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

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

Journal Title: Геодинаміка - Year 2015, Vol 19, Issue 2

Abstract

Purpose. In order to quantitatively estimate the dynamic effects of velocity structure under the three seismic stations of the Carpathian network spectral ratios between horizontal and vertical components of displacements on the free surface (frequency characteristics) depending only on parameters of the medium and defining its resonance properties have been calculated using the algorithm designed based on matrix method and the models constructed using the data of drilling in near surface layers. Methodology. In methodical aspect, it has been tested if interferential resonance frequencies in perfectly elastic layered medium calculated using the developed algorithm and plane wave as a source coincide with ones predicted by Nakamura’s technique. In the following paper, it is intended to check if the frequencies coincide with ones identified in experimental characteristics calculated using the real records of seismic noise and earthquakes, and the applicability of the methods in the regional seismological studies. Results. The major resonance maxima in spectral ratios between horizontal and vertical components of displacements calculated for stations Uzhgorod and Mizhgirya corresponded to the thinnest (less than ten meters) surface layers of predominantly clay deposits, and occurred consequently within the high-frequency parts of spectrum, virtually outside the working range of equipment, and with no effect on the results of observations at the stations. However, in contrast to the Nakamura’s technique, enabling to estimate thickness/resonance maximum of only one (usually surface) layer, it is possible, calculating spectral ratios using the matrix method, to identify other maxima, resulting from the more complex structure of the medium, and, in particular, such as under the station Uzhgorod, where the maxima have occurred within the working range of equipment and should be accounted for. Despite the simpler velocity structure under the two remaining stations (Trosnyk and Mizhgirya) the distinct resonance maxima within the working range of equipment have been identified there too. Originality. For the first time, frequency characteristics of the medium under the three seismic stations of the Carpathian network depending only on the parameters of the medium and defining its resonance properties have been calculated using the matrix method and velocity models of first hundreds of meters constructed based on the data of drilling. Practical significance. The resulting frequency characteristics enable to estimate the effect of the medium on the records of seismic signals, most significant at frequencies corresponding to interferential resonance maxima (and minima). The frequencies should be taken into consideration during the analysis and interpretation of seismic signals recorded at the stations, and in estimation of seismic risk and seismic hazard.

Authors and Affiliations

R. Gnyp

Keywords

EP ID EP459076
DOI 10.23939/jgd2015.02.072
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