Triggering of Microseismicity During Low Tides at the Equatorial
Mid-Atlantic Ridge, Inferred from the PI-LAB Experiment Data
Abstract
The combined gravitational pulls from the moon and the sun result in
periodical tidal stresses at rates potentially exceeding the tectonic
ones. Yet, tidal triggering of earthquakes in critically stressed faults
is still under debate and controversial results have been obtained,
depending upon specific physical properties and geological settings.
Although no universal triggering pattern between earthquakes and tides
has been observed in oceanic environments, previous research implies
relation between increased seismicity rates and low tides at particular
sites at fast-spreading ridges in the Pacific. We present a dataset of
4719 microearthquakes (-1.4≤ML≤4.0) recorded by an Ocean
Bottom Seismometer (OBS) network at the slow-spreading equatorial
Mid-Atlantic Ridge from March 2016 to February 2017. We use a
single-station template matching technique to focus on a small volume,
spreading within a ~5km radius from the station. The
origin time of the events and their epicentral location is sufficiently
determined for a robust comparison with the ocean tides. Our analysis
suggests a significant correlation between seismic potential and tidal
forces, with the majority of events occurring during or towards low
tides, i.e., during maximized extensional stress and maximized
extensional stress rate. The tidal dependence of magnitude distribution
is also investigated. Although the b-values are generally lower at low
tides, the differences are not sufficiently large to achieve statistical
significance. However, seismic bursts (enhanced activity rate clusters),
occurring at rates above the reference seismicity, are exclusively
initiated at extensional stress rates. Coulomb stress modelling implies
that slip is promoted during low tides at low-angle normal faults. Local
morphology, seismicity distribution and focal mechanisms suggest the
existence of high angle faults at shallower depths. Coulomb modelling
suggests slip on these faults should not be triggered at low tides
unless another factor is considered. One possibility is the presence of
a shallow magma chamber. Such a chamber has also been suggested by
previous seismic imaging results. Overall, the result yields new insight
into magmatic – tectonic cycles and seismicity triggering at mid-ocean
ridges.