RELATIVE LOCATION OF HYBRID EVENTS AT STROMBOLI VOLCANO, ITALY

In this work we analyzed distinctive seismic events known as hybrid events. They were observed at Stromboli Volcano, during the 2007 eruption. Hybrid events have intermediate characteristics between volcano-tectonic and long-period events (Martini et alii 2007). Three main swarms of hybrid events occurred during March 2007 on days 6-8, 20 and 22. The total number of hybrid events observed during this period was about 3,500. Different procedures were used for the analysis: first the clustering of waveforms, then the absolute and relative location of these events. The waveform clustering (using Self-Organizing Map neural networks) has shown that most of the events belong to three main families. These clusters have persisted for a long time interval (more than 20 days). This suggests that the source has not undergone significant changes over this period. During the three most intense swarms we recorded also other waveforms belonging to different families. For each family we performed waveform stacking to improve the signal/noise ratio. Then, using a manual procedure, we obtained a preliminary absolute location for each family. Relative waveform shift for each family have been computed using the cross-spectrum method. These data have been used to perform a relative hypocenter location of each family using a double-difference approach. We observe that almost all the hypocenters fall at very shallow depth. This observation suggests that their source is linked to processes occurring very close to the volcano surface. In the period from March 6 to 8 the formation of a macroscopic fracture system was observed at the summit of the volcano. We infer a causal link between these structures and the source of the hybrid events located in the same position