Tidal Triggering of Microseismicity at the Equatorial Mid-Atlantic
Ridge, Inferred from OBS Network
Abstract
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 mid-ocean, fast-spreading ridges in Pacific
ocean. Here, we present a microseismicity dataset (4719 events) from an
Ocean Bottom Seismometer (OBS) network at the equatorial Mid-Atlantic
Ridge, suggesting a significant correlation between seismic potential
and tidal forces. We show that low as well as decreasing ocean water
level results in elevated seismicity rates and lower b-values,
translated into considerably increased probabilities of stronger event
occurrence at or towards low tides. In addition, seismic bursts
(enhanced activity rate clusters), occurring at rates fairly above the
reference seismicity, are exclusively present during either high
extensional stresses or high extensional stress rates. Our results
exhibit remarkable 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 modelling. The triggering of slip on high
angle faults observed here, is not easily explained without another
factor. It may be related to fatigue and/or the presence of a shallow
magma body beneath the ridge, as suggested by previous seismic imaging
in the region.