Seismic Attenuation and Shallow Velocity Structures at Stromboli Volcano, Italy

Abstract

The surface-wave field associated with the explosive activity at Stromboli volcano is investigated using data recorded by two short-period seismic arrays, deployed on the north and west flanks of the volcano. The group-velocity dispersion curves for Rayleigh waves are derived using the multiple filter technique. The phasevelocity dispersion curves are recovered using a phase match filter and compared with that inferred from zero-lag cross-correlation analysis applied to the array data. These analyses indicate Rayleigh-wave group velocities ranging from 0.29 to 0.24 km/sec in the 1.5- to 8.0-Hz frequency band, and phase velocities ranging from 1 km/sec at 1.5 Hz to about 0.3 km/sec at frequencies above 5 Hz. In addition, the dispersive properties of the attenuation coefficient (c) for Rayleigh waves are inferred from application of the multiple filter technique to seismograms recorded at different distances from the source. These results are validated through examination of the spectral amplitude decay with distance for both body and Rayleigh waves. The values of the body-wave quality factor thus obtained are Qa=20 and Qa=6 for the north and west side of the island, respectively. The velocity and attenuation dispersion curves are inverted for the shear-wave velocity and Qb structures down to a depth of about 200 m. Shear-wave velocities for the west flank range from about 0.3 km/ sec for the uppermost 17-m-thick layer to 1.9 km/sec at depths greater than 200 m. Comparison with previous studies indicates a similar velocity structure for the north and west flanks. The attenuation structure for the west flank is described by a shallower, 36-m-thick layer with Qb=9, underlain by a half-space with Qb=50. On the north flank, Qb=40 for the shallower 30-m-thick layer and Qb=44 for the underlying half-space. Residuals from analysis of the spectral decay with distance are used to quantify site effects affecting the different array elements on the west flank. Local amplifications at that array are interpreted in terms of an edge effect associated with concave topography. Velocity similarities observed at the north and west flanks are compatible with surface geologic data. Discrepancies in attenuation properties at the two sites are interpreted in terms of different degrees of heterogeneity and crack density controlling the scattering quality factor Qs.