Anomalous character of the coda envelopes onMt Vesuvius explained in terms of depth dependent Q

Abstract

Many observations made on volcanic areas showed that a non-uniform distribution of coda wave energy is present between regions with differing geological structures. This observation was carried out in La Reunion and interpreted by Aki and Ferrazzini as due to a trapping process of the scattered waves in the volcanic cone, named by these authors as ‘coda localization’. The same properties were found in Mt Vesuvius examining codas from shots fired for active tomography. In this paper, we experimentally check whether the scattered wave field produced by local VT earthquakes is inhomogeneous as found for codas from shots in the same volcano. We examine the Log plot of S-wave direct amplitude normalized for the coda amplitude at a given lapse time (coda normalization) for all the recording stations and the site transfer functions calculated for the same stations using both the direct S-wave spectra and the coda wave spectra. Results show that the Log normalized amplitude increases with distance, the opposite of what commonly observed in non-volcanic zones. This effect may be caused by an effective inhomogeneity of the scattering wave field or, in different words, by a total-Q increasing with depth.