Abstract

An assessment and analysis of the amplitude-frequency composition of the background microseismic noise and the possible seasonal variations for 11 seismic stations from the network of the National Operating Telemetry System for Seismological Information (NOTSI) are presented. The paper uses the method of the spectral ratio of the horizontal to the vertical component of the microseismic noise (H/V method of Nakamura). Three-component continuous 24-hour seismic records from the 11 seismic stations were used as input. H/V ratio curves were obtained for the four seasons (winter, spring, summer and autumn), characterizing the amplitude-frequency composition of the microseismic fluctuations in the frequency range from 0.5 to 20 Hz. The obtained curves of the H/V ratio are conditionally divided into two groups - stable; and curves with weak seasonal variations. The first group includes the seismic stations VTS1, PSN, PRD, PLD, MMB and KDZ, which have stable amplitude-frequency characteristics of the microseismic signal regardless of the season. In general, the curves of the first group are easy to interpret and represent smooth curves slightly complicated by extremums of relatively small amplitude. The second group includes stations for which the obtained curves of the spectral H/V ratio have significant seasonal variations: KKB, PGB, PLN, SZK, TRAN. The obtained H/V ratio curves have relatively larger changes compared to the first group. The comparison of the location of the seismic stations with the obtained H/V curves shows the dependence of the amplitude-frequency characteristics of the microseismic signal on the local conditions.