Abstract. Sulphur dioxide (SO2) fluxes of active degassing volcanoes are routinely measured with ground-based equipment to characterize and monitor volcanic activity. SO2 of unmonitored volcanoes or from explosive volcanic eruptions, can be measured with satellites. However, remote-sensing methods based on absorption spectroscopy generally provide integrated amounts of already dispersed plumes of SO2 and satellite derived flux estimates are rarely reported. Here we review a number of different techniques to derive volcanic SO2 fluxes using satellite measurements of plumes of SO2 and investigate the temporal evolution of the total emissions of SO2 for three very different volcanic events in 2011: Puyehue-Cordón Caulle (Chile), Nyamulagira (DR Congo) and Nabro (Eritrea). High spectral resolution satellite instruments operating both in the ultraviolet-visible (OMI/Aura and GOME-2/MetOp-A) and thermal infrared (IASI/MetOp-A) spectral ranges, and multispectral satellite instruments operating in the thermal infrared (MODIS/Terra-Aqua) are used. We show that satellite data can provide fluxes with a sampling of a day or less (few hours in the best case). Generally the flux results from the different methods are consistent, and we discuss the advantages and weaknesses of each technique. Although the primary objective of this study is the calculation of SO2 fluxes, it also enables us to assess the consistency of the SO2 products from the different sensors used.

A dense seismic array maintained temporarily for 7 weeks provides an approach to deducing the 3-D heterogeneity of the structure of the upper crust beneath Mt Etna volcano, Sicily, Italy, from the inversion of P-and S-wave arrivals of local earthquakes. Independent data from a different source-array geometry using waves generated by distant shots at sea confirm a main lateral variation. A high-velocity body underlies part of the volcano. Its contrast with the surrounding thick sedimentary pile and its lo'cation extending from the present summit to the southeast suggests that it may have been built by the development and migration of past eruptive centres followed by caldera collapse. Local low-velocity anomalies near its sharp western edge may be associated with presently active features. These are: an extension of the region of the central craters from sea level to greater depth and a zone at 6 km depth, of possible proximity of magmatic material to sediments connected both to the upper central system and to greater depths. The structure under the western flank appears as highly heterogeneous.During the recording period, a sustained lava eruption of the Southeast Crater stopped and was followed by explosive activity at the central craters and a series of destructive earthquakes on the lower eastern flank in October 1984. No continuous migration occurred between these phases but they might be related through their locations with respect to structural heterogeneity. Seismic activity changed from a swarm of events just beneath the summit at sea level to a few isolated, much stronger events a few kilometres deeper and towards the edge of the central high-velocity zone and eventually to destructive single earthquakes further away under the lower slopes.

Sulphur dioxide (SO₂) fluxes of active degassing volcanoes are routinely measured with ground-based equipment to characterize and monitor volcanic activity. SO₂ of unmonitored volcanoes or from explosive volcanic eruptions, can be measured with satellites. However, remote-sensing methods based on absorption spectroscopy generally provide integrated amounts of already dispersed plumes of SO₂ and satellite derived flux estimates are rarely reported.

Here we review a number of different techniques to derive volcanic SO₂ fluxes using satellite measurements of dispersed and large-scale plumes of SO₂ and investigate the temporal evolution of the total emissions of SO₂ for three very different volcanic events in 2011: Puyehue-Cordón Caulle (Chile), Nyamulagira (DR Congo) and Nabro (Eritrea). High spectral resolution satellite instruments operating both in the UV-visible (OMI/Aura and GOME-2/MetOp-A) and thermal infrared (IASI/MetOp-A) spectral ranges, and multispectral satellite instruments operating in the thermal infrared (MODIS/Terra-Aqua) are used. We show that satellite data can provide fluxes with a sampling of a day or less (few hours in the best case). Generally the flux results from the different methods are consistent, and we discuss the advantages and weaknesses of each technique. Although the primary objective of this study is the calculation of SO₂ fluxes, it also enables to assess the consistency of the SO₂ products from the different sensors used

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