Periodic sulphur dioxide degassing from the soufriè're hills volcano related to deep magma supply

Soufrière Hills Volcano produced prodigious quantities of sulphur dioxide (SO₂) gas throughout 1995-2013. An unprecedented, detailed record of SO₂ flux shows that high SO₂ fluxes were sustained through eruptive pauses and for two years after the end of lava extrusion and are decoupled from lava extrusion rates. Lava extrusion rates have exhibited strong 1-to 2-year cyclicity. Wavelet analysis demonstrates periodicities of c. 5 months and c. 2 years within the SO2 time series, as well as the shorter cycles identified previously. The latter period is similar to the wavelength of cycles in lava extrusion, albeit non-systematically offset. The periodicities are consistent with pressure changes accompanying deformation in a coupled magma reservoir system whereby double periodic behaviour may arise from limited connectivity between two reservoirs. During periods of lava extrusion SO₂ is released together with the lava (yielding the c. 2year period), albeit with some offset. In contrast, when magma cannot flow because of its yield strength, SO₂ is released independently from lava (yielding the c. 5month period). Our results have implications for eruption forecasting. It seems likely that, when deep supply of magma ceases, gas fluxes will cease to be periodic.