Crustal weakening before rupture at volcanoes after long repose.

At volcanoes reawakening after prolonged quiescence, local seismicity commonly increases before eruption. The earthquakes occur as crust extends over a zone of pressurising fluid several kilometres underground. The deformation causes existing faults to slip and new fractures to open. An eruption is most likely when the fractures grow large enough to rupture the crust. Fracture growth coincides with a net decrease in the elastic energy stored in the crust. Here we present a method for following changes in stored energy through a balance between the energy supplied by the pressure source and lost by seismicity. We test the method against unrest sequences at two large calderas, Sierra Negra and Campi Flegrei. The results provide a practical procedure for identifying the onset of fracture growth and, hence, for recognising an increased potential for crustal rupture.