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

G. Ventura, G. Vilardo

Abstract


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

Keywords


hydrothermal fluids;faults;cracks;fluid pressure;volcanic/hydrothermal areas;seismicity

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References


DOI: https://doi.org/10.4401/ag-3235
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X