Geochemical changes at the Bagni di Triponzo thermal spring during the Umbria-Marche 1997-1998 seismic sequence

After the earthquakes of September 26, 1997, that hit the Umbria-Marche boundary (Apennine, Central Italy), with a maximum 6.0 M-w, a program of geochemical surveying together with a collection of hydrogeological changes episodes was extended throughout the epicentre-area, taking the yearly period of the seismic sequence as a whole. After a first areal screening, the Bagni di Triponzo thermal spring was selected for a discrete temporal monitoring (weekly and monthly basis), being the unique thermal spring throughout the epicentre area. This site deserves peculiar interest in deepening the knowledge about deep fluids circulation changing during seismicity. Laboratory and on-field analyses included major, minor and trace elements as well as dissolved gases (He, Ar, CH4, CO2, H2S, Rn-222, NH4, As, Li, Fe, B, etc..) and selected isotopic ratios (C, H, O, He, Sr, Cl), meaningful from tectonic point of view. The chemistry and isotopic chemistry of the spring were fully outlined and discussed, pointing out the main process involving the thermal aquifer: the water-rock interaction inside the Evaporite Triassic Basement (ETB), possibly involving also the Paleozoic Crystalline Basement. On the other hand, sudden and apparent geochemical and hydrogeological variations during the seismic sequence ruled out an evolution in the water-rock interaction processes. They occurred both at depth, i.e., induced by fluid remobilization within the crust explained by the Coseismic Strain Model and by the Fault Valve Activity Model, and in the shallow part of the reservoir (i.e., meteoric water contamination). A statistical multivariable analysis (Factor Analysis) was accomplished to better constrain the correlation between the paroxysmal phases of the seismic sequence and the observed trends and spike-like anomalies. The groundwater variations was inferred to occur mainly inside the ETB, from depth (1-2 km) up to surface, particularly in association of the Sellano earthquake (14/10/1997) and of the seismic re-activation of the sequence at the end of March 1998 (Gualdo Tadino-Rigali and Verchiano areas). The lack of deeper input from below the ETB (slight signature of PCB), as the lack of He mantle signature, during the seismic period as a whole, accounted for seismogenic fault segments rooted only in the crust. The results also provide useful information about the earthquake-related response mechanisms occurring at this site, that represent the basic task for planning and managing the impending hydro-geochemical network aimed at defining the relationships between seismic cycle, fluids and reliable earthquake forerunners.