Bimodal double-frequency microseisms beneath Arctic sea ice: The role of sediment modulation

Double-frequency (DF) microseisms, generated by the nonlinear interaction between ocean waves, constitute the most energetic component of the seismic ambient noise field. While their spectral energy is primarily governed by ocean-wave conditions at the source, it is also modulated by water column and sediment. However, the combined influence of the factors remains poorly constrained. In this study, we analyze under-ice ocean-bottom seismometer (OBS) records at the eastern Gakkel Ridge in the Arctic Ocean and identify a distinct bimodal DF microseism distribution with spectral peaks at short periods of approximately 1 s and 3 s. Polarization and correlation analyses reveal that the dominant energy of both peaks originates from the St. Anna Trough, a region located south-southwest of the study area that experiences intense ocean-wave activity in autumn. While the peak wave period of about 6 s in the source region accounts for the 3 s spectral peak, it cannot directly explain the 1 s peak. One-dimensional Rayleigh-wave dispersion modeling demonstrates that, at a water depth of ∼4 km and sediment thicknesses of ∼0–400 m observed at our OBS sites, a group-velocity minimum near 1 s occurs in the fundamental or first-overtone modes, leading to Airy-phase energy focusing. Consequently, the observed bimodal pattern reflects the combined effect of source-region ocean-wave conditions and local sediment modulation. A global analysis of 583 OBS deployments further reveals that bimodal patterns predominantly occur where sediment thickness is less than ∼500 m, indicating that sediment modulation of DF microseisms occurs globally.