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The quantification of the effects of external forcings such as seismicity and rainfall on slope destabilization is an open and important question. To investigate the role of these forcings, we analyze an unprecedented 10-year long catalog of the rockfalls occurring in the Piton de la Fournaise volcano crater. Indeed, the dense seismic network operated by the Piton de la Fournaise Volcano Observatory (La Réunion Island) makes it possible to precisely locate the rockfalls and recover the volume of each event. We use statistical tools originally developed for earthquakes to study the spatio-temporal evolution of the rockfall activity and to unravel the impact of the external forcings. Our results highlight the predominant effect of low amplitude seismicity on the slope destabilization, via a progressive damaging of the slopes. Moreover, we show that the efficiency and the time delay of this dynamic triggering depends on the stability state of the slopes, i.e. the distance to failure. To better understand our observations, we compare them with laboratory experiments on granular avalanches triggered by ultrasound.

Piton de la Fournaise is a shield volcano located on La Réunion island in the Indian Ocean and most recently tends to erupt once - twice annually. Besides volcanic tremor during eruptions and rockfall, long-period (LP) and volcano-tectonic (VT) earthquakes are dominating signals on the island.In October 2022, a rotational sensor and an array of seven seismometers were installed within the Enclos Fouqué, the youngest caldera of volcano Piton de la Fournaise. We record volcano-seismic signals that were also detected by the seismic network of the Observatoire Volcanologique du Piton de la Fournaise (OVPF). Our aim is to test the performance of the rotational sensor and the conventional seismic array with respect to these events.Local VT and rockfall events have been detected on all instruments and could be compared by calculations of backazimuth (BAZ) and signal-to-noise ratio (SNR). We derive the rotational rate using three array stations for array derived rotation (ADR). First results indicate an agreement between the BAZ obtained from the rotational sensor, the array and the location using the OVPF network for strong rockfall events. Summit VT and weak local earthquakes could furthermore be located by the array BAZ. Preliminary SNR results from all considered events indicate higher values for the array stations. Since the instruments could not be buried on site and the rotational sensor is likely affected by wind noise, it is assumed that comparison between the instruments will work best for strong and/ or close events.

SUMMARYThe brutal onset of seismicity offshore Mayotte island North of the Mozambique Channel, Indian Ocean, that occurred in May 2018 caught the population, authorities and scientific community off guard. Around 20 potentially felt earthquakes were recorded in the first 5 d, up to magnitude Mw 5.9. The scientific community had little pre-existing knowledge of the seismic activity in the region due to poor seismic network coverage. During 2018 and 2019, the MAYOBS/REVOSIMA seismology group was progressively built between four French research institutions to improve instrumentation and data sets to monitor what we know now as an on-going exceptional submarine basaltic eruption. After the addition of 3 medium-band stations on Mayotte island and 1 on Grande Glorieuse island in early 2019, the data recovered from the Ocean Bottom Seismometers were regularly processed by the group to improve the location of the earthquakes detected daily by the land network. We first built a new local 1-D velocity model and established specific data processing procedures. The local 1.66 low VP/VS ratio we estimated is compatible with a volcanic island context. We manually picked about 125 000 P and S phases on land and sea bottom stations to locate more than 5000 events between February 2019 and May 2020. The earthquakes outline two separate seismic clusters offshore that we named Proximal and Distal. The Proximal cluster, located 10 km offshore Mayotte eastern coastlines, is 20–50 km deep and has a cylindrical shape. The Distal cluster start 5 km to the east of the Proximal cluster and extends below Mayotte's new volcanic edifice, from 50 to 25 km depth. The two clusters appear seismically separated, however our data set is insufficient to firmly demonstrate this.

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.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.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).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.

The island of La Réunion is made up of two main shield volcanoes: Piton des Neiges and Piton de la Fournaise. A wide variety of seismic events is observed, including but not limited to: tectonic events, volcano-tectonic earthquakes, volcanic tremors, long-period and very-long-period earthquakes. The seismicity of La Réunion island illuminates different levels of its volcanic plumbing system. In the shallowest part of Piton de la Fournaise, earthquakes are clustered below the volcano summit area and often occur as intense seismic swarms during magma intrusions. Seismic activity is also observed below the East flank of Piton de la Fournaise, shaping an eastward dipping structure possibly accommodating seaward displacements of the volcano. At larger depths, episodes of upward seismicity migrations indicate magmatic recharges of the shallow reservoir from deeper parts of the volcano plumbing system. Earthquakes are also observed below the oceanic crust in a zone of underplating between Piton des Neiges and Piton de la Fournaise. A major event at Piton de la Fournaise was the collapse of its summit caldera in April 2007. This activity resulting in a summital depression of 330 m was accompanied by very-long-period earthquakes with magnitudes ranging from MW = 4.4 to MW = 5.4. Another noticeable activity in La Réunion island is an active seismic swarm located within the oceanic crust under the North flank of Piton des Neiges. The origin of this sustained seismic activity is still unclear and is currently investigated, thanks to the deployments of additional stations in the region., Comptes Rendus Geoscience, 353 (S1), ISSN:1778-7025, ISSN:1631-0713

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 8hr preceding the 26 May 2016 eruption at Piton de la Fournaise (PdF) volcano (La Reunion, 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 800m 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 approximate to 5hr, 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. Plain Language Summary Basaltic magma stored beneath volcanoes reaches the surface by fracturing the Earth's crust. As experienced in May 2018 at Kilauea volcano (Hawaii, USA), magma can travel kilometers from the reservoir and fissure opening may threaten man-made structures. Anticipating where and when eruptive fissures open requires better understanding of the factors controlling magma propagation. During the 26 May 2016 eruption of Piton de la Fournaise, Reunion Island,the preeruptive crisis spanned 8hr25min from the first signal recorded by the observatory to the eruption onset.We determine the magma paths and propagation timing, which led to this eruption using complementary satellite data of ground surface displacement, combining radar interferometry, which provides high spatial resolution, with GPS, which provides high temporal resolution. We highlight complex magma propagation within the subareal volcano, showing two direction changes, an arrest and an acceleration. Flank slip and magma degassing seem to play a key role in controlling both the geometry and the timing. Based on this scenario, this event was close to turn into a failed eruption as there was a 5-hr pause in propagation before magma finally reached the surface. Understanding such unusual eruptions is a challenge for observatories as it may lead to repeated false alerts.

