3D scattering image of the Campi Flegrei caldera (Southern Italy)New hints on the position of the old caldera rim

Abstract

A scattering image of the Campi Flegrei caldera has been obtained using coda wave envelopes from local microearthquake data recorded at a portable 12-station digital network. This was operated during the paroxysmic phase of the ground uplift (bradyseism) that occurred in the years 1983–1984. The Nishigami technique has been used to obtain the 3D distribution of the strong scatterers in an earth volume with a horizontal extension of 20 km×20 km and a vertical thickness of 6 km. This earth volume was segmented in blocks of 2 km×2 km×2 km for the outer section, and in blocks of 1 km×1 km×1 km for the inner section. The inversion was performed using a hybrid jackknife-SVD method that has allowed qualitative control of the stability and robustness of the solution. The results show that the scatterers revealed by the coda envelopes, which were band-pass filtered at a center frequency of 18 Hz, match the inner border of the caldera rim, which is well outlined on land by geological observations. The scatterer position also delineates this border well beneath sea level. These results are in close agreement with recently obtained seismic velocity tomography. The scatterers imaged at the 6 and 12 Hz band-pass frequencies are located around the zone of maximum uplift, beneath Solfatara crater, where most of the present residual volcanic activity is concentrated.