Generation mechanism of the 26 s and 28 s tremors in the Gulf of Guinea from statistical analysis of magnitudes and event intervals

Abstract

The earth is a dynamic planet with abundant vibrating processes. Besides the earthquakes, volcanoes and other activities, there is a special type of sources called persistent localized microseismic source (PL) with long-period almost harmonic signals and fixed location. The 26 s (0.038 Hz) and 28 s (0.036 Hz) tremors in the Gulf of Guinea are two typical PL sources in the world, but their generation mechanisms are still enigmatic. Moreover, understanding behaviors of these two sources helps to reduce their interference to ambient noise tomography. We implemented an algorithm to detect events in the PL signals for the past 30 years, and then performed statistical analysis of magnitude-cumulative number (M-N) and interval time-number (T-N). We found that the magnitude distribution is similar to the Gutenberg-Richter relation (G-R relation or power law) and the distribution of interval between events is consistent with a Poisson process. We propose that the two sources are probably related to underground complex crack networks featuring fractal characteristics, and are dominantly driven by temporally random dynamic processes. However, ocean swell might affect the 26 s source occasionally while the primary microseism seems to modulate the two PL sources.