also valid for the energy density w G M in the geomagnetic field, defined by:
w G M = W V = 1 2 μ 0 B 2 . (2)
Possible relative changes in the density of the energy contained in the geomagnetic field, which occur because of tectonic tensions, are very small. However, they are a part of the noise. Their evaluation follows from the measurements of changes in the geomagnetic field. To evaluate relative changes in the density of energy contained in the geomagnetic field it would only be necessary to calculate relative changes in the intensity of the geomagnetic field (flux density), which arise from the absolute values of the geomagnetic field vector B.
The introduction of the geomagnetic index followed the idea of better reviewing and easier comparison of measurements of the geomagnetic field acquired at various locations. Geomagnetic observatories use proton magnetometers to measure the absolute value of the geomagnetic field vector. A 3-axis magnetometer fluxgate measures changes in all three components of the geomagnetic field (east, and nadir) or DIF (declination, inclination and absolute value). In accordance with international recommendations, one-second measurements follow the filtering procedure using a Gauss digital filter [
The geomagnetic index K (German: Kennziffer) describes the current state of the measured geomagnetic field at the specific location. K allows different types of geomagnetic days to be described. The day enumerated zero (0), is the magnetically calmest day, five (5) is the type of day when a magnetic storm comes into its mildest form, while the numbers from six (6) to nine (9) describe those days with strong geomagnetic activities (i.e. geomagnetic storms). The intensity of geomagnetic storms increases from the geomagnetic equator towards the northern/southern regions and, therefore, each observatory has its own index K. The scale of the geomagnetic index K is logarithmic, and has been determined based on measurements of the geomagnetic field components acquired from 3-hour intervals. In the diurnal variations during magnetically calm days, the influence of the Moon and the effect of a calming down after the end of the geomagnetic storms should be excluded from the measurements.
The determination of a magnetically calm day (Sq) follows from detection of changes in the geomagnetic field during days without unusual solar phenomena (sudden ionospheric disturbances). The magnetically calmest five or eight days during the month, with the index K below the selected minimal value [
In order to use geomagnetic measurements in short term earthquake detection, geomagnetic activity should be defined by geomagnetic data processing from a continuous time series and further analysis of indexes of geomagnetic activity.
When processing measurements from a continuous time series of geomagnetic field records, it is important to define the width of the time windows [
In order to exclude the daily effect from solar phenomena, the width of time window used should be less than 24 hours (
The computation of the standard deviation of one-minute average values of the geomagnetic field vector’s component X a v g follows Equation (3). The relative average value X a v g (Equation (4)) of the specific window is calculated in the time interval (data window) with the previously selected width (
Δ X w = 1 N − 1 ∑ j = 1 N ( X j − X a v g X a v g ) 2 , (3)
X a v g = 1 N − 1 ∑ j = 1 N x j (4)
with pre-determined samples j within the specific time interval. The standard deviations of all three components (separate in each direction X, Y and Z) of the geomagnetic field vector represent the basis for further determination of the geomagnetic activity index Activity(w) in the time interval (data-window) with the selected width during the day under consideration (Equation (5)) and diurnal geomagnetic activity index in this day Activity(day) (Equation (6)). w stands for the number of time intervals (data windows) in a single day.
A c t i v i t y ( w ) = Δ X w 2 + Δ Y w 2 + Δ Z w 2 (5)
A c t i v i t y ( d a y ) = 1 w ∑ n = 1 w A c t i v i t y ( w ) n . (6)
The reference value of the diurnal geomagnetic activity index A c t i v i t y ( d a y ) can only be observed in comparison with the geomagnetic activity index during the previous day A c t i v i t y ( d a y − 1 ) . A comparison of those indexes indicates the index of geomagnetic activity, which enables the determination of whether the stress in the Earth’s crust caused an increase or decrease in the geomagnetic activity (Equation (7)). Activity presents the ratio of the resulting subtracting geomagnetic activities (for the observed and previous day), based on the result of subtracting the equivalent daily amplitudes A (for the observed and previous day):
A c t i v i t y = f [ A c t i v i t y ( d a y ) − A c t i v i t y ( d a y − 1 ) , A ∗ k ] . (7)
A stands for the amplitude and k stands for the coefficient of the geomagnetic storm influence.
In the following sections, geomagnetic measurements and computations of geomagnetic activity for the Slovenian observatory PIA will be introduced.
