Abstract

We thank (Cayol and Battaglia, 2009) (hereafter referred to as CB2009) for commenting on our work (Houlie and Montagner, 2007). We start our response by reviewing our model and then discuss the potential of discrimination of tilt and displacement at a VBB seismometer from seismic records. There appears to be a misunderstanding of the approach and the interpretation of our results presented in Houlie and Montagner (2007). We do not question the discovery by Battaglia et al. (2000) of transient deformation before volcanic eruptions, as well as the existence of 2 types of signals, but the interpretation is not necessarily as simple as proposed in their paper. We note that the three components in the seismic record of displacement might have different explanations. From a theoretical point of view, any perturbation of displacement can be expressed as follows: δuiðtÞ = ∑j ∂uiðtÞ xj dxj. It means that δui(t) can be expressed by 9 independent terms. Since σzj=0 at the surface of the Earth, there remain 6 degrees of freedom. Usually, the symmetric part (deformation eij) related to the stress tensor (in the elastic case) is separated from the anti-symmetric part that is related to rotation or tilt. Consequently, distinguishing displacement and tilt is a real scientific challenge. Very long period (period TN100 s) transients are interpreted in terms of tilts (Battaglia et al., 2000), but we explore the possibility that a large part of seismic record is related to translational displacement in addition of or rather than tilt (rotation). Following this simple idea, we draw the consequences of this hypothesis and propose a model which differs from previous approaches (Battaglia et al., 2000; Battaglia and Bachelery, 2003; Battaglia et al., 2005). There are two extreme models for interpreting the data: the first one is only based on tilt (associated with the dislocation model (Okada, 1992)) and a second one is based on ground displacement generated by a spherical source. CB2009 claim that the whole signal can be explained by tilt, but we claim that a significant part of the signal is related to ground displacement and that actual data cannot yet discriminate between these two models. It is likely that both effects are present, and the next challenge will be to determine where the right loci of the true solution are between these 2 extrememodels. The intention of our paper (Houlie andMontagner, 2007) was to draw attention to this fundamental issue, which has important consequences for the understanding of volcano eruption dynamics. We suggest that the actual data are not sufficient to provide a definitive answer to this issue. Our work shows that an interpretation of the currently available seismic record in terms of translational displacements is perfectly valid and that further work will be needed to discriminate between tilt and displacement models and combinations of the two.

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