Ambient vibration measurements to support morphometric analysis of a pyroclastic cone

Abstract

Pyroclastic cones are a typical feature on volcanoes characterized by flank activity. Their distribution and orientation are important markers to obtain information on the maximum horizontal compressional stress acting on a volcano. A geophysical survey was performed on the pyroclastic cone of Mt. Vetore (Mt. Etna volcano, Southern Italy) to obtain information on its internal structural setting and to support the standard morphometric analysis. Results highlighted evident frequency peaks at 1.0 Hz inside the cone, which are attenuated away from it. The random decrement method was applied to this peak to compute damping and then to exclude links with anthropogenic sources. Moreover, time-frequency polarization analysis revealed that ambient vibrations are strongly polarized in a narrow frequency band, centered at a frequency of 1.0 Hz, with a preferred oscillation azimuth of 70–90° N. Array measurement of ambient vibrations was also used to obtain a shear wave velocity profile and then to retrieve the main interfaces with high seismic impedance. Results suggest a cone structure having a feeder dike consisting of fractured rocks with thickness of about 50 m surrounded by pyroclastic material lying on a high-velocity substrate. Finally, a 3D model of Mt. Vetore cone was built employing the finite element method to reproduce an experimental modal frequency of the cone itself. The numerical results successfully reproduced the experimental ones collected by the geophysical survey.