Seismic Signals Associated with Landslides and with a Tsunami at Stromboli Volcano, Italy

In this article, we analyze the seismic signals produced by two landslides that occurred at the Stromboli volcano on 30 December 2002, recorded by both broadband and short-period seismic stations located in the 2.5–22-km distance range from the source. For both landslides, the characteristics of the low-frequency seismograms indicate a complex time history in the release of seismic energy. The first landslide occurred over the submerged part of the northwest sector of the volcano and had associated a large-amplitude, low-frequency pulse representative of the abrupt detachment of a large mass. Lower amplitude phases in the following 3 minutes possibly indicate minor detachment events. The highest amplitude, low frequency signals are well described by a single-force source model. The second mass-failure episode is also characterized by a complex source and can be interpreted as a multiple event, with a less abrupt onset and at least four detachments occurring during 4–5 minutes and producing low-frequency signals. Synthetic seismograms generated by a shallow single force located in the submerged area of Sciara del Fuoco and directed upslope, fit well the first low-frequency seismic pulse recorded at Stromboli and Panarea by three-component stations. From this simulation, we estimated the force exerted by the first mass failure. The estimate of the volume through two different procedures, gives values in the range of 1.0–1.5 million m3 and about 14 million m3, respectively. The landslides, which involved both the submarine and the subaerial northwest flank of the volcano, produced a tsunami that struck the coast of Stromboli Island and in a few minutes reached the other islands of the Aeolian Archipelago. Three broadband seismic stations installed on land about 100 m from the coastline at Panarea Island, located 20 km southwest of Stromboli, recorded very long period seismic signals produced by the tsunami waves. Analysis of these signals gives invaluable information on the spectral content and propagation properties of tsunami waves and on their interaction with the ground at a short distance from the coast. Synthetic tsunami waves, obtained by a landslide source model and taking into account the bathymetry of the sea surrounding Stromboli and Panarea Islands, fit the observed phenomena and the experimental data very well.