The estimation of volume change $\Delta V$ and its location using a broadband seismometer data is still approximate in the case of the Piton de la Fournaise and the volumes estimated here are constituting an upper bound limit.We are aware that this approach is simplistic but we believe that it is justified by the fact that the Mogi's model is still routinely used in volcano observatories to determine the pressure change in the edifice. A large number of deformation models are available in the literature (Yang and Davis, 1986; Gudmundsson, 1987). Some additional experiments will be necessary to discriminate the relative contributions of tilt and displacement during the transfer of magma out of the upper reservoir. The transient deformations extend over a region larger than 10 km across and with a typical duration of 500-1000 s. The similar pattern of deformation observed suggests that the source of the events has been stable over the last 15 yr. Large scale deformations were already suggested based on GPS benchmark time-series located on the western part of the Enclos Fouqué. We also confirm here that the EastŠWest displacement of the GPS benchmark 1B80 (Figure 2.46) could be related to the magma chamber pressure state and not to the slip along a discontinuity located along the border of the Enclos Fouqué, as previously suggested (Houlié, 2005). The GPS station located at the summit of the edifice constitutes a complementary tool to detect the long-term deformation episodes ($T>6$ months). The GPS receivers at the summit are only sensitive to the long-term component of the deformation of the magma chamber. Due to their location above the source, their sensitivity is reduced to the vertical component accuracy of the GPS (±15 mm) (Houlié, 2005). The variability of the signal on the eastern component of RER suggests that it might be possible to locate the source of the over/under pressure provided that an additional broadband seismometer survey takes place.

Figure 2.46: Displacement of the site 1B80 during 1997-2002 period. The shear modulus $\mu$ is estimated to be 1.95 GPa at the Piton de la Fournaise. This low value is similar to the one used in Hawaii (Rubin and Pollard, 1987) to model the injection of basalt at Kilauea volcano. Figure from Houlié (2005). Data: OVPF/IPGP.
\begin{figure}\epsfig{file=EPSL8680-F04_1b80.eps, width=8cm}\end{figure}

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