Magma pressurisation sustains ongoing eruptive episode at dome-building Soufrière Hills Volcano

Dome-building volcanoes, where long-term eruptive episodes can be interspersed with periods of intra-eruptive repose, are particularly challenging for volcanic hazard assessment. 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 rapid transition from prolonged, seemingly low-risk, repose 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 long-term post-2010 deformation at 12 cGPS stations 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. Our results show promise for application to other dome-building volcanoes.