Seismological Insight on the Shallow Magma System

We present an overview of the volcanic seismicity recorded at Stromboli from January to September 2003. The data set starts few weeks after the onset of the eruption and covers most of the effusive phase and the subsequent recovery of the explosive activity. The most important variations occurred between May and July coinciding with the waning of the lava flow and the reappearance of Strombolian activity at the summit craters. All the parameters indicate that the shallow magmatic system has not undergone permanent changes during this period. The only significant variation related to the shallow conduit is the increase in volcanic tremor amplitude and the change in the spectral content of long-period events during the transition between effusive and explosive activity. A slight increase in the very-long-period (VLP) events source elevation seems to mark the rise of the magma at the end of the effusive phase. The variations in the VLP events occurrence rate are more likely to be attributed to changes in the gas flow rate and the bubble coalescence mechanism, therefore, to a deeper portion of the magmatic system. The 5 April paroxysm is associated only with a small increase of the activity in the following days