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Local seismicity around the Chain Transform Fault at the Mid-Atlantic Ridge from OBS observations
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  • David Schlaphorst,
  • Catherine Rychert,
  • Nicholas Harmon,
  • Stephen Hicks,
  • Petros Bogiatzis,
  • John Michael Kendall,
  • Rachel Abercrombie
David Schlaphorst
University of Lisbon

Corresponding Author:dschlaphorst@fc.ul.pt

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Catherine Rychert
University of Southampton
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Nicholas Harmon
University of Southampton
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Stephen Hicks
Imperial College London
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Petros Bogiatzis
University of Southampton
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John Michael Kendall
University of Oxford
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Rachel Abercrombie
Boston University
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Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly useful due to their relatively simple structure in comparison to continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there have not been many studies of slower spreading centres. Here we present the first local seismicity catalogue based on event data recorded by a temporary broadband network of 39 ocean bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from March 2016 to March 2017. Locations are constrained by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. Depths and focal mechanisms of the larger events are refined using deviatoric moment tensor inversion. We find a total of 972 events in the area. Most of the seismicity is located at the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterised by normal faulting and most of the transform events are characterised by strike slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 ± 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600˚C-isotherm from a simple thermal model. The deeper events could be explained by the control of seawater infiltration on the brittle failure limit.