13:45 〜 15:15
[SVC33-P04] Intrusions and flank slip interactions at Piton de la Fournaise evidenced by InSAR monitoring and stress modeling
キーワード：Piton de la Fournaise, Ground deformation, Flank instability, InSAR, Inverse modeling, Stress field
Predicting the location of eruptive vents and the occurrence of flank slip events are major challenges for risk mitigation. Magma intrusions and flank slip events are interrelated hazards which are mainly controlled by the internal structure and the stress field of the edifice. Therefore, their forecasting relies on both the imaging of active structures and the monitoring of the stress field.
At the Piton de la Fournaise volcano (Réunion island, France), a major hazard is the propagation of intrusions beyond the limits of the caldera, which may feed lava flows in inhabited areas. Another hazard, indicated by the bathymetry, is the occurrence of large flank slip events potentially leading to earthquakes and flank collapse accompanied by tsunamis. In March-April 2007 the largest eruption of the last hundred years took place close to the caldera wall, and was associated to an aseismic slip of the eastern flank of up to 1.4 m towards the sea. Since then, InSAR and GNSS data show that slip is going on at a rate of 1.4 cm/yr. These events highlighted the necessity to anticipate magma propagation direction, eastern flank instability and their interactions. This work proposes to investigate these processes through inverse modeling of displacement and through stress modeling. 60 intrusive events that took place between 1998 and 2021 at the volcano are studied.
The inversions were carried out using InSAR and GNSS data combined with versatile inverse modeling based on 3D boundary element to determine the intrusion geometry of each event. The modelled intrusions reveal a major NE-SE intrusive zone, a sill zone, and four secondary intrusion zones radiating from the summit cone. The NE-SE major intrusive zone is connected at depth to the sill zone located below the eastern flank. Together they form a continuous intrusive zone characterized by a convex, seaward spoon shaped structure suggestive of a rotational landslide. This structure has sub-vertical walls at its head, accommodating the opening of the dykes. Towards the east, the intrusive structure becomes horizontal with sills affected by coeval normal displacement and seaward slip. Overall, the structure is characterized by a continuum of displacement from no slip, to sheared sills and finally pure slip.
In addition to this imaging of the main intrusion zones, stress modeling on the identified intrusion zones allowed to follow the dynamics of the intrusions and flank slip events and their interactions. We evidence that normal stress change inherited from past intrusions is not able to explain the 1998-2007 intrusive sequence. However, in the 2007-2021 sequence, the location of intrusions is consistent with decrease in normal stresses (unclamping) induced by previous intrusions. During this sequence, the stress evolution within the rift zones explains (1) the migration of activity from the summit to the distal areas, (2) the alternation between intrusions in the northern and the southern flanks, (3) the shift of the activity between the rift zones and the sill zone, (4) the occurrence of flank slip events.
We also document a positive feedback between intrusions in the vertical intrusion zones and slip events of the eastern flank. Therefore, the spoon-shaped structure could acts as a destabilization surface where the occurrence of catastrophic flank collapses is encouraged by both magma intrusions and flank slip events through unclamping and Coulomb stress increase at the base of the unstable sector.
At the Piton de la Fournaise volcano (Réunion island, France), a major hazard is the propagation of intrusions beyond the limits of the caldera, which may feed lava flows in inhabited areas. Another hazard, indicated by the bathymetry, is the occurrence of large flank slip events potentially leading to earthquakes and flank collapse accompanied by tsunamis. In March-April 2007 the largest eruption of the last hundred years took place close to the caldera wall, and was associated to an aseismic slip of the eastern flank of up to 1.4 m towards the sea. Since then, InSAR and GNSS data show that slip is going on at a rate of 1.4 cm/yr. These events highlighted the necessity to anticipate magma propagation direction, eastern flank instability and their interactions. This work proposes to investigate these processes through inverse modeling of displacement and through stress modeling. 60 intrusive events that took place between 1998 and 2021 at the volcano are studied.
The inversions were carried out using InSAR and GNSS data combined with versatile inverse modeling based on 3D boundary element to determine the intrusion geometry of each event. The modelled intrusions reveal a major NE-SE intrusive zone, a sill zone, and four secondary intrusion zones radiating from the summit cone. The NE-SE major intrusive zone is connected at depth to the sill zone located below the eastern flank. Together they form a continuous intrusive zone characterized by a convex, seaward spoon shaped structure suggestive of a rotational landslide. This structure has sub-vertical walls at its head, accommodating the opening of the dykes. Towards the east, the intrusive structure becomes horizontal with sills affected by coeval normal displacement and seaward slip. Overall, the structure is characterized by a continuum of displacement from no slip, to sheared sills and finally pure slip.
In addition to this imaging of the main intrusion zones, stress modeling on the identified intrusion zones allowed to follow the dynamics of the intrusions and flank slip events and their interactions. We evidence that normal stress change inherited from past intrusions is not able to explain the 1998-2007 intrusive sequence. However, in the 2007-2021 sequence, the location of intrusions is consistent with decrease in normal stresses (unclamping) induced by previous intrusions. During this sequence, the stress evolution within the rift zones explains (1) the migration of activity from the summit to the distal areas, (2) the alternation between intrusions in the northern and the southern flanks, (3) the shift of the activity between the rift zones and the sill zone, (4) the occurrence of flank slip events.
We also document a positive feedback between intrusions in the vertical intrusion zones and slip events of the eastern flank. Therefore, the spoon-shaped structure could acts as a destabilization surface where the occurrence of catastrophic flank collapses is encouraged by both magma intrusions and flank slip events through unclamping and Coulomb stress increase at the base of the unstable sector.