Stephen Paul Hicks , Pablo J. González , Anthony Lomax , Ana Ferreira, Ricardo S Ramalho, Neil Mitchell, Graça Silveira, Nuno Afonso Dias, Joao Fontiela, Rui Fernandes, Susana Custódio, Maria Tsekhmistrenko, Virgilio Mendes, Adriano Pimentel, Rita Silva, Gonçalo Prates, William Sturgeon, Augustin Marignier, Fernando Carrilho, Rui Marques, Miguel Miranda, Arturo M Garcia

Understanding the signatures and mechanisms of failed volcanic eruptions is vital for mapping magma plumbing systems and forecasting volcanic hazards. Geological structures like fractures and faults are key to guiding magma, but their mechanisms remain unclear due to limited 3-D mapping of faults in volcanic regions and sufficiently precise earthquake locations. The triple-junction setting of the Azores Archipelago, where volcanic systems and seismogenic crustal faults coexist, offers a unique window into how faults impact magmatism. Using ~12,000 earthquakes relocated to ultra-high precision with onshore and ocean-bottom seismometer data, along with geodetic observations and seismic autocorrelation imaging, we analyse a failed eruption in 2022 on São Jorge Island. Magma from the upper mantle ascended rapidly and largely aseismically over several days along a crustal fault, before stalling beneath the island edifice. Adjacent seismicity with rotated focal mechanisms suggests that the ascending magma became less buoyant due to devolatilisation, with fluids leaking laterally along the fault zone, triggering an intense, months-long seismic swarm. This study reveals the dual role of fault zones in both facilitating and arresting magma ascent, highlighting the interplay between tectonism and magmatism.