Stability of the seismic source during effusive and explosive activity at Stromboli volcano

In December 2002 Stromboli Volcano erupted triggering a sector collapse and a tsunami. During the 7 month-long effusive eruption, the typical persistent and moderate explosive activity of Stromboli ceased. However, very long period (VLP) seismic events, typically associated with the explosions at Stromboli, were recorded during the effusive eruption. These were clustered mainly in two groups with stable waveforms. This suggests that strombolian activity never ceased but was confined deep within the magma conduit. A location method, based on the seismic directivity recorded with an asynchronous seismic deployment, positioned the two clusters ∼60 m apart from each other at ∼510 ma.s.l. and ∼180 m NW from the summit craters. However, seismic polarization reveals a ∼50 m upward migration of the VLP source at the end of the eruption when volcanic activity moved from the effusive style to more typical strombolian activity.