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Caractérisation des déplacements liés aux coulées de lave au Piton de la Fournaise à partir de données InSAR

Abstract : Since 2003, the observation national service for the volcanology (SNOV) OI2 have been analyzing the ground surface displacements due to the volcanic eruptions at Piton de la Fournaise using the Interferometry by Synthetic Aperture Radar (InSAR). One of major results is the observation of Eastern seaward flank sliding during the 2007 eruption (around 1.4 m of horizontal displacements). More recent results demonstrated that this sliding continued between 2009 and 2014 with a decreasing of displacement rates. However, this signal is combined with the displacements caused by the lava flows, (thermo-mechanical compaction, substratum flexure under the lava-flow weight), and the signal deconvolution is complex. Consequently, the thesis objective is to characterize these displacements to remove the contribution of lava flows from our observations. Therefore, we should only observe the signals related to the internal volcanic processes. To characterize the lava flows, we have to estimate their displacements, their locations, their surface areas and thicknesses. From the InSAR coherence data, the developped algorithms enable us to extract the lava-flow outlines and estimate the surface area value of the lava flow with the associated uncertainties. They use an iterative and probabilistical approach. These algorithms are now used during effusive crisis at Piton de la Fournaise and are become a product within OI2. The vertical-displacements evolutions of the October 2010 lava flow spanning from mid-2011 to mid-2015 follow an exponential decreasing form with rates of 10’s milimetre per year. Over the same period, the horizontal displacements are negligeable. The results about the displacement rates for lava flows emplaced between 2014 and 2018 show that these displacements are caused by the thermo-mechanical compaction, the substratum flexure, the slope and the lava field structures (as lava tunnels). One year after the emplacement of the lava flow, the vertical displacement rates can reach some 10’s centimetre per year and these values are similar for the horizontal displacements. Modelling of vertical displacements has been made using the lava thicknesses as input parameter. The preliminar results show that the flank sliding amplitude must be reduced of around 80%, (10 mm.yr-1 ) and its location is different from that of the post-2007 period. Finally, the displacements outside lava flows allow a characterization of the substratum rheology. This last one appears variable. Our models estimate that the poroelasticity, with a rheological constrast, (about 100 m-deep), is a possibility to represent the rheology of susbtratum. Ultimately, the precise substratum characterization from the co-eruptive displacements will enable us to better constrain the geometry of magmatic intrusion.
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Alexis Hrysiewicz. Caractérisation des déplacements liés aux coulées de lave au Piton de la Fournaise à partir de données InSAR. Sciences de la Terre. Université Clermont Auvergne, 2019. Français. ⟨NNT : 2019CLFAC062⟩. ⟨tel-02519025v1⟩

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