Investigation of pre-seismic activity effect on recorded VLF signals in Tehran receiver

Electromagnetic phenomena such as disturbance of VLF (very low frequency) radio signals, have been used for prediction of short-term earthquakes in the world from some years ago. VLF signals are reflected from the lower bound of the ionosphere, so any change in the ionosphere D region leads to changes in the conditions of the VLF wave propagation. One of the factors that influences the various parameters of the ionosphere is the processes surrounding the earthquake center before it occurs. So far, several hypotheses have been proposed for the mechanism of energy penetration from the earthquake to the ionosphere in theory. One of the most appropriate hypotheses is the role of atmospheric gravitational waves in this regard. However, there are not many observational evidence. In this study, after a review of variations in VLF signals received at the receiver of Tehran University's Institute of Geophysics (TEH), significant disturbances were observed several days prior to the onset of the Mn 5.8 Sirch earthquake that occurred on 22 July, 2018, in Kerman province, southeastern Iran. These abnormalities appeared as decreasing in the night range and increasing in the amplitude of the day and were only in the VTX3-TEH pathway, so they did not have any relation to the disorder in the transmitter or receiver. The association of these abnormalities with the factors affecting the ionosphere, including Solar flares, Lightning, Meteorological and geomagnetic activities was also studied and risk associated with these factors were rejected. These abnormalities began about four days before the earthquake and continued until the day of the earthquake; therefore, they are probably related to the precursor effects of the Sirch Kerman earthquake. Spectral analysis of signals was also performed and a 20 minutes harmonic was found in the spectrum of days before the earthquake. This period is not observed in the quiet days. This result, in addition to denying the probability of a geomagnetic effect on the signal, shows the effect of atmospheric gravitational waves in the lithosphere-atmospheric-ionospheric coupling mechanism for this earthquake. Based on these observations, the radio anomaly under study can be considered as a precursor of the Sirch earthquake in Kerman. The 22 July 2018 Mn 5.8 Sirch earthquake disturbances in Kerman provide another evidence of radio frequency disturbances at the VLF frequency before the earthquake. However, for more accurate monitoring of these signals, regular monitoring of long-term data as well as the number of more receivers in the country is required. In this case three important features, an earthquake pre-indicator, the time, location and magnitude of the earthquake in VLF/LF anomalies, occur from about a week to a maximum of 10 days before the earthquakes (Biagi, 2009) and are suitable for short-term forecasting, but still the exact time of earthquake is not clear. The disturbances do not appear for earthquakes with magnitude less than 5.5 and for earthquakes between 5-6 disturbances are less severe than earthquakes larger than 6, so this method may estimate the magnitude of the earthquake. Also, using an appropriate network coverage of the VLF/LF receivers and the use of appropriate processing methods, it is possible to locate somewhat an earthquake. Finally, it can be said that this new field of science is considered as a promising candidate for short-term earthquake prediction to reduce earthquake damage in active area such as Iran.