James Hickey, Karen Pascal, Matthew Head, Joachim Gottsmann, Nico Fournier, Sigrun Hreinsdottir, Racquel Syers

Dome-building volcanoes are particularly challenging for volcanic hazard assessment, where long-term eruptive episodes can be interspersed with periods of intra-eruptive repose. Defining the end of eruptive episodes is vitally important for the socio-economic recovery of affected communities, but highly problematic due to the potential for prolonged, seemingly low-risk, repose to rapidly transition to dangerous effusive or explosive activity. It is currently unclear what constitutes the end of repose and an eruptive episode. Here we show that analysis of surface deformation can characterise repose and help define an eruptive episode. At Soufrière Hills volcano (SHV) the observed long-term deformation requires the pressure in the magma system to increase with time; time-dependent stress relaxation or crustal creep cannot explain the deformation trends alone. Continued pressurisation within the magmatic system during repose could initiate a renewed eruption, qualifying as sustained unrest and therefore continuation of the eruptive episode. For SHV, persistent magma pressurisation highlights the need for sustained vigilance in the monitoring and management of the volcano and its surroundings, despite the last eruptive activity ending in 2010.