The use of seismic arrays to study the seismo-volcanic source. The example of Mt Etna and Stromboli Volcano.

The properties of volcanic tremor wavefields at Mt Etna Volcano, Italy, are investigated using data from two dense, small aperture arrays of short-period seismometers deployed on the North and South flank of the volcano. Spectral analysis shows that most of the seismic energy is associated with several, narrow spectral peaks spanning the 1–5 Hz frequency band. Analysis of simultaneous recordings indicates that most of these peaks are common to different sites, thus suggesting a source effect as the origin of this energy. Frequency-slowness analyses show a complex wavefield, where body- and surface-waves alternatively dominate depending on the frequency band and the component of motion taken into account. Using a probabilistic approach, we invert slowness data measured at two dense arrays for retrieving source location and extent. The joint inversion of slowness data from the two arrays points to different source locations. This observation is interpreted in terms of ray bending associated with lateral heterogeneity and/or strong topographic effects on wave propagation. Once the propagation effects are taken into account, the most probable source location is a shallow region encompassing the summit craters and the eruptive fissures active at the time of the experiment. Data from two dense arrays of short-period seismometers are used to retrieve source locations of the explosion quakes at Stromboli volcano. Slowness vectors estimated at both arrays with the zero-lag cross-correlation technique constitute the experimental data set. A probabilistic approach based on a grid search spanning the volcano interior is used to calculate the probability of the source location. Results show a shallow source, located beneath the crater area, at depths not greater than 500 m below the surface