International Congress on Rock Mechanics 1999

Limits and benefits from ground deformation measurements for volcanological monitoring

François Beauducel, Pierre Briole, François H. Cornet, and Giuseppe De Natale

9th International Congress on Rock Mechanics, Paris, August 25-29, 1999. Paper presented on workshop "Movements of rock masses" (invited seminar).

Abstract. Magma transport towards the surface prior to eruptions generate structure changes and detectable ground movements. At the moment, seismicity is the most developed and most efficient method used for the monitoring on active volcanoes. Low frequency deformations, typically with periods from few seconds to several years, are associated with activity changes like internal pressure and stress variations into the edifice. Therefore, they can be used to understand sources characteristics, substructures geometry and the mechanical behaviour of the volcano. At present, one of the main tasks needed for volcanic risk assessment problem is the determination of the boundary and initial conditions for dynamical models of eruptions. Deformation studies are then useful for both direct monitoring and fundamental volcanological researches. We propose here a short list of both classical and new geodetic methods used in volcanology. Since there is presently no available techniques capable to produce an accurate and continuous deformation field, the integration of different methods is needed. Basically, they can be sorted into different types of measured parameters (displacement or strain), spatial and temporal sampling values, remote sensing or ground based systems (linked to ground coupling and field-access problems). In the light of artefact considerations, a critical review of measurement errors related to each of these methods is presented. Surface observations cannot specify, in a unique way, the stress-strain distribution at depth. Thus, the validation of different types of observations, determining a unique deformation field for a particular volcano, constitutes a key tool of a sound geophysical interpretation. Modelling of ground deformations in volcanic areas started with very simple models (like nucleus of isotropic strain into homogeneous elastic medium, i.e. Mogi’s model). Theses analytical models are still in use for data interpretation. Recent studies have shown that more complex models are mandatory to obtain meaningful results about the shape and location of sources implied. Taking into account three-dimensional topography, discontinuities (like fractures and caldera borders) and multiple type of sources (pressure and stress variation into magma chamber and conduit), we have found that elastic behaviour matches the reality in many cases. The use of these models, properly introduced into inversion processes and constrained by data, allow (1) to give estimations of magma flux related parameters, (2) to outline the role of main subsurface discontinuities, (3) either to give evidence for rock slope problems. Study of deformation field on large scale volcanic areas needs the consideration of hydrothermal phenomena to allow discrimination between competitive models, for instance magmatic or phreatic sources. This can be solved by a thermo-poro-elastic modelling, which will be one of the next target for deformation studies on volcanoes.

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