Sudden shallow dyke intrusion at São Jorge Island (Azores) after 60 years of repose

Eruptions at long-inactive volcanoes are usually preceded by days to months of unrest as magma migrates gradually to shallower depths. This is built into plans by civil protection agencies for societal response. Here we show that at São Jorge, Azores, after 60 years of repose, magma reached almost the surface in a vertical dike intrusion within a few hours of the seismicity onset with no previous precursory signals. São Jorge lies in a rift zone where extensional stress is expected to be built over time to accommodate magma at depth. Recent eruptions at São Jorge have produced pyroclastic density currents, and the potential for an eruption to occur with little warning poses a significant risk. Deformation associated with the event reached other neighboring islands over a distance of at least 45 km away from São Jorge. Deformation was high on the first day of activity (>50 mm within March 19-20) and significantly decreased afterward. The combined analysis of GNSS and InSAR data allows using a model of segmented rectangular dislocations with multiple patches for data inversion. A maximum opening of 1.7 m at 4-6 km depth is inferred from the modeling. We interpret the cause of the initial vertical shallow injection to be due to host rock failure conditions triggered by deviatoric stresses. We investigate why lateral spreading of the dike occurred soon after the initial injection. Using a FEM simulation, we show how the tension at the tip of a vertical propagating dike is high at the start and decreases with shallower depths, reaching similar levels of tension found at the lateral parts of the dike and increasing the probability of lateral propagation.