| Summary: | Abstract A location in the Gulf of Guinea, which emits monochromatic seismic waves at 26-second period, seemingly continuously, was identified in the 1960s. However, the origin of these seismic waves remains enigmatic to date. Here we use three-component data from two seismic arrays in Africa, as well as additional seismic data compiled from around the world, to investigate the tremors. We identify frequency glides accompanying the previously known 26 s microseism which start at the same frequency and originate in the same, fixed location in the Gulf of Guinea. The stable characteristics of the tremors, their low frequency range, the implied large spatial scale, and the decades-long timescales where this phenomenon seems to have been active, all point towards a gap in our understanding of long period oceanic and volcanic signals. Since tremor is an important tool to monitor volcanoes, understanding this phenomenon may affect future forecasting of volcanic activity.
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