Geochemical evolution of groundwaterof the Iblean Foreland (Southeastern Sicily) after the December 13, 1990 earthquake (M = 5.4)

Geochemical surveys were performed by the Istituto Nazionale di Geofisica (ING), between December 1990 and July 1991, in the framework of an interdisciplinary task study throughout the Siracusa epicentral area: these studies were aimed at col1ecting specific information on the geochemical patterns of fluids, in relation to the geodynamic and seismic evolution of the nol1hern Iblean Foreland area, stal1ing from the December, 13, 1990 Syracuse earthquake (M = 5,4). The results of the hydrogeochemical surveys, discussed in this paper, were in part unexpected. In particular, a steady decrease of the PCO2 va1ues, after the eal1hquake, in ground- waters of the epicentral area, along a NNW-SSE fault bordering the Augusta Graben (Brucoli Sulphureous Spring), was observed. This observation enabled us to reconstruct the geochemical processes triggered by the earthquake: a sudden and strong release of CO2 of deep origin, probably related to a pore pressure uprising and/or to a water/rock interaction changes in the vicinity of the seismogenic structure. The existence of deep- fluid uprising (CO2, 222Rn, NHj, H2S), as well as the variation in time of geochemica1 flows accompanying seismic activity along this NNW-SSE anomalous-sites 1ine, within the whole Iblean Foreland, witnes., the activity (as concern as fluidodynamic and geochemistry) of the NNW-SSE striking Ibleo-Maltese Escarpment fault system. This fact can be taken into account in locating the seismogenic structure responsible for the 1990 earthquake, like a contribution of the geochemical methods applied to seismotectonics. During June 1992, a more complete ana1ysis of the Iblean Foreland groundwaters was performed, co1lecting data on the geochemi- ca1 feature, of the different aquifers in aseismic period. Mu1tivariable statistics, chemical equilibria studies and mapping with our geochemical data, were also performed.