A sudden release of energy build-up within Earth's crust manifests itself as an earthquake. The seismic waves traveling through the ground are able to cause large destruction and loss of lives. In the last decades, science tries to develop an earthquake prediction system. However, up to current date, these efforts are still lacking of general success, with. different earthquake prediction methods currently in rudimentary stages [1].
There are multiple angles of approach to this problem. Among others, there is a relatively new research that focuses on the possible causal relation between increased solar activity and the occurrence of major earthquakes on the Earth. In [2], authors reviewed latest information accumulated on the subject, stating that the effects of solar influence on the Earth's seismicity could be separated by agents of energy transfer, which could be electromagnetic emission of the Sun, particle fluxes of solar wind, solar proton events, modification of radiation belts and indirect impacts through the intermediate agent such as atmosphere disturbances and modification of atmosphere circulation as effect of solar activity.
Solar flares are eruptions of strong and localized electromagnetic radiation from surface of the Sun, driven by sudden change in magnetic field, usually from the area associated with sunspots. Powerful flares are usually accompanied by Coronal Mass Ejections (CMEs), where large amount of plasma is launched from corona into space. CMEs can sometime occur in the absence of a flare [3]. The frequency of CMEs depends on solar activity variations within solar cycles. On average, during solar maxima the Sun produces around three CMEs per day, while during solar minima one flare occurs in several days [4]. CMEs increase amount of solar wind reaching the Earth. Solar wind is a stream of charged particles, mostly protons and electrons, originating from solar surface and traveling through space [4]. The number of protons within unit of volume is termed as the solar Proton Density (PD). The general similarity of clustering of major earthquakes of M≥8.0 and the sunspot number during 20th century is demonstrated in [2]. Here, the increase in frequency of M≥8.0 earthquakes in the middle of century corresponds to the period when the Sun has been most active [5]. However, in this research author found that temporal distribution of large global earthquakes is well-described by a random process plus localized aftershocks, and apparent clustering is due to random variability. However, the conclusion is given with a limitation because of too small length of instrumental seismological series for reliable conclusions. In [6], a dependence of earthquake counts and seismic energy released for earthquakes of M≥4.5 was researched against sunspot number data. Here, the inversed relationship is found; with decreasing the sunspot number between 1973 and 2011, the number of earthquakes increased. However, the deviation of counts from a trend is mostly positive near the minimum of solar cycles, when the flux of the galactic cosmic rays is increased. There are several papers indicating an increase in charged particles’ bursts from the Sun before the major earthquake occurrences [7,8,9,10,11,12]. In [2], authors speculate that both the cosmic rays and the high energy particles precipitating from the radiation belt of inner magnetosphere in times of strong geomagnetic storms, may be a trigger of earthquakes.
In the recent results [13], 20 years of solar PD and velocity data as recorded by the SOHO mission were analyzed together with the worldwide seismicity in the corresponding period. Here, the relationship of major earthquakes and changes in density of incoming solar particles was investigated. A clear correlation between PD and occurrences of major earthquakes (M>5.6) was confirmed, with increase in PD preceding occurrence of major earthquakes by approximately one day. The significance of a correlation is found to be very high, with the inaccuracy probability lower than 10–5. Also, the correlation increases with the magnitude, thus the most destructive earthquakes were found to be most reliably preceded by the increased PD values. The authors propose a theoretical model that could explain the finding in terms of reverse piezoelectric effect induced by the applied electric field related to the PD. They conclude that the results open new perspectives in seismological interpretations, as well as in earthquake forecasting.
This paper contributes to the findings of relationship between major earthquakes and the increased solar activity. Here, the PD time-series before major earthquake (M=6.4) at the end of 2020 in Croatia were examined. The findings agree with previous work published in [13], as well as other previously mentioned researches. The increase in PD values has been found within one day before the major earthquake strike.