Understanding the interplay between pressure evolution in magma bodies in volcano roots and caldera collapses is important to for the general understanding of volcanoes and how calderas form. We use lessons-learned regarding caldera collapse dynamics and inferred 2014-2015 pressure evolution in a magma body under the Bardarbunga caldera, Iceland, to revisit the dynamics of the 2007 caldera collapse at Piton de La Fournaise volcano, La Reunion, in a project supported by EUROVOLC trans-national access. At Piton de la Fournaise, (rising to 2632 m above sea leve) a shallow and small magma body (close to sea-level; volume less than one cubic kilometer) played a central role. The overpressure compared to lithostatic prior to collapse is inferred to have been small (< 5 MPa), based on models of driving pressure for minor eruptions that occurred on 18-19 February and 30 March prior to the caldera forming lateral flank eruption that occurred 2 April – 1 May, 2007. The site of the lateral flank eruption occurred at an elevation of 500 m, much lower than the summit. This elevation difference is inferred to a key factor for creating high driving pressure for magma flow. We infer that rapid flow of magma led to fast drop in pressure in a shallow magma body under the caldera, triggering inflow of magma from a deeper magma body under Piton de la Fournaise, that was in important element of the 2007 eruptive activity. This deep inflow did, however, not sustain enough the pressure in the shallow magma body during the eruption, causing development of significant under-pressure leading to the collapse.

Building on previous work aimed at identifying and characterizing the potential mechanical trigger controlling eruptions and destabilization at Piton de la Fournaise, we study the mechanical behavior of the volcanic edifice on a crustal scale. Do the recurrent earthquake pattern correspond to a destabilization structure, precursor of a large-scale flank sliding? Or instead to a reactivated area of magma storage (partially crystallized “sill”)? To answer these questions, we design numerical models which estimate the stress field associated with the volcanic complex. We use the ADELI finite element method in three dimensions, which handles elasto-visco-plastic rheologies. In these models, we take into account 1) the topographic load, 2) the major density and resistance heterogeneities within the volcano obtained from previous studies, and 3) the overpressure induced by the intrusion of a dike of arbitrary geometry.The modeled dike injection generates deformation and stress fields such that their isocontours highlight an ellipsoidal cup structure extending from the central cone to a depth close to 0 and reaching the ends of the eastern flank. This zone could be assimilated to the zone of seismicity observed and described previously. Together with several systematic test cases, we will discuss the significance of these results, such as whether it reveals a rheological delimitation zone of the hydrothermalized bedrock, resulting from the combined influence of the topographic load and that of a magmatic injection.

International audience; We apply three different methods based on the analysis of the multi-component seismic data to detect seismovolcanic tremors and other seismovolcanic signals, to propose an approach to classify them and to locate their sources. We use continuous seismograms recorded during one year by 21 stations at the Piton de la Fournaise volcano (La R ́eunion, France). The first method allows to detect seismovolcanic signals based on stability in time of the inter-components cross-correlations function. Two other methods based on the simultaneous analysis of the whole network can be used to detect seismovolcanic signals and to locate their sources. In the first network-based method, the seismic wave-field is analyzed by calculating the width of the network covariance matrix eigenvalue distribution. The second network-based method consists in performing the 3D back-projection of the inter-stations cross-correlations in order to calculate the network response function. Simultaneous analysis of the parameters measured by the three different methodscan be used to classify different types of seismovolcanic tremors. Our results demonstrate that all three methods efficiently detect seismovolcanic tremors accompanying the 2010 eruptions and the preceding pre-eruptive seismic swarms. Furthermore, methods 2 and 3 based on simultaneous analysis of the whole network detect a large number of volcanic earthquakes. Our location results show that each seismovolcanic tremor is located in a distinct region of the volcano, close to the eruptive site at a shallow depth and the pre- ceding seismic crisis is located deeper at about the sea level under the summit crater.

International audience; Present-day volcano imaging and monitoring relies primarily on ground surface and satellite remote sensing observations. The overall understanding of the volcanic edifice and its dynamics is thus limited by surface investigation, spatial resolution and penetration depth of the ground methods, but also by human and material resources, and harsh environments. Here, we show for the first time that an airborne electromagnetic survey provides a 3D global resistivity model of an active volcano. The high-Present-day volcano imaging and monitoring relies primarily on ground surface and satellite remote sensing observations. The overall understanding of the volcanic edifice and its dynamics is thus limited by surface investigation, spatial resolution and penetration depth of the ground methods, but also by human and material resources, and harsh environments. Here, we show for the first time that an airborne electromagnetic survey provides a 3D global resistivity model of an active volcano. The high-resolution survey acquired at the Piton de la Fournaise volcano on La Réunion Island, Indian Ocean, shows unprecedented details of the internal structure of the edifice, highlighting the upwelling hydrothermal system below the craters, magma intrusion pathways and inherited faults. Together with surface monitoring, such airborne imagery have a high potential to better characterize volcano internal structure and magmatic processes, and therefore to better anticipate catastrophic events such as phreato-magmatic eruptions or volcano destabilizations.survey acquired at the Piton de la Fournaise volcano on La Réunion Island, Indian Ocean, shows unprecedented details of the internal structure of the edifice, highlighting the upwelling hydrothermal system below the craters, magma intrusion pathways and inherited faults. Together with surface monitoring, such airborne imagery have a high potential to better characterize volcano internal structure and magmatic processes, and therefore to better anticipate catastrophic events such as phreato-magmatic eruptions or volcano destabilizations. Structures of active volcanoes result from many successive constructive and destructive processes that are interconnected 1-3. Understanding their internal structure is a priority issue to monitor volcanic activity and anticipate catastrophic events such as phreatic to phreato-magmatic explosions, edifice collapses or flank destabilizations. Volcano edifices grow from both exogeneous and endogeneous processes through accumulation of lava flows, explosive deposits and magmatic intrusions. Successive magma intrusions and magma storages at depth generate accumulated deformations and mechanical stresses that weaken the volcano edifice stability, together with structures inherited from previous destructive events 1,4. Water circulation also contributes to the overall complexity and instability of volcanoes. Heating of the water table by magma injections and accumulations at depth generates extensive hydrothermal systems underneath active volcanoes 5. The induced circulation of acid hydrothermal fluids leads to intense leaching and alteration 6. All these internal processes promote surface rupture 7 and significant edifice spreading 8 that can cause catastrophic flank destabilization 9. Imaging the interior of active volcanoes is thus fundamental to determine their internal structure, the dynamic of their magmatic and hydrothermal systems , their potential points of failure and their associated volcano hazards such as phreatic/phreato-magmatic eruptions 10 , large-scale deformation 11,12 and flank instabilities 2,13. For decades, geophysical techniques have been developed and applied to study both the internal structure and the dynamics of active volcanoes. Among others, ground resistivity imagery techniques, such as 2D or 3D electrical resistivity tomography (ERT) 10,14,15 , magnetotellurics (MT) and transient electromagnetic soundings (TDEM) 16,17 , are renowned for being well-suited and reliable in such settings. While ERT oftenly provides high resolution imagery of the first hundred meters 18,19 , MT and TDEM provide a smoother image of the bulk resis-tivity up to few kilometres depth 20. All these methods have been improved during the last decades to image

