Magma Propagation at Piton de la Fournaise From Joint Inversion of InSAR and GNSS

Abstract

Magma propagation is an unsteady process controlled by magma‐crust interaction. To provide information on its dynamics, we invert complementary ground deformation data spanning the 8 hr preceding the 26 May 2016 eruption at Piton de la Fournaise (PdF) volcano (La Réunion, France). Data are inverted using 3‐D boundary element models combined with a Monte Carlo inversion method. The final geometry of the displacement source is determined based on four interferograms spanning the whole propagation phase while the dynamics of the propagation is inferred from temporal inversion of continuous Global Navigation Satellite System (GNSS) data, using the final geometry as an a priori to constrain the source. The best modeled magma path consists in a 2,700‐m‐long sill located 800 m above sea level and connected to the eruptive fissure by a subvertical dike. The quick opening of the horizontal part of the intrusion could have been favored by limited flank sliding during the early stage of propagation. The intrusion then stalled for ∼5 hr, while pressure increased slightly, until final upward propagation and eruption. Volume budget suggests that the eruption was fed by a single batch of magma quickly disconnected from its source. The delay prior to the eruption may reflect a limited magma supply. Finally, two mechanisms, potentially acting together, might have favored the eruption: a driving role of magmatic gas and/or, as often observed at Piton de la Fournaise, an eastward flank slip.

Combining InSAR and GNSS to Track Magma Transport at Basaltic Volcanoes

“…Since then 16 eruptions have occurred on the flanks of the cone, inside the caldera-shaped depression including the May 2016 eruption studied here. After 10 days of minor inflation and a low seismicity rate, a seismic crisis started on May 25 at 19:40 (all times are U.T.C) accompanied by rapid deformation, which corresponded to magma transfer towards the surface [16]. At 04:05 on May 26, a fissure opened, 2800 m to the southeast of the summit cone, at an elevation of 1890 m producing lava fountaining and a 0.5 Mm 3 lava flow, which is an unusually small volume for a Piton de la Fournaise eruption [8].…”

“…At 04:05 on May 26, a fissure opened, 2800 m to the southeast of the summit cone, at an elevation of 1890 m producing lava fountaining and a 0.5 Mm 3 lava flow, which is an unusually small volume for a Piton de la Fournaise eruption [8]. The eruption stopped after 27 h. InSAR acquisitions and GNSS measurements provide an extensive dataset which can be used to understand the magma transfer during this eruption [16].…”

“…The main sources of error in InSAR and GNSS data derive from atmospheric conditions (water content of the troposphere and ionosphere) and models are simplifications of a complex reality: the number of sources is unknown and their shapes are probably more complex than assumed. A previous study of the 26 May 2016 eruption at Piton de la Fournaise [16] showed that, despite the very dense network of permanent GNSS stations present on this volcano, magma propagation could only be successfully tracked when adding a priori information on the intrusion shape provided by the inversion of co-eruptive interferograms. This conclusion was reached using the intermediate Sentinel-1 acquisition, which allowed different strategies to invert GNSS data to be quantitatively evaluated.…”

“…This phase, which is crucial to understanding the dynamics of magma emplacement, is usually recorded by continuous, ground-based sensors such as GNSS stations, tiltmeters or seismometers for example, References [9,[13][14][15]]. It is not always possible to place ground-based captors in ideal locations with respect to the intrusion due to field conditions [16]. Moreover, seismicity does not always permit to track magma propagation, specifically when microearthquakes are too shallow and have a too small magnitude [17].…”

“…This requires as many parameters as there are dislocations, which leads to statistically less likely solutions [30] and solutions physically less meaningful than pressure sources [31]. In our previous study [16], we addressed these issues by determining the pressurized area corresponding to the active intrusion, where the a priori intrusion was derived from InSAR and a circular pressure source was projected onto the mesh representing this intrusion. This method had two drawbacks: 4 parameters were needed when the number of permanent GNSS measurements is 30 and a projection from a plane on the intrusion geometry was needed, which could exclude part of the intrusion and thus induce errors if the intrusion geometry was curved.…”