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An analytical approach to estimate the relative contribution of the fluid pressure and tectonic stress in hydrothermal/&#13; volcanic areas is proposed assuming a Coulomb criterion of failure. The analytical procedure requires the&#13; coefficient of internal friction, cohesion, rock density, and thickness of overburden to be known from geological&#13; data. In addition, the orientation of the principal stress axes and the stress ratio must be determined from the&#13; inversion of fault-slip or seismic data (focal mechanisms). At first, the stress magnitude is calculated assuming&#13; that faulting occurs in 'dry' conditions (fluid pressure=0). In a second step, the fluid pressure is introduced performing&#13; a grid search over the orientation of 1) fault planes that slip by shear failure or 2) cracks that open under&#13; different values of fluid pressure and calculating the consistency with the observed fault planes (i.e. strike&#13; and dip of faults, cracks, nodal planes from focal mechanisms). The analytical method is applied using fault-slip&#13; data from the Solfatara volcano (Campi Flegrei, Italy) and seismic data (focal mechanisms) from the Vesuvius&#13; volcano (Italy). In these areas, the fluid pressure required to activate faults (shear fractures) and cracks (open&#13; fractures) is calculated. At Solfatara, the ratio between the fluid pressure and the vertical stress ?is very low for&#13; faults ( ?=0.16) and relatively high for cracks ( ?=0.5). At Vesuvius, ?=0.6. Limits and uncertainties of the&#13; method are also discussed

Estimates of fluid pressure and tectonic stress in hydrothermal/volcanic areas:a methodological approach