Ten years of extensometry at Soufrière of Guadeloupe: New constraints on the hydrothermal system

Thomas Jacob, François Beauducel, Gilbert Hammouya, Jérôme G. David, Jean-Christophe Komorowski

Soufriere Hills Volcano - Ten Years On international workshop, Seismic Research Unit, University of West Indies, July 24-30, 2005.

Abstract. Since 1995, the displacement field has been monitored on the lava dome’s fractures at Soufrière of Guadeloupe volcano, using manual extensometry on 12 sites and five main radial fractures. In this study we analyse the data on four of the most important fractures: the North-west fracture, the North fracture, the 30th of August fracture and the 8th of July fracture, totalizing 7 stations. Three main trends are observed: (1) a period of extension from 1995 to 1999, (2) a period of contraction from 1999 to 2004, and (3) a new period of extension from 2004 until present. Given the small scale of these displacements, less than 2 cm, and given the real three dimensional context, we have used for modelling an elastostatic boundary elements code that takes into account 3-D topography, fractures and complex pressure source geometry: the mixed boundary element method (MBEM). Based on the hydrothermal activity of the volcano and recent results in electrical tomography, main source of displacements can be accounted by a hydrothermal reservoir undertaking pressure changes. Source overpressure value, geometry and location are inverted to simulate the complex observed deformation pattern. The best fit model for periods of extensions yields a shallow pressure source of ellipsoidal shape centered within the lava dome, at about 100-m depth, undergoing an overpressure rate of 0.21 and 0.48 MPa/yr for the two periods of extension respectively. At this given rate, if the extension trend remains constant in the future, the pressure in the superficial reservoir will exceed lithostatic pressure in less than ten years.

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