Detecting renewal of volcanic activity is a challenging task and even more difficult in tropical settings. Continuous measurements of soil CO2 flux were carried out at the Piton de la Fournaise volcano during 2013-2016. Since this site is in the tropics, periods of heavy rainfall are in the norm. Measurements covered volcanic unrest after a hiatus of 3.5 years. We find that, while temperature has the strongest effect, extreme rainfall causes short-term noise. When corrected and filtered from the environmental influence soil CO2 time series permit to detect a major deep magmatic event during March-April 2014, three months before the first eruption of the new activity phase. Correlation with geophysical datasets allow timing of further stages of upward fluid ascent. Our study validates soil CO2 flux monitoring in tropical environments as a valuable tool to monitor magma transfer and to enhance understanding of volcano unrest down to the lithospheric mantle.

Monitoring the endogenous seismicity of volcanoes helps to forecast eruptions and prevent their related risks, and also provides critical information on the eruptive processes. Due the high number of events recorded during pre-eruptive periods by the seismic monitoring networks, cataloging each event can be complex and time-consuming if done by human operators. Automatic seismic signal processing methods are thus essential to build consistent catalogs based on objective criteria. We evaluated the performance of the “Random Forests” (RF) machine-learning algorithm for classifying seismic signals recorded at the Piton de la Fournaise volcano, La Reunion Island (France). We focused on the discrimination of the dominant event types (rockfalls and volcano-tectonic earthquakes) using over 19,000 events covering two time periods: 2009–2011 and 2014–2015. We parametrized the seismic signals using 60 attributes that were then given to RF algorithm. When the RF classifier was given enough training samples, its sensitivity (rate of good identification) exceeded 99%, and its performance remained high (above 90%) even with few training samples. The sensitivity collapsed when using an RF classifier trained with data from 2009 to 2011 to classify data from 2014 to 2015 catalog, because the physical characteristics of the rockfalls and hence their seismic signals had evolved between the two time-periods. The main attribute families (waveform, spectrum, spectrogram or polarization) were all found to be useful for event discrimination. Our work validates the performance of the RF algorithm and suggests it could be implemented at other volcanic observatories to perform automatic, near real-time, classification of seismic events.

International audience; early detection of the onset of a caldera collapse can provide crucial information to understand their formation and thus to minimize risks for the nearby population and visitors. Here, we analyse the 2007 caldera collapse of piton de la Fournaise on La Réunion Island recorded by a broadband seismic station. We show that this instrument recorded ultra-long period (ULP) signals with frequencies in the range (0.003-0.01 Hz) accompanied by very-long period (VLP) signals (between 0.02 and 0.50 Hz) prior to and during the caldera formation suggesting it is possible to detect the beginning of the collapse at depth and anticipate its surface formation. Interestingly, VLP wave packets with a similar duration of 20 s are identified prior to and during the caldera formation. We propose that these events could result from repeating piston-like successive collapses occurring through a ring-fault structure surrounding a magma reservoir from the following arguments: the source mechanism from the main collapse, the observations of slow source processes as well as observations from the field and the characteristic ring-fault seismicity. Caldera collapses are rare (only seven events over the last 100 years) and particularly destructive volcanic events that can induce catastrophic changes in the shape of a volcanic edifice and its environment 1. Identifying the occurrence of the first collapse at depth is of major importance in evaluating the triggering factors and anticipating the caldera surface formation. This detection can help to predict or at least indicate early future caldera collapses and subsequent consequences such as explosive eruptions, (e.g. those that followed the major Kīlauea Caldera collapse in 1470-1510 2), or atmospheric impacts 3. In the past two decades only four caldera collapses have been monitored by dense geophysical networks: chronologically, the 2000 Miyake-jima, Japan; the 2007 Piton de la Fournaise, La Réunion/France; the 2014-2015 Bárðarbunga, Iceland; and the 2018 Kīlauea, Hawai'i 4-7. Laboratory experiments and numerical analyses predicted the occurrence of precursory collapses at depth before the onset of the surface subsidence 8-12 but observations obtained from adequate broadband seismometers were lacking. Until the present study, the Miyake-jima caldera formation was the only known case with evidence of such deep collapses before the faults reached the surface. The deep collapses were suggested from observations of VLP seismic pulses of 20 s width 13. However, the timing of the first collapse at depth was not reported. VLP signals observed at volcanoes are generally considered having frequencies between 0.01-0.5 Hz (ref. 14) and until the 2000 Miyake-jima event, they were generally considered to result from inertial forces associated with changes in the flow of magma and gases through conduits 14. The VLP signals detected during the Miyake-jima caldera formation were explained by different physical mechanisms: (i) a buried geyser model 15 , (ii) a piston-like model 4,13,16 , and (iii) ring-faulting mechanisms related to shear failure on curved or cone-shaped fault structures 17,18. 46 step-like tilt changes (TC) were observed during the Miyake-jima caldera formation and among them 39 were accompanied by the VLP seismic signals 4,19,20. The origin of these TC associated with the VLP pulses have been attributed to either a piston model with a vertical rock column intermittently sinking into a magma reservoir 4,21 or to a magma sheet model with a large sill-like

International audience; We have analyzed the impact of different forcings, such as rain and seismicity, on slope instabilites on an active volcano. For this, we compiled a catalog of the locations and volumes of rockfalls in the Piton de la Fournaise crater using seismic records. We validated it by comparing the locations and volumes to those deduced from photogrammetric data. We analyzed 10,477 rockfalls, spanning the period 2014 to 2016. This period corresponds to the renewal of volcanic activity after a 41-month rest. Our analysis reveals that renewed eruptive activity has unsettled the crater edges. External forcings such as rain and seismicity are shown to potentially increase the number and the volume of rockfalls, with a stronger impact on the volume. Preeruptive seismicity seems to be the main triggering factor for the largest volumes, with a delay of one to several days. Rain alone does not seem to trigger especially large rockfalls. We infer that repetitive vibrations from the many seismic events, combined with the action of rain, induce crack (or slip) growth in highly fractured (or granular) materials, leading to the collapse of large volumes. Regarding their spatial distribution before an eruption, the largest rockfalls seem to migrate toward the location of magma extrusion.

