Oceanic seismotectonics from regional earthquake recordings: the 4-5˚N Mid-Atlantic Ridge

Uncertainties in epicentral locations and hypocentral depths often prevent earthquakes from being associated with individual or group of faults in bathymetric data, thus limiting the understanding of tectonic behavior. Ocean bottom seismometers (OBSs) can overcome this problem, but they take significant efforts to build and deploy, so information from them covers only a minor part of the earthquake record of mid-ocean ridges. As an alternative, a combination of records from seismometers at regional distances and appropriate processing methods can yield location and depth estimates that are useful because they provide extensive data. We illustrate this with a study of magnitude of the seismicity in the 4–5°N Mid-Atlantic Ridge using data from seismometers in Brazil, Cape Verdes, and Africa coast. The seismicity occurred in swarms in 2012 (seven events), 2014 (five events), 2016 (62 events), and 2019 (eight events). We compare the seismicity with features using bathymetric data collected with a multibeam sonars, which reveal two detachment fault surfaces (“megamullions”), one close to the modern rift valley floor but offset by ~10 km from it. The located seismicity is shallow (best estimate less than 8 km below seafloor). The swarms occurred over two segments of the ridge and, in the 2016 case, clearly involved movements on widely distributed multiple faults, including faults on both sides of the valley. Although the methods used produce epicenters and hypocenters with uncertainties that are still larger than those of OBS experiments, they could provide a way to study whether seismicity is systematically deep in certain parts of the ridge where megamullions are observed.