Fehler, M. C.

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.

In this article we apply a passive scattering-imaging method, derived from the method developed by Nishigami (1991) to data from the coda of the local volcano-tectonic (VT) earthquakes of Mt. Vesuvius. This method provides the space distribution of the strong scatterers together with a rough estimate of their strength. In the development of our method we use a realistic raytracing calculated with a raybending approach in the 3D velocity model of Mt. Vesuvius structure obtained with travel-time inversion. The inversion procedure adopted for the scattering imaging in the present study is based on the conjugate gradient method (CGM). The volume under study is divided into cubic cells with different dimensions in a multiscale approach. We obtain the best resolution (900 m cubic cell size) in the central part of the volume under study (roughly in a radius of 4 km centered in the crater) within a depth of 5 km. We analyzed the coda signals after filtering in two frequency bands, the first centered at 12 Hz and the second at 18 Hz, where most of the seismic energy is concentrated. Results show that most of the strong scatterers are located in the depth range between the surface and 3000 m below the sea level, in correspondence with the crater axis where most of the seismicity occurs. Part of the scatterers are located in the zones characterized by the maximum velocity contrasts.