Seismic ambient noise cross correlation is increasingly used to monitor volcanic activity. However, this method is usually limited to volcanoes equipped with large and dense networks of broadband stations. The single-station approach may provide a powerful and reliable alternative to the classical “cross-station” approach when measuring variation of seismic velocities. We implemented it on the Piton de la Fournaise in Reunion Island, a very active volcano with a remarkable multidisciplinary continuous monitoring. Over the past decade, this volcano has been increasingly studied using the traditional cross-correlation technique and therefore represents a unique laboratory to validate our approach. Our results, tested on stations located up to 3.5 km from the eruptive site, performed as well as the classical approach to detect the volcanic eruption in the 1–2 Hz frequency band. This opens new perspectives to successfully forecast volcanic activity at volcanoes equipped with a single three-component seismometer.

International audience; Identifying the onset of volcano unrest and providing an unequivocal identification of volcano reawakening remain challenging problems in volcanology. At Piton de la Fournaise, renewal of eruptive activity in 2014–2015, after 41 months of quiescence and deflation, was associated with long-term continuous edifice inflation measured by GNSS. Inflation started on June 9, 2014, and its rate progressively increased through 2015. Inflation onset was rapidly followed by an eruption on June 20–21, 2014, showing that volcano reactivation can be extremely fast, even after long non-eruptive phases. This short-lived eruption involved a shallow source (1.3–1.9 km depth below the summit). The inflation that followed, and eruptions in 2015, involved a larger depth range of fluid accumulation, constrained by inverse modeling at ca. 3.9 to 1.2–1.7 km depth. This time evolution reveals that volcano reawakening was associated with continuous pressurization of the shallowest parts of its plumbing system, triggered by progressive upwards transfer of magma from greater depth. A deep magma pulse occurred in mid-April 2015 and was associated with deep seismicity (3 to 9.5 km depth) and CO2 enrichment in fluids emitted by summit fumaroles. From this date, ground deformation accelerated and the output rates of eruptions increased, culminating in the long-lasting, large-volume, August–October eruption (~ 36 Mm3). This evolution suggests that deep magma/fluid transfer through an open conduit system first provoked the expulsion of the top of the plumbing system in June 2014, and then induced the progressive vertical transfer of the entire plumbing system down to 9 km (four eruptions in 2015). The new sustained feeding of the volcano was also at the origin of the hydrothermal system perturbation and the acceleration of the eastern flank motion, which favor lateral dike propagation and the occurrence of frequent and increasingly large eruptions. Our results highlight the fast and progressive way in which basaltic magmatic systems can wake up.

We apply a template matching method to detect and locate preeruptive earthquakes at Piton de la Fournaise volcano in 2014 and 2015. This approach enabled the detection of many events and unveiled persistent seismicity features through multiple eruptions. Shallow earthquakes define a ring-shaped structure beneath the main crater. The repetitive occurrence of events along this structure suggests that it corresponds to a preexisting zone of weakness within the edifice. We also show evidence of deep magma transfer in 2015. More than 5000 deep earthquakes define an upward migration immediately followed by the occurrence of shallow events leading to an eruption 20 days later. This suggests the creation of a hydraulic connection between the lower part of the volcanic system and a magma reservoir located near sea level. We can envisage than such replenishments of the shallow reservoir occurred in the past but were undetected because of limited deep earthquake detections.

International audience; Body wave reconstruction from ambient seismic noise correlations is an important step toward improving volcano imaging and monitoring. Here we extract body and surface waves that propagate in Piton de la Fournaise volcano on La Réunion island using ambient noise cross correlation and array-processing techniques. Ambient noise was continuously recorded at three dense arrays, each comprising 49 geophones. To identify and enhance the Green's function from the ambient noise correlation, we apply a double beamforming (DBF) technique between the array pairs. The DBF allows us to separate surface and body waves, direct and reflected waves, and multipathing waves. Based on their azimuths and slownesses, we successfully extract body waves between all the combinations of arrays, including the wave that propagates through the active magmatic system of the volcano. Additionally, we identify the effects of uneven noise source distribution and interpret the surface wave reflections.

International audience; Piton de la Fournaise (PdF) is recognized as one of the world's most active volcanoes in terms of eruptive frequency and the substantial quantity of lava produced. Yet with the sole exception of rather modest intracrateric fumarole activity, this seems to be in contrast with an apparent absence of any type of natural fluid emission during periods of quiescence. Measurement campaigns were undertaken during a long-lasting quiescent period (2012–2014) and just after a short-lived summit eruption (June 2014) in order to identify potential degassing areas in relation to the main structural features of the volcano (e.g., rift zones) with the aim of developing a broader understanding of the geometry of the plumbing and degass-ing system. In order to assess the possible existence of anomalous soil CO2 flux, 513 measurements were taken along transects roughly orthogonal to the known tectonic lineaments crossing PdF edifice. In addition , 53 samples of gas for C isotope analysis were taken at measurement points that showed a relatively high CO2 concentration in the soil. CO2 flux values range from 10 to 1300 g m^-2 d^-1 while δ13 C are between -26.6 and -8‰. The results of our investigation clearly indicate that there is a strong spatial correlation between the anomalous high values of diffusive soil emissions and the main rift zones cutting the PdF massif and, moreover, that generally high soil CO2 fluxes show a δ13 C signature clearly related to a magmatic origin.

International audience; Many active European volcanoes and volcano observatories are island-based and located far from their administrative “mainland”. Consequently, Governments have developed multisite approaches, in which monitoring is performed by a network of individuals distributed across several national research centers. At a transnational level, multinational networks are also progressively emerging. Piton de la Fournaise (La Réunion Island, France) is one such example. Piton de la Fournaise is one of the most active volcanoes of the World, and is located at the greatest distance from its “mainland” than any other vulnerable “overseas” site, the observatory being 9365 km from its governing body in Paris. Effusive risk is high, so that a well-coordinated and rapid response involving near-real time delivery of trusted, validated and operational product for hazard assessment is critical. Here we review how near-real time assessments of lava flow propagation were developed using rapid provision, and update, of key source terms through a dynamic and open integration of near-real time remote sensing, modeling and measurement capabilities on both the national and international level. The multi-national system evolved during the five effusive crises of 2014–2015, and is now mature for Piton de la Fournaise. This review allows us to identify strong and weak points in an extended observatory system, and demonstrates that enhanced multi-national integration can have fundamental implications in scientific hazard assessment and response during an on-going effusive crisis

