3-D modelling of Campi Flegrei ground deformations: Role of caldera boundary discontinuities

François Beauducel, Giuseppe De Natale, Franco Obrizzo, and Folco Pingue

Pure and Applied Geophysics, Vol 161, N°7, Pages 1329-1344, 2004.

Abstract. Campi Flegrei is a caldera complex located west of Naples, Italy. The last eruption occurred in 1538, but the volcano has produced unrest episodes since then, involving rapid and large ground movements (up to 2 m vertical in two years), accompanied by intense seismic activity. Surface ground displacements detected by various techniques (mainly InSAR and levelling) for the 1983 to 1996 period can be modelled by a shallow point source in an elastic half space, but the source depth is not compatible with seismic and drill hole observations, which suggest a magma chamber just below 4 km depth. This apparent paradox has been explained as due to the presence of boundary fractures marking the caldera collapse. We present here the first full 3-D modelling for the unrest of 1982-1985 including the effect of caldera bordering fractures and of the topography. To model the presence of topography and of the complex caldera rim discontinuities, we used a mixed boundary elements method. The a priori caldera geometry is determined initially from gravimetric modelling results, and refined by inversion. The presence of the caldera discontinuities allows a fit to the 1982-1985 levelling data as good as, or better than, in the continuous half-space case, with a much different source depth, which fits the actual magma chamber position as seen from seismic waves. These results show the importance of volcanic structures, and mainly of caldera collapses, in ground deformation episodes.

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