AGU Fall Meeting 1999

Seven years of SAR interferometry on Etna: Geophysical results and methodological problems

Pierre Briole, François Beauducel, Alessandro Bonforte, Christophe Delacourt, Jean-Luc Froger, and Giuseppe Puglisi

AGU Fall Meeting 1999, Special Session V03: InSAR Imaging of Active Magmatic Processes.

Abstract. The first result obtained using SAR interferometry on Etna was a volcano-wide fringe pattern interpreted as a deflation associated with the second half of the large 1991-1993 eruption. The deformation field was interpreted using a simple Mogi model and the inferred source depth was about 15km. Further studies showed the existence of local deformation fields corresponding to the compaction of recent lava fields, in particular those of 1989, 1986-1987, 1983, 1981 and 1971. In some cases a subsidence of the area surrounding the recent flows was observed and interpreted as a visco-elastic relaxation of the surrounding medium. Creep along active faults is observed in several places of the volcano in the period 1995-1999, but no creep is observed in the period 1992-1995. This observation confirms the non steady-state behaviour for creeping faults on Etna. This property was already known from numerous field observations done in the past, but the SAR interferometry allows to quantify the phenomena in size and time. However interferograms of Etna are affected by tropospheric effects due to the topography of the volcano. Those effects must be properly eliminated in order to correctly interpret the fringes pattern. The problem of the troposphere was first investigated from its theoretical point of view and using existing local meteorological data as well as radio-soundings data. Recently, thanks to the large amount of available interferograms, another approach has been investigated, consisting in the research of a correlation fringe-elevation in the interferograms themselves. This approach proved to be efficient for most of the coherent interferograms. After removal of the tropospheric correction, a revised evolution of the large scale deformation of the volcano at large scale between 1992 and 1999 has been issued. The subsidence occurring during the second half of the 1992 eruption as well as the uplift preceding the 1995 unrest of the Southeast crater are visible, but their amplitude is less than previously estimated. The depth of the modelled source of subsidence-uplift related to the large scale deformation is now of the order of 6-8km. The study of the correlation fringe-elevation was possible only after a detailed analysis of the spatial and temporal properties of coherence of the Etna area. Indeed, the technique of fringe unwrapping for fringe-elevation correlation analysis is possible only if the poorly coherent pixels are eliminated. A map of the most coherent pixels of the volcano was produced.

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