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 bou ndary (Apennine, Central Italy), with a maximum 6.0 M-w, a program of geoch emical surveying together with a collection of hydrogeological changes epis odes was extended throughout the epicentre-area, taking the yearly period o f 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 know ledge about deep fluids circulation changing during seismicity. Laboratory and on-field analyses included major, minor and trace elements as well as d issolved gases (He, Ar, CH4, CO2, H2S, Rn-222, NH4, As, Li, Fe, B, etc..) a nd selected isotopic ratios (C, H, O, He, Sr, Cl), meaningful from tectonic point of view. The chemistry and isotopic chemistry of the spring were ful ly outlined and discussed, pointing out the main process involving the ther mal aquifer: the water-rock interaction inside the Evaporite Triassic Basem ent (ETB), possibly involving also the Paleozoic Crystalline Basement. On t he other hand, sudden and apparent geochemical and hydrogeological variatio ns during the seismic sequence ruled out an evolution in the water-rock int eraction processes. They occurred both at depth, i.e., induced by fluid rem obilization 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 betwe en the paroxysmal phases of the seismic sequence and the observed trends an d spike-like anomalies. The groundwater variations was inferred to occur ma inly inside the ETB, from depth (1-2 km) up to surface, particularly in ass ociation of the Sellano earthquake (14/10/1997) and of the seismic re-activ ation of the sequence at the end of March 1998 (Gualdo Tadino-Rigali and Ve rchiano areas). The lack of deeper input from below the ETB (slight signatu re of PCB), as the lack of He mantle signature, during the seismic period a s a whole, accounted for seismogenic fault segments rooted only in the crus t. 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 earth quake forerunners.