Synthetic and experimental frequency characteristics of near-surface layers under the seismic stations Trosnyk, Uzhgorod, and Mizhgirya

Purpose. In the paper, experimental spectral ratios between horizontal and vertical components of displacements from local earthquakes and noise recorded at the stations Uzhgorod, Trosnyk and Mizhgirya (frequency characteristics of the medium under the stations) are compared to synthetic ratios calculated using the velocity models built based on the data of drilling in near-surface layers; correlation between the resonance frequencies is evaluated, as well as the applicability of experimental and synthetic frequency characteristics in the regional seismological investigations. Methodology. The synthetic spectral ratios between horizontal and vertical components of displacements on the free surface of perfectly elastic horizontally-layered half-space were calculated using the algorithm designed based on matrix method and plane P-wave as a source and compared subsequently to the experimental ones obtained, after the deconvolution of seismograph response, from the portions of records of small earthquakes and noise as short as to eliminate the deep phases, possibly not accounted for in the synthetic ratios owing to insufficient total thickness of the models, and phases arising after the arrival of direct S-wave from the earthquake. Similarity between the synthetic and experimental frequency characteristics was evaluated by calculating a function of cross-correlation between them. Results. The highest rates of cross-correlation were obtained between experimental frequency characteristics of the medium under the stations of Carpathian network estimated from the records of local earthquakes and synthetic characteristics calculated at horizontal phase velocities corresponding to velocities of first arrivals of P-waves from the earthquakes at the stations. That the correlation was not of incidental character might be concluded both from high values of corresponding maxima, achieving 0.8, and more, and from insignificant values of their frequency shift,  0.2 Hz, and less. The high rates of correlation might be considered as an adequacy test for the velocity models, built based on the data of drilling and used subsequently in calculation of synthetic frequency characteristics, on the one hand, and as a test on reliability of resonance frequencies estimated from the experimental ratios in the locations where data of drilling would not be available, on the other. The high rates of cross-correlation also obtained between experimental spectral ratios for small earthquakes recorded at the stations and the ratios for noise seemed to justify only the using of noise for estimation of resonance properties of near-surface medium. Originality. For the first time, resonance properties of near-surface layers under the seismic stations Mizhgirya, Trosnyk and Uzhgorod have been estimated based on two alternative approaches – by calculating the synthetic spectral ratios between horizontal and vertical components of displacements on the free surface using velocity models built based on the data of drilling, and by calculating the experimental ratios between the components of records of small earthquakes and noise at the stations. It has been confirmed that the results of both the approaches converge well, which justifies their eventual application in seismological investigations in the Carpathian region. Practical significance. The frequencies of interferential resonances in a layered medium under the seismic stations should be accounted for during the analysis of seismic signals recorded at the stations, in estimation of parameters of seismic load etc. Reliable and effective methods have been suggested for estimation of the frequencies and of the thicknesses of near-surface layers by theoretical modeling of frequency characteristics of the medium, and experimentally, from the records of earthquakes and noise at the stations.