Local seismicity around the Chain Transform Fault at the Mid-Atlantic Ridge from OBS observations

Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly informative due to their relatively simple structure in comparison to their continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there been few studies of transform faults in slow spreading ridges. Here, we present the first local seismicity catalogue based on event data recorded by a temporary broad-band network of 39 ocean-bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from 2016 to 2017 March. We locate 972 events in the area by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. We refine the depths and focal mechanisms of the larger events using deviatoric moment tensor inversion. Most of the earthquakes are located along 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 characterized by normal faulting and most of the transform events are characterized 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 degrees C-isotherm from a simple thermal model. The deeper events could be explained by the control of sea water infiltration on the brittle failure limit.