New phenomenon in the Gulf of Guinea sheds light on 70 year old mystery

Abstract

More than 70 years ago, seismologists discovered 26 second signal that can be detected on seismic stations almost globally. The signal is excited seemingly continuously from an area in the Gulf of Guinea. Even after 70 years, we still don’t know what is causing this 26 second signal or what physical mechanism is responsible for it. We investigate its evolution, and discover that energetic bursts of higher amplitude in the 26 s signal are often accompanied by a spectral glide effect. We apply array methods to constrain the source location and wave types of the sustained seismic signals, and find that the source of both signal types excites Love and Rayleigh waves and are connected spatially, with a fixed source location. The gliding tremor has a very low frequency onset, lasts for days and is extremely repeatable. Similar to the 26 s microseism, the glides are occasionally so strong they can be detected almost globally. The gliding tremor reminds us of signals detected close to active volcanoes. After careful consideration of different oceanic and volcanic mechanisms, we propose a combined source mechanism for the gliding and continuous tremor, of a hydrothermal system consisting of a layered structure or channel with a resonance period of 26 s. The channel is sealed off by a fractured plug that acts as valve through which gas can escape intermittently, thereby producing pressure pulses with a repetition period stabilized by the resonance of the channel. However, the low frequency, decades long duration that these phenomena appear to have been active and the repeatability of the glides cannot be explained by known volcanic tremor mechanisms. This points towards the need to view tremor in a new way. Since volcanic tremor is an important tool for monitoring volcanic activity, our discovery may affect future forecasting of activity at volcanoes.