Seismic instruments: article

Аnnotation: The results of studies on the search for the influence of microseismic noise on the number of recorded earthquakes in Tajikistan using a seasonal benchmark are presented. To do this, we tested the hypothesis that seasonal variations in the number of weak earthquakes are caused by variations in the intensity of seismic noise, called the hypothesis of noise discrimination of seismic signals. As part of the research, seasonal variations in the number of weak earthquakes recorded at the Garm test site in Tajikistan from 1954 to 1991 were analyzed. A similar analysis of seasonal variations was carried out for microseismic noise in the frequency range of 0.01-40 Hz, recorded by a network of broadband seismic digital stations in Tajikistan in 2005-2020. The presence of distinct seasonal variations in both cases has been reliably established. This is confirmed by the results of a spectral analysis of noise intensity variations and the number of earthquakes: the spectra clearly show a peak with a period of about a year, indicating the presence of a high-amplitude seasonal (annual) harmonic. At the same time, in seismicity, a powerful annual peak is observed only in samples of weak earthquakes. An analysis of the nature of seasonal changes in the compared data using graphs constructed by the epoch superposition method made it possible to identify regular changes during the year. The features of seasonal variations in the number of earthquakes and seismic noise are considered. For noise, the anti-phase behavior of seasonal variations at frequencies below 0.5 Hz and above 3 Hz was established: the amplitude of high-frequency noise in the middle of the year is maximum, and, on the contrary, low-frequency noise is minimal. The number of recorded earthquakes in the middle of the year is minimal. The deviations of the number of earthquakes and noise intensity from their average annual value for high-frequency and low-frequency noises are compared. The results obtained are largely consistent with the hypothesis of noise discrimination of seismic signals. In this case, high-frequency noise acts as an active agent, and this is understandable, since its spectrum overlaps with the spectrum of seismic waves of local earthquakes. However, a large scatter of data does not completely exclude the possibility of the presence of some other, additional, causes of seasonal variations in seismicity.