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Triggering of Microseismicity During Low Tides at the Equatorial Mid-Atlantic Ridge, Inferred from the PI-LAB Experiment Data
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  • Konstantinos Leptokaropoulos,
  • Nicholas Harmon,
  • Stephen Hicks,
  • Catherine Rychert,
  • David Schlaphorst,
  • John-Michael Kendall
Konstantinos Leptokaropoulos
University of Southampton

Corresponding Author:k.leptokaropoulos@soton.ac.uk

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Nicholas Harmon
University of Southampton
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Stephen Hicks
Imperial College London
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Catherine Rychert
University of Southampton
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David Schlaphorst
Universidade de Lisboa
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John-Michael Kendall
University of Oxford
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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.