Fault-mediated magma propagation and triggered seismicity revealed by the 2022 São Jorge Azores unrest

Understanding failed volcanic eruptions is key to mapping magma plumbing and forecasting hazards. Faults and fractures guide magma, but their mechanisms remain unclear due to the lack of precise earthquake locations and limited 3-D fault mapping in volcanic regions. The triple-junction setting of the Azores Archipelago, where volcanic systems and seismogenic faults coexist, offers a natural laboratory to study fault-magma interactions. We analysed ~18,000 earthquakes relocated to high precision using onshore and ocean-bottom seismometers, combined with geodetic data and seismic autocorrelation imaging, during a failed 2022 eruption on São Jorge Island. A magmatic dike ascended rapidly and mostly aseismically from the upper mantle, intruding a crustal fault before stalling ~1,600 m below the surface. Seismicity indicates that magma branching and lateral fluid escape along the fault triggered an intense, months-long swarm with rotated focal mechanisms. This study demonstrates the dual role of faults in facilitating and arresting magma ascent.