Detection and Characterization of Monochromatic Microseismic Activity in the Gulf of Guinea

This study investigates the persistent monochromatic microseisms originating from the Gulf of Guinea, a phenomenon first observed in the 1960s. These signals stand out for their narrow spectral content, long-term stability, and global detectability. Despite decades of investigation, however, their generation mechanism remains debated, with hypotheses ranging from oceanic wave interactions to deep geological processes. We analyzed 25 years of continuous data from the global seismic network (2000–2024) using a coherence-based methodology designed to detect and characterize weak but persistent sources. The approach included daily coherence computation and a selective event detection procedure based on coherence thresholds, which allowed the isolation of over 3,400 high-coherence events. This methodology enhanced spectral resolution and enabled the study of temporal, seasonal, and spatial variations with unprecedented detail. The results confirm the remarkable persistence of the 26 s and 28 s components, refine the spectral structure of the 16 s band, and reveal a previously unnoticed 21 s peak. Grid-search localization consistently placed all signals in the same Gulf of Guinea continental margin region, demonstrating remarkable spatial stability throughout the observation period. Seasonal analysis further demonstrates systematic modulation linked to Southern Ocean swells. Together, these observations show that the Gulf of Guinea is not the origin of a single anomalous signal, but rather a stable, multi-modal resonant system. The combined evidence supports a hybrid generation mechanism in which a thin crustal plate overlying a subsurface volcanic sill acts as a natural resonator, modulated by long-period ocean waves.