HR: 14:10h
AN: V52E-03    [PDF]
TI: Anelastic Deformation on Montserrat
AU: * Cayol, V
EM: cayol@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, CNRS, Univ. B. Pascal, OPGC, 5, rue Kessler, Clermont-Ferrand, 63000 France
AB: GPS measurements at Soufriere Hills volcano on Montserrat show that displacements that occurred since 1995 are irreversible. Displacements measured between June 1996 and June 1997 (Shepherd et al., 1998) are re-interpreted using a three-dimensional Boundary Element Method (3D BEM) for elastic medium. Using a spherical source model, it is shown that the source of the displacements is located at the base of the dome (350 meters below the dome summit in 1997), therefore these displacements could be created by the dome weight. Further models show that the weight of the dome on the crater floor only accounts for 14 percent of the measured displacements. We have then assumed that the dome was fed by a vertical cylindrical conduit so that increase in the dome volume increases pressure in the conduit. We find that this pressure could explain the GPS observations if the conduit radius was 350 meters in diameter. In 1996 and 1997, tiltmeters recorded cyclic deformations of the edifice (Voight et al, 1999). Two tiltmeters operated simultaneously at different distances from the summit allowing determination of the source depth. Assuming these deformations are created by the pressurization of a portion of a cylindrical conduit in an elastic medium and using again a 3D BEM, we show that this source is located at 900 meters depth below the dome summit. Scaling the conduit diameter so that the elastic rock matrix does not fail, the measured tilt amplitudes are explained by a conduit diameter larger than 100 meters. These interpretations of Montserrat deformations lead us to estimate a conduit diameter much larger than the 30 meter-diameter estimated from geological and petrological interpretations (Devine et al., 1998). This paradox can be explained if the conduit size estimated from the deformations corresponds to a "pseudo-pressurized-conduit" made of a central cylindrical conduit where magma is flowing, embedded in a visco-plastic cylindrical medium where stresses are close to hydrostatic so that magma pressures are transmitted to the surrounding elastic rock matrix. This explanation is compatible with the geological and petrological interpretations as these methods provided the conduit diameter required to transport magma to the surface. This anelastic rheology is particularly relevant to studies of the stability of the edifice and further risk assessments.
DE: 3909 Elasticity and anelasticity
DE: 8164 Stresses--crust and lithosphere
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting