Unraveling the Piton de la Fournaise eastern flank structure by reconciling data from multiple geophysical methods

Long-term deformation often occurs in lava fields at volcanoes after flow emplacements. The investigation and interpretation of deformation in lava fields is one of the key factors for the assessment of volcanic hazards. As a typical Hawaiian volcano, Piton de la Fournaise volcano’s (La Réunion Island, France) main eruptive production is lava. Characteristics of the lava flows at Piton de la Fournaise, including the geometric parameters, location, and elevation, have been investigated by previous studies. However, no analysis focusing on the long-term post-emplacement deformation in its lava fields at a large spatial extent has yet been performed. One of the previous studies revealed that the post-emplacement lava subsidence played a role in the observed Eastern Flank motion by conducting a preliminary investigation. In this paper, an InSAR time series analysis is performed to characterize the long-term deformation in lava fields emplaced between 1998 and 2007 at Piton de la Fournaise, and to conduct an in-depth investigation over the influence of post-emplacement lava subsidence processes on the instability of the Eastern Flank. Results reveal an important regional difference in the subsidence behavior between the lava fields inside and outside of the Eastern Flank Area (EFA), which confirms that, in addition to the post-lava emplacement processes, other processes must have played a role in the observed subsidence in the EFA. The contribution of other processes is estimated to be up to ~78%. The spatial variation of the observed displacement in the EFA suggests that a set of active structures (like normal faults) could control a slip along a pre-existing structural discontinuity beneath the volcano flank. This study provides essential insights for the interpretation of the Eastern Flank motion of Piton de la Fournaise.

Piton de la Fournaise is one of the few volcanoes in the world to be monitored on a regular basis from space, by use of Interferometric Synthetic Aperture Radar (InSAR) data. Thirty of the 39 eruptions that occurred during the 1998–2010 period were imaged by various radar satellites. In this chapter, we present one of the best examples of surface deformation mapping achieved at Piton de la Fournaise volcano. From March 30 to May 1 2007, Piton de la Fournaise experienced a major eruptive crisis, characterized by the largest historical flank eruption (210 × 106 m3), and by a 340 m caldera collapse. The event was captured by InSAR data from the ENVISAT and ALOS-1 satellites. Large and unusual displacements of the Central Cone and the Eastern Flank of Piton de la Fournaise occurred during the co-eruptive period and in the following months. Analysis of the InSAR displacement combined with other geophysical and field observations allows us to propose conceptual models to explain the Central Cone and the Eastern Flank displacements. The April 2007 caldera collapse induced a sudden decompression of the hydrothermal system contributing to the strong centripetal subsidence of the Central Cone. This then decreased exponentially with poro-elastic compaction and creep of the Central Cone. For the Eastern Flank, the displacement is related to the combined effect of a sill and slip on a detachment. Dense olivine-rich magma from the deeper part of the main magma storage was injected as a dyke which may have encountered a pre-existing structural discontinuity, intruded it as a sill and activated it as a detachment surface. Magma then further propagated to the April 2 eruption site.

Recent advances in volcano seismology such as the measurement from seismic noise of subtle decreases of seismic velocities preceding volcanic eruptions (Brenguier et al. 2008; Duputel et al. 2009) or the precise localization of volcano-hydrothermal microseismic tremor sources (Taisne et al. 2011; Cros et al. 2011) have emerged from the analysis of long-term, continuous, and high-quality seismic data. There is thus an increasing need for high-resolution continuous seismic data in volcanic environments to improve our knowledge of fundamental volcanic processes such as deep magma transfers, volcanic unrest, magma transport, and eruptions. With 30 eruptions since 2000, Piton de la Fournaise volcano (La Reunion Island) has been among the most active volcanoes worldwide. This volcano emits basaltic lavas with volumes ranging from less than 1 million cubic meters to hundreds of millions of cubic meters. Piton de la Fournaise volcano has been monitored since 1979 from a permanent observatory that since 1999 has recorded continuous seismic and deformation data. Since 2009, we have deployed new broadband seismic and GPS stations under the framework of the UnderVolc research project. Most of the temporary stations are being converted to permanent stations, and the continuous seismic waveforms are publicly available by standard netdc requests. Within this paper, we will provide technical information about the deployed UnderVolc broadband seismic and GPS stations. We will also present data quality measurements from a noise spectral analysis using PQLX (McNamara and Buland 2004). We will show examples of seismic records during volcanic crisis and present novel results from the new seismic velocity monitoring method (Brenguier et al. 2008) using the new network. Finally, we will present some results of long-term displacement rates of the Piton de la Fournaise edifice flank using GPS data. Piton de la Fournaise volcano is a hot-spot shield volcano located on La …

<p>Geophysical measurements from the networks of instruments maintained by volcano observatories for several decades provide a large database that is rich in information concerning magma transport from deep storage zones to its shallow propagation before eruptions. In this study, we analyze multi-year time series of GNSS and seismic data acquired at Piton de la Fournaise (PdF) volcano (La Réunion, France) from 2014 up to now. These observations are sensitive to the dynamics of the magma within the volcanic system and their detailed study allows us to better apprehend its behavior both during pre-eruptive periods, by informing us about the preparation phases before an eruption and also during co-eruptive periods, by following the eruptions time-evolution and the corresponding dynamics.</p><p>We propose to scan continuously GNSS data by inverting them in time windows ranging from minutes to days using a point compound dislocation model (pCDM). This approach provides analytical expressions for surface displacements due to a complex source of deformation with variable geometry to model different shapes such as dikes, prolate ellipsoids, or pipes. As a result, we image a deep reservoir around 7-8 km below the PdF summit, as well as, in some cases, the upward magma migration dynamics in the crust over several days toward a shallow reservoir at sea level and the final dyke propagation over a few hours that ultimately feeds the eruptive site.</p><p>These observations are systematically compared to seismic data over the same time period and are jointly interpreted. We use both the seismicity catalog of "regular" volcano-tectonic events as well as the results of cross-correlations network-based methods obtained with the CovSeisNet package allowing the detection of “un-regular” signals and the location of their sources, such as micro-seismicity generated during dyke propagation, and long-period seismicity (tremor and LP events).</p><p>The joint use of information from geodetic and seismic networks constitutes an important step in improving our knowledge of volcanic systems. While the analysis of GNSS network data enables the imaging of active pressure-sources in the system with an estimation of the volumes of involved magma, the seismic network analysis allows for a more detailed view of the magma dynamics in the volcanic edifice.</p>

Long-term ground displacement observations using InSAR and GNSS at Piton de la Fournaise volcano between 2009 and 2014

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.

The aim of this work is to propose a general model of Piton de la Fournaise volcano using information from geological and geophysical studies. Firstly, we make a graphical compilation of all available geophysical information along a W–E profile. Secondly, we construct a geological section that integrates both the geophysical information and the geological information. The lithosphere beneath Piton de la Fournaise is not significantly flexed, and the crust is underlain by an underplating body, which might represent the deep magma reservoir for La Reunion volcanism. Piton de la Fournaise is a relatively thin volcano lying on a huge volcanic construction attributed mostly to Les Alizes volcano. Indeed, if the differentiated rocks observed at the bottom of the Riviere des Remparts are the top of Les Alizes volcano, the interface with Piton de La Fournaise may be located at about sea level beneath the summit area. The endogenous constructions (intrusive complexes) related to Les Alizes and Piton de la Fournaise volcanoes represent a large volume. The huge intrusive complex of Les Alizes volcano probably rests on the top of the oceanic crust and appears to have a buttressing effect for the present eastern volcano-tectonic activity of Piton de la Fournaise. The early Piton de la Fournaise edifice was built around a focus located beneath the Plaine des Sables area. The center subsequently moved 5–6 km eastward to its current location. The dense, high-velocity body beneath the Plaines des Sables and the western part of the Enclos probably corresponds to the hypovolcanic intrusive complex that developed before the volcanic center shifted to its present-day position. Magma reservoirs may have existed, and may still exist, as illustrated by the March 1998 crisis, at the mechanical and density interface between the oceanic crust and the Les Alizes edifice. Strong evidence also exists for the presence of a shallower magma reservoir located near sea level beneath the summit. The March 1998 pre-eruptive seismic pattern (location and upward migration) seems to be evidence for a transfer of magma between the two reservoirs. The dominant structural feature of the central zone is a collapse structure beneath the summit craters, above the inferred magma reservoir near sea level. The collapsed column constitutes a major mechanical heterogeneity and concentrates most of the seismic, intrusive, and hydrothermal activity because of its higher permeability and weaker mechanical strength.

Basaltic volcanoes are among the largest volcanic edifices on the Earth. These huge volcanoes exhibit rift zones and mobile flanks, revealing specific stress field conditions. In this paper, we present new deformation data issued from the Global Navigation Satellite Systems (GNSS) network installed on Piton de la Fournaise. Density of the GNSS stations allowed us to reach a sufficient resolution to perform a spatially significant analysis of strain at the scale of the active part of the volcano. Since 2007, summit inflation during preeruptive/eruptive sequences (summit extension/cone flanks contraction) alternates with summit deflation during posteruptive/rest periods (summit contraction/cone flanks extension) and generates a “pulsation” of the volcano. This volcano “pulsation” increases rock fracturing and damage, decreases the rock stiffness, and increases the medium permeability. The deformation regime of the mobile eastern flank evidences mass transfer in depth from the summit to the east. During the long‐term summit deflation recorded between 2011 and 2014, the upper eastern flank extended steadily eastward whereas the lower eastern flank contracted. Simultaneous extension and eastward displacement of the upper eastern flank and eastward contraction of the middle and lower eastern flank contributes to build the Grandes Pentes relief, steeping the topographic slope. We relate the eastern flank topographic slope spatial variations to rock or basal friction angle changes. The lower flank contraction process is an evidence of its progressive loading by the upper eastern flank, which brings this flank closer to an eventual instability.

We study the Piton de la Fournaise (PdF) volcano dynamics through the observation of continuous seismic velocity changes from 2000 to 2013. We compute the cross correlations of ambient seismic noise recorded at more than 30 short‐period and broadband stations of the UnderVolc temporary seismic experiment and of the PdF volcano observatory network. The velocity changes are estimated from the travel time delay measured on the cross correlations computed between pairs of stations. We average the relative velocity changes for all pairs of stations and obtain a time series of the velocity change of Piton de la Fournaise volcano over 13 years. From the period 0.5 to 4 s, the depth sensitivity of the velocity change is ranging from approximately 100 m to 2500 m. A slow decrease of velocity is measured from 2000 and ends with a major eruption that occurred in April 2007. This eruptive episode is followed by an increase of the velocity. These long‐term changes are compared to the deformation of the Piton de la Fournaise edifice estimated from geodetic measurements. An analysis of baseline change between GPS stations indicates an inflation of the volcanic edifice prior to April 2007 followed by a deflation since then. This deflation predominantly affects the terminal cone. Seismic velocity changes and deformation have similar long‐term trends with velocity decrease observed during inflation and velocity increase during deflation. However, the velocity change magnitude is about 2 orders of magnitude greater than the deformation. This suggests nonlinear relation between velocity changes and deformation.

In volcanic domains, magma transport and pressure build-up induce high stress-strain perturbations in the surrounding volcanic edifice that may lead to volcanic flank movements and possible instability. In this study, we focus on the 2007 March-April episode of volcanic activity at Piton de la Fournaise (PdF) Volcano, La R'eunion Island. This episode was associated with a large volume of emitted lava (240 × 106 m3) and a 340-m caldera collapse. We present observations of continuous seismic velocity changes measured using cross-correlations of ambient seismic noise over 10 yr at PdF. Overall, we observe a large velocity reduction starting a few days prior to the major 2007 April 2 eruption. Comparison of seismic velocity change measurements with observations of deformation from InSAR and GPS shows that the seismic velocity reduction coincided with a widespread flank movement starting at the time of injection of magma to feed an initial eruption, a few days before the 2007 April 2 eruption. We emphasize the potential of noise-based seismic velocity change measurements, together with geodetic observations, to detect and monitor possibly hazardous slope instabilities.

Magma transport and storage at Piton de La Fournaise (La Réunion) between 1972 and 2007: A review of geophysical and geochemical data

Structure of piton de la fournaise volcano (La Reunion island, Indian ocean) from magnetic investigations. An illustration of the analysis of magnetic data in a volcanic area

Rift zones and magma plumbing system of Piton de la Fournaise volcano: How do they differ from Hawaii and Etna?

Explosive activity of the summit cone of Piton de la Fournaise volcano (La Réunion island): A historical and geological review

Eruptive history of the Piton de la Fournaise volcano, Reunion Island, Indian Ocean