International audience; On ocean basaltic volcanoes, magma transfer to the surface proceeds by subvertical ascent from the mantle lithosphere through the oceanic crust and the volcanic edifice, possibly followed by lateral propagation along rift zones. We use a 19-year-long database of volcano-tectonic seismic events together with detailed mapping of the cinder cones and eruptive fissures to determine the geometry and the dynamics of the magma paths intersecting the edifice of Piton de la Fournaise volcano. We show that the overall plumbing system, from about 30 km depth to the surface, is composed of two structural levels that feed distinct types of rift zones. The deep plumbing system is rooted between Piton des Neiges and Piton de la Fournaise volcanoes and has a N30–40 orientation. Above 20 km below sea level (bsl), the main axis switches to a N120 orientation, which permits magma transfer from the lithospheric mantle to the base of the oceanic crust, below the summit of Piton de la Fournaise. The related NW-SE rift zone is 15 km wide, linear, spotted by small to large pyroclastic cones and related lava flows and emits slightly alkaline magmas resulting from high-pressure fractionation of clinopyroxene ± olivine. This rift zone has low magma production rate of ~ 0.5–3.6 × 10 −3 m 3 s −1 and an eruption periodicity of around 200 years over the last 30 ka. Seismic data suggest that the long-lasting activity of this rift zone result from regional NNE-SSW extension, which reactivates inherited lithospheric faults by the effect of underplating and/or thermal erosion of the mantle lithosphere. The shallow plumbing system (b11 km bsl) connects the base of the crust with the Central Cone. It is separated from the deep plumbing system by a relatively large aseismic zone between 8 and 11 km bsl, which may represent a deep storage level of magma. The shallow plumbing system feeds frequent, short-lived summit and flank (NE and SE flanks) eruptions along summit and outer rift zones, respectively. Summit rift zones are very active (~0.1–0.25 m 3 s −1), short (2–3 km), and present an orthogonal pattern confined to the central active cone of Piton de la Fournaise. Outer NE and SE rift zones are much less active (~4–7.3 × 10 −3 m 3 s −1) and extend from inside the Enclos Fouqué caldera to bound the mobile eastern volcano flank. We show that the outer rift zones are almost aseismic and are genetically linked to the seaward flank displacements, whose most recent events where detected in 2004 and 2007. East flank sliding is itself triggered by shallow (b 2 km depth) sill intrusions. We propose that the subvertical magma intrusions along the perpendicular summit rift zones, sill intrusions, and subsequent magma injections along the outer rift zones are controlled by cycles of stress permutations. We thus tentatively propose that as for Piton de la Fournaise, the regional stress field acting on Etna and Hawaiian volcanoes is an important parameter in the control of the magma transfer along their deep plumbing system whereas the dynamics of their summit system is governed by shallow stress variations (above 5–10 km bsl) due to edifice deformation and magma transfers.

The Piton de la Fournaise volcano exhibits frequent eruptions preceded by seismic swarms and is a good target to test hypotheses about magmatically induced variations in seismic wave properties. We use a permanent station network and a portable broadband network to compare seismic anisotropy measured via shear wave splitting with geodetic displacements, ratios of compressional to shear velocity (Vp/Vs), earthquake focal mechanisms, and ambient noise correlation analysis of surface wave velocities and to examine velocity and stress changes from 2000 through 2012. Fast directions align radially to the central cone and parallel to surface cracks and fissures, suggesting stress-controlled cracks. High Vp/Vs ratios under the summit compared with low ratios under the flank suggest spatial variations in the proportion of fluid-filled versus gas-filled cracks. Secular variations of fast directions (ϕ) and delay times (dt) between split shear waves are interpreted to sense changing crack densities and pressure. Delay times tend to increase while surface wave velocity decreases before eruptions. Rotations of ϕ may be caused by changes in either stress direction or fluid pressure. These changes usually correlate with GPS baseline changes. Changes in shear wave splitting measurements made on multiplets yield several populations with characteristic delay times, measured incoming polarizations, and fast directions, which change their proportion as a function of time. An eruption sequence on 14 October 2010 yielded over 2000 shear wave splitting measurements in a 14 h period, allowing high time resolution measurements to characterize the sequence. Stress directions from a propagating dike model qualitatively fit the temporal change in splitting.

The dynamics of the 2–12 January 2010 effusive eruption at Piton de la Fournaise volcano were examined through seismic and infrasound records, time-lapse photography, SO2 flux measurements, deformation data, and direct observations. Digital elevation models were constructed for four periods of the eruption, thus providing an assessment of the temporal evolution of the morphology, the volume and the extrusion rate of the lava flow. These data were compared to the continuous recording of the seismic and infrasonic waves, and a linear relationship was found between the seismic energy of the tremor and the lava extrusion rate. This relationship is supported by data from three other summit eruptions of Piton de la Fournaise and gives total volume and average lava extrusion rate in good agreement with previous studies. We can therefore provide an estimate of the lava extrusion rate for the January 2010 eruption with a very high temporal resolution. We found an average lava extrusion rate of 2.4 m3s−1 with a peak of 106.6 m3s−1 during the initial lava fountaining phase. We use the inferred average lava extrusion rate during the lava fountaining phase (30.23 m3s−1) to estimate the value of the initial overpressure in the magma reservoir, which we found to range from 3.7×106 Pa to 5.9×106 Pa. Finally, based on the estimated initial overpressure, the volume of magma expelled during the lava fountaining phase and geodetic data, we inferred the volume of the magma reservoir using a simple Mogi model, between 0.25 km3 and 0.54 km3, which is in good agreement with previous studies.

International audience; Monitoring of large basaltic volcanoes, such as Piton de la Fournaise (La Réunion Island, France), has revealed preeruptive accelerations in surface displacements and seismicity rate over a period of between 1 h and several weeks before magma reaches the surface. Such eruptions are attributed to ruptures of pressurized magma reservoirs. Elastic models used to describe surface deformation would assume that accelerations in surface deformation are due to increases in reservoir pressure. This assumption requires changes in magma or pressure conditions at the base of the magma feeding system that are unrealistic over the observed timescale. Another possible cause for these accelerations is magma pressure in the reservoir weakening the volcanic edifice. In the present study, we modeled such weakening by progressive damage to an initially elastic edifice. We used an incremental damage model, with seismicity as a damage variable with daily increments. Elastic moduli decrease linearly with each damage increment. Applied to an initially elastic edifice with constant pressure at the base of the system, this damage model reproduces surface displacement accelerations quite well when damage is sufficient. Process dynamics is controlled by the damage parameter, taken as the ratio between the incremental rupture surface and the surface to be ruptured. In this case, edifice strength and magma reservoir pressure decrease with decreasing elastic moduli, whereas surface displacement accelerates. We discuss the consequences of pressure decreases in magma reservoirs.

International audience; Peripheral diffuse degassing of CO2 from the soil occurs across the western flank of Piton de la Fournaise volcano (La Réunion Island, Indian Ocean) along a narrow zone. In this area, carbon isotopic analysis on soil gas samples highlights significant mixing between magmatic and organic end-members. The zones with the strongest mag- matic signature (highest δ13C) overlap spatial distribution of hypocenters recorded shortly before and during vol- cano reactivation and allow discriminating a N135° degassing lineament, with a minimum length of 11 km and 140 ± 20 m-width. Such orientation is in accordance with that of an old dyke network along the rift zone and with N120–130° and N140–155° lineaments related to the inheritance of oceanic lithosphere structures.Our findings show that this N135° lineament represents a preferential magmatic pathway for deep magma transfer below the volcano flank. Moreover, spatial distributions of recent eccentric cones indicate a well-founded possibil- ity that future eruptions may by-pass the shallow plumbing system of the central area of the volcano, taking a lat- eral pathway along this structure. Our results also confirm that Piton de la Fournaise activity is linked to a laterally shifted plumbing system and represent a major improvement in identifying the main high-risk area on the densely populated western flank of the volcano.

We implemented the first operational automated seismic‐event classification system for monitoring activity at the Piton de la Fournaise volcano observatory (OVPF, La Reunion Island). Our classifier is based on the Random Forest algorithm. It distinguishes between eight classes of seismic signals: summit and deep volcano tectonic events, local, regional, and teleseismic earthquakes, T phases, rockfalls, and sound waves. It adopts a multistation approach and automatically selects the best features for each station and combination of stations from a large set of waveform‐ and spectrum‐based features. It reaches peak performance when it runs on a three‐station combination: one station on the summit of Piton de la Fournaise, one in its caldera, and one on the volcano flank. We interfaced our classification system with the observatory management interface WebObs used at OVPF.

International audience; Seismic data have been used to catalog the location and volume of most of the rockfalls that occurred at the Piton de la Fournaise volcano from May 2007, just after the major collapse of the Dolomieu summit crater floor, to May 2011. This catalog made it possible to compare the evolution of the number and volume of rockfalls at a high temporal resolution and to investigate their links with eruptions, seismicity, deformations and rainfalls affecting the Piton de Ia Fournaise volcano. Results show that the purge of unstable areas created by the Dolomieu crater floor collapse occurred in two phases: a first phase, lasting three months, during which the intense rockfall activity immediately following the collapse decreased abruptly and a second phase, lasting more than two years, during which the daily volume of the rockfalls slowly decreased before reaching a steady state. A detailed study of 4 time periods, including 3 eruptive cycles, indicates that strong seismicity can increase the number of rockfalls. Furthermore, when a dike reaches the surface at the summit of the central cone, giving birth to an eruption, the associated local forcing can in some cases increase the volume of rockfalls, possibly by creating or expanding weak zones. In most cases, this is not observed for dike intrusions that do not reach the surface. The newly created weak zones are most often far from the eruptive fissures of the ongoing eruption, but close to the location of the next eruption. This suggests that these distant zones are also weakened at depth, thus creating preferred paths for future dikes propagating toward the surface. The impact of rainfall on rockfall activity was also studied, given that La Reunion Island is subject to very intense rainfall events. Our results show that rainfall can in some cases trigger rockfalls rapidly, with response times of less than a day, but not systematically, and no triggering rainfall threshold was found.

International audience; Noise-based seismology is proving to be a complementary approach to active-source or earthquake-based methods for imaging and monitoring the Earth’s interior. Until recently, however, noise-based imaging and monitoring relied mostly on the inversion of surface waves reconstructed from correlations of mostly microseismic seismic noise (around 5 s of period). Compared to body-wave tomography and considering similar wavelengths, surface-wave tomography succeeds in retrieving lateral subsurface velocity contrasts but is less efficient in resolving velocity perturbations at depth. Recently, the use of large-N seismic arrays has proven to be of great benefit for extracting body waves from noise correlations by stacking over a large number of receiver pairs and by applying array processing. In this article, we describe a recent large-N array experiment that we conducted on Piton de la Fournaise (PdF). Our main goal was to extract body waves traveling directly in the vicinity of the active magma reservoir located at ∼2.5 km depth below the summit crater using noise correlations between arrays of seismic sensors. Within this article, we provide technical information about the VolcArray experiment, which consisted of the deployment of 300 seismic nodes during one month on PdF. We also present data-quality measurements and show how the short-period seismometers that we used compare to standard broadband seismic records. Finally, we show noise array beamforming results to study the content of seismic noise at frequencies between 1 and 12 Hz and discuss the ability to use these data to recover body waves between arrays from the correlation of ambient seismic noise.

International audience; Several patterns of magma paths have been proposed since the 1980s for Piton de la Fournaise volcano. Given the significant differences, which are presented here, we propose a reappraisal of the magma intrusion paths using a 17-years-long database of volcano-tectonic seismic events and a detailed mapping of the scoria cones. At the edifice scale, the magma propagates along two N120 trending rift zones. They are wide, linear, spotted by small to large scoria cones and related lava flows and involving magma resulting from high-pressure fractionation of ol ± cpx and presents an eruption periodicity of around 200 years over the last 30 kiloyears. The upper plumbing system originates at the base of the edifice below the Enclos Fouqué caldera. It feeds frequent (1 eruption every 9 months on average), short-lived summit and distal (flank) eruptions along summit and outer rift zones, respectively. Summit rift zones are short and present an orthogonal pattern restricted to the central active cone of Piton de la Fournaise whereas outer rift zones extend from inside the Enclos Fouqué caldera to the NE and SE volcano flanks. To sum up, rift zones of Piton de la Fournaise present strong geometrical and dynamical differences. On the one hand, the lower plumbing system feeds rift zones showing striking similarities to those developed in Hawaii during the alkaline postshield stage. On the other hand, the rift zones connected to upper plumbing system can be compared the rift system of Mount Etna, whose dynamics is know to be linked with the lateral displacement of the east flank.

Piton de la Fournaise (La Reunion Island) is amongst the most active volcanoes in the world, having had about 130 eruptions during the last century. Over the last 50 years it has produced 970 Mm3 of magma, at a mean eruption rate of 19.4 Mm3 per year, of which 489 Mm3 had been erupted over the last 15 years (at a rate of 32.7 Mm3 per year). This indicates a relative increase in its output. In this paper we present a detailed description of the high volcanic activity spanning the 1998–2013 period (eruptive precursors, location, characteristics and volume of emitted lavas, …). This period of high volcanic activity has been the subject of numerous studies that have led to significant advances in the knowledge of the shape and dynamics of the shallow plumbing system of the volcano.

In this article, we summarize some recent results of measurements of temporal changes of active volcanoes using seismic noise cross-correlations. We first present a novel approach to estimate volcano interior temporal seismic velocity changes. The proposed method allows to measure very small velocity changes (≈ 0.1%) with a time resolution as small as one day. The application of that method to Piton de la Fournaise Volcano (La Reunion Island) shows velocity decreases preceding eruptions. Moreover, velocity changes from noise cross-correlations over 10 years allow to detect transient velocity changes that could be due to long-lasting intrusions of magma without eruptive activity or to pressure buildup associated to the replenishing of the magma reservoir. We also present preliminary results of noise cross-correlation waveform perturbation associated with the occurrence of dike injection and volcanic eruption. We show that such an analysis leads us to locate the areas of dike injection and eruptive fissures at Piton de la Fournaise Volcano. These recent results suggest that monitoring volcanoes using seismic noise correlations should improve our ability to forecast eruptions, their intensity and thus potential environmental impact.

We present a study of the tremor that accompanied several eruptions of the Piton de la Fournaise volcano. We locate tremor sources in several frequency bands with a method based on the use of seismic amplitudes corrected for the recording site effect. The location of the sources is obtained by approximating the decay of the amplitude as a function of the hypocentral distance using the body wave amplitude decay. For frequencies above 1.5 Hz, sources are generally found at hallow depth and close to the location of the eruptive vents or fissures. The best correlation between the position of the vents and tremor sources is obtained for the 5–10 Hz frequency band suggesting the tremor in that band is directly generated at the eruption sites. Lower frequencies seem to be related to deeper processes than the eruption process observed at the surface. As sources are found at shallow depth, locating the tremor origin does not appear to be efficient for mapping the underground plumbing system of the volcano. On the other hand, during some eruptions, it appears as an efficient tool for monitoring the evolution of the eruption such as the opening of new eruptive fissures.

SUMMARY We present a study of the seismicity which preceded the 1998 March 9 eruption of the Piton de la Fournaise volcano. The eruption was preceded by a seismic crisis unusually long for this volcano which lasted for more than 35 hr and included about 3100 events with a maximum magnitude of 2.2. We located 583 volcano-tectonic (VT) events by manually picking their arrival times. The locations outline a monotonic upward migration of the hypocentres from about 5 km below sea level to the surface, leading to the onset of the eruption. This good correlation of the seismicity with eruptive activity suggests that the seismic migration was triggered by the ascent of magma toward the eruption site. The location of the earthquakes outlines an almost vertical conduit below the volcano providing new insights into the volcanic plumbing system.

International audience; Since the collapse of the Dolomieu crater floor at Piton de la Fournaise Volcano (la Réunion) in 2007, hundreds of seismic signals generated by rockfalls have been recorded daily at the Observatoire Volcanologique du Piton de la Fournaise (OVPF). To study rockfall activity over a long period of time, automated methods are required to process the available continuous seismic records. We present a set of automated methods designed to identify, locate, and estimate the volume of rockfalls from their seismic signals. The method used to automatically discriminate seismic signals generated by rockfalls from other common events recorded at OVPF is based on fuzzy sets and has a success rate of 92%. A kurtosis-based automated picking method makes it possible to precisely pick the onset time and the final time of the rockfall-generated seismic signals. We present methods to determine rockfall locations based on these accurate pickings and a surface-wave propagation model computed for each station using a Fast Marching Method. These methods have successfully located directly observed rockfalls with an accuracy of about 100 m. They also make it possible to compute the seismic energy generated by rockfalls, which is then used to retrieve their volume. The methods developed were applied to a data set of 12,422 rockfalls that occurred over a period extending from the collapse of the Dolomieu crater floor in April 2007 to the end of the UnderVolc project in May 2011 to identify the most hazardous areas of the Piton de la Fournaise volcano summit.

[1] Volcanic eruptions impact on societal risk, and volcanic hazard assessment is a necessary ingredient for decision-makers. However, the prediction of volcanic eruptions remains challenging due to the complexity and the non-linearity of volcanic processes. Identified forerunners such as increasing seismicity or deformation of the volcanic edifice prior to eruption are not deterministic. In this study, we use statistical methods to identify and discriminate precursory patterns to eruptions, on three sets of observables of Piton de la Fournaise volcano. We analyzed the short-term (i.e. the inter-eruptive period) time series of the seismicity rate, the deformation and the seismic velocity changes (deduced from seismic noise cross-correlations) over the period 1999–2006, with two main goals. First, we characterize the average pre-eruptive time patterns before 22 eruptions using superposed epoch analysis for the three observables. Using daily rate values, we resolve (1) a velocity change within 100–50 days from the eruptions onsets, then a plateau value up to eruption onset; (2) a power law increase in seismicity rate from noise level 15–10 days before eruption time; (3) an increase of displacement rate on the eruption day. These results support a three step mechanism leading to magma transfers toward the surface. Second we use pattern recognition techniques and the formalization of error diagrams to quantify the predictive power of each forerunner either as used independently or as combined to each other. We show that when seismicity rate alone performs the best prediction in the failure to predict versus alarm duration space, the combination of the displacement and seismicity data reduces the false alarm rate. We further propose a tool which explores the prediction results in order to optimize prediction strategy for decision-makers, as a function of the risk value.

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 …

International audience; The incremental caldera collapses of Fernandina (1968), Miyakejima (2000), and Piton de la Fournaise (2007) are analyzed in order to understand the collapse dynamics in basaltic setting and the associated edifice deformation. For each caldera, the collapse dynamics is assessed through the evolution of the (1) time interval T between two successive collapse increments, (2) amount of vertical displacement during each collapse increment, and (3) magma outflow rate during the whole collapse caldera process. We show from the evolution of T that Piton de la Fournaise and Fernandina were characterized by a similar collapse dynamics, despite large differences in the caldera geometry and the duration of the whole collapse caldera process. This evolution significantly differs from that of Miyakejima where T strongly fluctuated throughout the whole collapse process. Quantification of the piston vertical displacements enables us to determine the magma outflow rates between each collapse increment. Displacement data (tiltmeter and/or GPS) for Piton de la Fournaise and Miyakejima are used to constrain the edifice overall deformation and the edifice deformation rates. These data reveal that both volcanoes experienced edifice inflation once the piston collapsed into the magma chamber. Such a deformation, which lasts during the first collapse increments only, is interpreted as the result of larger volume of piston intruded in the magma chamber than magma withdrawn before each collapse increment. Once the effect of the collapsing rock column vanishes, edifice deflates. We also determine for each caldera the critical amount of magma evacuated before collapse initiation and compare it to analog models. The significant differences between models and nature are explained by the occurrence of preexisting weak zones in nature, i.e., the ring faults, that are not taken into account in analog models. Finally, we show that T at Piton de la Fournaise and Fernandina was equally controlled by the frictional resistance along the ring faults and the magma outflow rate. In addition to these two parameters, the collapse dynamics of Miyakejima was also influenced by variations of the magma bulk modulus, which changed after the influx of deep gas-rich magma into the collapse-related magma chamber. Altogether, our results show that the dynamics of caldera collapse in basaltic volcanoes proceeds in two phases: Phase 1, starting with the first collapse, is characterized by the largest collapse amplitude, an incremental edifice inflation, and a step-by-step increase of the rate of magma outflow. Phase 2 shows a rapid decrease of the magma discharge rate to a low level concomitant with the continuous edifice deflation. If deep magma is injected into the magma chamber, as at Miyakejima, an additional phase occurs (phase 3).

[1] At shallow depth beneath the Earth's surface, magma propagates through strongly heterogeneous volcanic material. Inversion of buoyancy and/or solidification have strong impacts on the dynamics of propagation without any change of magma supply. In this paper, we study the spatial and time evolution of magma intrusions using induced seismicity. We propose a new method based on ratio analysis of estimates of radiated seismic intensities recorded at different stations during seismic swarms. By applying this method to the January 2010 Piton de la Fournaise volcano eruption, we image complex dike propagation dynamics which strongly differ from a model of constant velocity dike propagation. We provide a new method to image in real time the dynamics of dike propagation and to infer the position of eruptive fissures.

International audience; Analysis of seismic activity associated with the eruptions of 2007, which led to the collapse of the Dolomieu crater on April 5th, reveals the link between the seismicity and the magma transfers at Piton de la Fournaise. Three eruptive phases occurred on February 18th, March 30th and April 2nd, 2007, at the summit, 2,000 m, and 600 m high on the South-East flank respectively, illustrating the three types of eruptions defined for the current Piton de la Fournaise activity. We use cross-correlation of seismic waveforms and clustering to improve the earthquake locations and determine the best-constrained focal mechanisms (with an average of 78 P phase polarities). The pre-eruptive seismicity of the February and March eruptions is composed of time extended clusters that also preceded other distal eruptions from 2000 to 2007. Our analysis shows that the seismic swarm prior to the February eruption initiated the intrusion that led to the April 2007 eruption. The seismicity preceding the Dolomieu crater collapse consists of numerous, but time-limited, clusters and specific seismic activity that accompanied the Dolomieu crater collapse. From April 1st to April 5th, earthquakes with CLVD mechanisms combined with normal faulting sources occurred between 0.8 and 0 km asl, until the complete rupture of the shallow magma storage roof. This collapse induced the propagation of a de-pressurization front and triggered a migration of seismicity from 0 to − 8 km along a very narrow path.

International audience; Piton de la Fournaise is a highly active basaltic volcano located on La Réunion island. With the aim of gaining information on its 3D internal structure, we carried out a 3D tomographic inversion based on P-wave first arrival times for earthquakes recorded by the local seismic monitoring network of the Observatoire Volcanologique du Piton de la Fournaise (OVPF).We used data recorded between 1996 and 1999 during intereruptive periods as well as during the pre-eruptive swarms which preceded the eruptions in March 1998, July and September 1999. Volcano-tectonic activity below the volcano is mostly located at shallow depth, above sea level and below the central cone. However, the seismic crisis which preceded the March 9, 1998 eruption included a large number of events at greater depth providing an unprecedented data set which sheds light on the deep structure of the volcano. The tomographic technique which we used is based on an accurate finitedifference travel-time computation and a simultaneous probabilistic inversion of both velocity models and earthquake locations. The inversion is carried out using an improved technique which allows to well constrain the inversion parameters. This processing provides high quality stable tomographic images. The obtained P-wave velocity model confirms the presence of a high-velocity plug above sea level, under the summit craters. This anomaly is interpreted as corresponding to an intrusive, solidified dyke-and-sill complex with little fluid magma storage. The high number of hypocenters in the zone above sea level accounts for massive rock fracturing when magma rises toward the surface. The shallow high-velocity plug is surrounded by a lowvelocity ring known as being fractured and vesicular lavas or scorias located on volcano flanks. Two low-velocity anomalies are found below the summit caldera. The most superficial spreads from 1 km to 0 a.s.l.. The second is located below 1 km b.s.l.. These volumes may correspond to magma storage systems. They are separated by a relatively high-velocity volume at sea level with a strong velocity gradient reaching 0.6 km/s per km

International audience; We present the results of a real time method based on coda-wave interferometry from seismic noise cross-correlation functions for relative seismic velocity variations monitoring on a volcanic edifice. The ambient seismic noise at the Piton de la Fournaise volcano on La Réunion island is analyzed from January 2006 to June 2007. During this period, five eruptions occurred showing a great diversity in eruption duration, intensity and eruptive fissure location. Two different methods are used to compute the velocity variations in order to compare their stability in quasi real-time routine. We compare the obtained velocity variations with the surface deformation observed by GPS and extensometers networks. This allows us to identify and quantify three major processes at the origin of seismic wave velocity variations in the edifice. Firstly, the observed pre-eruptive summit inflation is accompanied by a decrease in seismic velocity. Secondly, the edifice deflation following the opening of an eruptive fissure is characterized by an increase of the velocity. Finally, the summit caldera collapse generates a strong velocity drop. Coda-wave interferometry from seismic noise cross-correlation functions in quasi-real time may allow us to forecast eruption and constrain the processes taking place in the volcanic plumbing system.

International audience; After 10 years of high activity and 24 eruptions, Piton de la Fournaise (PdF) produced on April 2 to May 1, 2007 one of its most voluminous and intense eruption since at least one century. The eruption focused at 590 m elevation in the Grand Brûlé on the south east flank of Piton de la Fournaise, 7 km away from summit. It was located close to the southern cliff of the Grand Brûlé volcano tectonic collapse, the Rempart du Tremblet, and to the nearby village called Le Tremblet, and it highly rattled and incommoded the inhabitants there. Eleven hours after the beginning of the eruption the lava flows reached the sea and created 0.45 km2 of new land until the end of eruption. In the night of April 5 the rock column beneath Dolomieu crater started to collapse with a piston like mechanism into the magma chamber, forming within less than 24 h the most recent and well documented caldera. We report in this paper the chronology of these two related major events in the recent activity of PdF seen by the scientific networks of the volcanological observatory (OVPF/IPGP) and field observations, which allowed to back up the scenario of the caldera formation and to quantify these two exceptional episodes.