An atypical swarm at the North Mid-Atlantic ridge indicating spreading events

A large seismic swarm affected the North Mid-Atlantic ridge between September and November 2022, with an outstanding seismicity rate and a cumulative moment equivalent to a magnitude Mw 6.3. We performed a detailed seismological analysis using regional, teleseismic and array data to reconstruct the spatiotemporal evolution of the seismicity. Combining template matching, relative location and full moment tensor inversion, we identify that most seismicity was located in a narrow band along the ridge, with typical normal faulting mechanisms. However, some of the latest and strongest events occurred up tp 25 km off the ridge axis, with thrust mechanisms that are atypical at mid-ocean ridges and inconsistent with the extensional tectonics. Seismicity also present a clear migration pattern, propagating over ~60 km from North to South, with the thrust mechanisms only occurring in the late phase of the swarm and only in the central-southern section. We hypothesize a magmatic intrusion as driver of the seismicity, with a vertical dyke first propagating southward, accompanied by normal faulting earthquakes, and then thickening, to produce a stress perturbation able to trigger thrust earthquakes on pre-existing structures on the side of the dike. The 2022 unrest provides evidence for sporadic spreading accompanied by large swarm episodes driven by magma intrusions at the mid-ocean ridge.