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