Tidal Triggering of Microseismicity at the Equatorial Mid-Atlantic Ridge, Inferred From the PI-LAB Experiment

The gravitational pulls from the moon and the sun result in tidal forces which influence both Earth's solid and water mass. These stresses are periodically added to the tectonic ones and may become sufficient for initiating rupture in fault systems critically close to failure. Previous research indicates correlations between increased seismicity rates and low tides for fast- and intermediate-spreading mid-ocean ridges in the Pacific Ocean. Here, we present a microseismicity data set (4,719 events) recorded by an ocean bottom seismometer deployment at the equatorial Mid-Atlantic Ridge. We show that low, as well as decreasing ocean water level, result in relatively elevated seismicity rates at higher magnitudes (lower b-values), translated into increased probabilities of stronger event occurrence at or towards low tides. Moreover, seismic bursts (enhanced activity rate clusters), occurring at rates well above the reference seismicity, are exclusively present during values of either high tidally induced extensional stresses or high extensional stress rates. Although the b-value differences are not significant enough to be conclusive, the seismicity rate variations exhibit statistical significance, supporting the previous findings for tidal triggering at low tides within normal-faulting regimes and extending the range of observations to slow-spreading ridges. Observed triggering of slip on low angle faults at low tides is predicted by Coulomb stress modeling. The triggering of slip on high angle faults observed here, is not easily explained without another factor. It may be related to the presence of a shallow magma body beneath the ridge, as supported by previous seismic imaging in the region.