The Slovenian territory includes the fast-raising Alpine belt and the boundaries of the Pannonian basin, which sweeps downwards relatively slowly [
Construction of the observatory began in 2014. Since 1 January 2015, the measurements from the observatory have been continuously dispatching to the global network for the cooperation of digital magnetic observatories, INTERMAGNET (International Real-time Magnetic Observatory Network). At the moment, the observatory is still in its testing phase. The geomagnetic observatory is located in the village of Sv. Peter near Piran, Slovenia (f = 45.459˚N, λ = 13.685˚E, H = 196 m) (
The PIA observatory is equipped with a 3-axis fluxgate magnetometer, which was selected based on the results of previous measurements performed during a period of strong seismic impulses [
To reduce the influence of electrical discharges in the air at atmospheric pressure, the PIA observatory is buried [
The PIA observatory lies on the Adriatic microplate near the boundary of the Adriatic and Eurasian lithospheric plate (
From
According to the automatic classification of seismic events within the Slovenian seismograph network, in the year 2015 there were 1944 registered earthquakes on the territory of Slovenia, which equates to an average of 5.3 per day. The magnitude M for 378% or 19.4% of these earthquakes exceeded the magnitude ML = 1.0 [
According to statements by the Seismology and Geology Office at the Slovenian Environment Agency, on 1 November 2015 at 07:52 UTC the seismographs of the national network of seismic observatories recorded a moderate earthquake shockwave. Inhabitants of the whole of Slovenia, western Croatia, Istria and Trieste in Italy experienced the earthquake. The preliminary estimation of the earthquake’s magnitude was 4.2. Several tens of aftershocks followed the main earthquake [
Changes in the geomagnetic field’s values showed that 1 November 2015 was a magnetically calm day with the equivalent daily amplitude A = 8.6 (
The observatory’s distance from the epicentre of the earthquake, which occurred on 1 November 2015 at 07:52 UTC, was about 150 km (azimuth: 69.8˚). At the area near the PIA observatory the earthquake was experienced first after 18.8 seconds (α ≤ 8.0 km/s) and last after 1 minute and 2.9 seconds (β ≥ 2.4 km/s) [
However, when comparing relative changes in the geomagnetic field’s energy density from several days before and after the earthquake (1 November 2015) occurrence (
Time period | Samples per day | Windows | Aavr | A c t i v i t y a v r ⋅ 10 − 6 |
---|---|---|---|---|
9th June to 22nd June | 1440 | 24 | 12.1 | -15.038 |
25th October to 7th November | 1440 | 24 | 12.4 | 6.366 |
Difference | 0.3 | 21.404 |
values increased by more than 21 units according to the reference values in June. Interestingly, the activity index remained low on the day of the earthquake.
From the results we can conclude, that the geomagnetic index that describes the change in energy of the geomagnetic field may be used for the short-term prediction of an earthquake. However, the higher values, which indicate an increase in tectonic stress, are not enough to indicate an earthquake. The fact is that the density of energy in the geomagnetic field increases when an earthquake comes into existence. However, increased values do not necessarily mean that an earthquake will occur.
The current measurements of changes in the geomagnetic field are particularly aimed at the measuring of conditions in the Earth’s inwardness and solar winds. Therefore, it would be necessary to introduce adequate measuring instruments [
To confirm the increased tectonic stress, it would be necessary to introduce parallel measurements of other geophysical indicators, such as changes in the electric resistance of the Earth’s strata and that of the resonant phenomena on extremely long electromagnetic waves, ELF (extremely low frequency) with wavelengths from 10,000 to 100,000 kilometres. There are receivers of the electromagnetic waves with extremely low frequencies (ELF). These frequencies appear in a cavity resonator between the Earth and its ionosphere. About 2000 storm cells, distributed over the Earth, cause more than 50 atmospheric electric discharges every second, which maintain this natural resonance [
In this paper the authors presented the situation of significant changes in the strength of the geomagnetic field at the PIA observatory. A higher level of noise compared to the geomagnetic measurements of the surrounding observatories in Italy and Croatia was observed. According to the authors’ current knowledge and experience, it can be assumed that the presence of a higher level of geomagnetic noise could be due to natural sources, i.e. local tectonics. This assumption is based on the fact that the PIA observatory is situated on the Adriatic microplate, which causes significant seismic events on the territory of Slovenian.
At the moment, the motivation question about the geomagnetic measurement noise at PIA cannot be answered completely. For this, additional analysis and correlation determination of different sorts of data, for example GNSS time-series of continuously operating reference stations, should be performed. However, we can confirm that geomagnetic measurements at the observatory PIA could be used as an additional and valuable data source of early warning information for increase seismic activity at the territory of Slovenia.
The following study of geomagnetic measurements in the period before and after the 4.2 magnitude earthquake that occurred in Slovenia on 1 November 2015, presented an aspect of short-term prediction of an earthquake from geomagnetic observations. A few days prior to and after the earthquake, higher energy density of the Earth—as expected—indicated some unexplained but significant changes in the local geomagnetic field, which might have occurred due to the increased stress on the Earth’s crust. However, to confirm the assumption, it would be highly recommendable to introduce parallel measurements of other geophysical indicators that may influence the geomagnetic field at this location. In future, this will also be one of the major areas of concern as well as the recognition of all possible natural sources and/or human activities that might significantly affect the geomagnetic measurements at the PIA observatory.
The authors declare no conflicts of interest regarding the publication of this paper.
Pavlovčič-Prešeren, P., Čop, R. and Kuhar, M. (2020) The Use of Geomagnetic Measurements to Study Local Tectonics: Case for the NE Part of the Adria-Eurasia Collisional Zone. Open Journal of Earthquake Research, 9, 83-99. https://doi.org/10.4236/ojer.2020.92006