Characteristics of secondary microseisms generated in the Bohai Sea and their impact on seismic noise

In this study, we use the Bohai Sea area as an example to investigate the characteristics of secondary microseisms and their impact on seismic noise based on the temporal frequency spectral analysis of observation data from 33 broadband seismic stations during strong gust periods, and new perspectives are proposed on the generation mechanisms of secondary microseisms. The results show that short-period double-frequency (SPDF) and long-period double-frequency (LPDF) microseisms exhibit significant alternating trends of strengthening and weakening in the northwest area of the Bohai Sea. SPDF microseisms are generated by irregular wind waves during strong offshore wind periods, with a broad frequency band distributed in the range of 0.2–1 Hz; LPDF microseisms are generated by regular swells during periods of sea wind weakening, with a narrow frequency band concentrated between 0.15 and 0.3 Hz. In terms of temporal dimensions, as the sea wind weakens, the energy of SPDF microseisms weakens, and the dominant frequencies increase, whereas the energy of LPDF microseisms strengthens and the dominant frequencies decrease, which is consistent with the process of the decay of wind waves and the growth of swells. In terms of spatial dimensions, as the microseisms propagate inland areas, the advantageous frequency band and energy of SPDF microseisms are reduced and significantly attenuated, respectively, whereas LPDF microseisms show no significant changes. And during the propagation process in high-elevation areas, LPDF microseisms exhibit a certain site amplification effect when the energy is strong. The results provide important supplements to the basic theory of secondary microseisms, preliminarily reveal the relationship between the atmosphere, ocean, and seismic noise, and provide important theoretical references for conducting geological and oceanographic research based on the characteristics of secondary microseisms.