22 years of satellite imagery reveal a major destabilization structure at Piton de la Fournaise

Abstract

Volcanic activity can induce flank failure, sometimes generating large earthquakes and tsunamis. However, the failure structures have never been fully characterized and the failure mechanism is still debated. Magmatic activity is a possible trigger, either through fault slip, which might be induced by dyke intrusions, or through sill intrusions, which might be undergoing coeval normal displacements and slip. At the Piton de la Fournaise volcano, satellite imagery combined with inverse modeling highlights the pathways of 57 magmatic intrusions that took place between 1998 and 2020. We show that a major arcuate dyke intrusion zone is connected at depth to a sill intrusion zone, which becomes a fault zone towards the sea, forming a spoon-shaped structure. Some sills are 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. Repeated intrusions into this spoon-shaped structure could trigger catastrophic collapses.

Fig. 1. Cross-sections of conceptual models of flank destabilization triggered by magma intrusions.

a Dyking in vertical rift zones is coupled to a low-angle fault, inducing flank movements, as found at Kilauea^,, and Etna using geodetic data. b Sheared sills intruding a preexisting fault induce flank destabilization, as observed at Piton des Neiges from field studies^,. Adapted from Chaput et al..

Fig. 2. Location map.

a Location of the three volcanic edifices on Réunion island. The black rectangle indicates the area shown in (b). b Map and structural features of Piton de la Fournaise edifice. c Main rift zones as outlined in previous studies^,–. Rift Zone (RZ), East Rift Zone (ERZ), South-south-west volcanic alignment (SSW).

Fig. 3. Internal structure of Piton de la Fournaise.

Earthquake locations^,,,. Blue surfaces show the 2007 post-eruptive slip surface determined for two different sets of model parameters. Figure modified from Lénat et al..

Fig. 4. Intrusion pathways from inverse modeling.

a Model locations are shown in six different colors based on the intrusion zone to which they belong. The number of events in each intrusion zone is indicated in the legend. Bold lines show the eruptive fissures, using the same color code as for the intrusions. Contours of the co- and post-eruptive model of March–May 2007^, are shown for comparison by dashed and solid gray lines, respectively. For each intrusion zone, the poles of triangular elements of models within their 95% confidence interval are given in a stereographic projection on the lower hemisphere (see “Methods”). b Variation of the elevation with the dip of triangular elements belonging to each intrusion zone.

Fig. 5. 3D geometry of the main NE-SE preferential intrusion zone.

a Best-fit models for the 29 intrusions emplaced in the major NE-SE and sill intrusion zones. The colors show the opening of the modeled intrusions (normalized for clarity of the representation). Magenta vectors indicate displacement of the sheared sills. b Spread of models within their 95% confidence interval. Colorscale shows the normalized density of points for each model (point densities under 0.2 are not shown for visibility). For (a) and (b), thin straight lines on the map views delineate the width of the slice shown in the adjacent cross-sections. The thin black curve shown in the cross-sections highlights the surface determined by polynomial regression of the models mesh points.

Fig. 6. Dip distributions from field data and intrusion models in this study.

a Shallow intrusions (depths <1 km). Data correspond to volcanoes worldwide. b Deep intrusions (depths >1 km). Data correspond to Piton des Neiges. Field data are taken from Cayol et al.. They are compared to dips computed from triangular elements of intrusion models within their 95% confidence intervals. n is the number of field data or the number of triangular elements of source models.

Fig. 7. Main intrusive structure and mechanism of eastern flank displacement.

a Map view of the identified rift zones (RZ). Flank slip associated with sheared intrusions took place in January 2004, March–May 2007, October 2019, and September 2020. b Cross-section showing the main intrusive destabilization structure, color-coded to reflect the amount of slip. Main geological features from Fig. 3 are indicated for reference. Earthquake locations^,,, are marked using the same legend as in Fig. 3. The spoon-shaped geometry is from the polynomial regression shown in Fig 5. c Mean opening and slip from all inverted models computed with a moving mean of 200 m radius.