Faults modulate magma propagation and triggered seismicity: the 2022 São Jorge (Azores) volcanic unrest

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 events. Geological structures, such as faults and fractures, play a crucial role in guiding magma, but their mechanisms remain unclear due to the lack of 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, provides a unique insight into the interaction between faults and magma. Using ~18,000 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. A magmatic dike, likely originating in the upper mantle, ascended rapidly, largely aseismically, and without apparent precursory surface deformation, into a crustal fault, before stalling beneath the island edifice, 1,600 m below the surface. Adjacent seismicity suggests that the ascending magma stalled, probably due to minor melt branching and fluids escaping laterally along the fault zone, triggering an intense, months-long seismic swarm, comprising rotated focal mechanisms. Our study reveals the dual role of fault zones in both facilitating and arresting magma ascent, highlighting the interplay between tectonism and magmatism.