Aline Peltier

Long‐term dynamics of Piton de la Fournaise volcano from 13 years of seismic velocity change measurements and GPS observations

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.

Subtle precursors of volcanic eruptions at Piton de la Fournaise detected by extensometers

The highly active Piton de la Fournaise volcano is an excellent field laboratory to develop and test, in a short time span, new methods for detecting volcanic precursors. Since 1995, a network of four extensometers has been installed and has detected extensional, shear and vertical movements of fractures in relation to ground deformations of the volcano. This study describes new insights in the distribution of stresses in the volcanic edifice between eruptions and during magma intrusions. Continuous measurements show that all eruptions were preceded several months before by significant fracture movements: a constant opening associated with a dextral movement on the south flank and a sinistral movement on the north flank of the volcano. These movements can be attributed to a shallow pressure source below the summit craters.

A damage model for volcanic edifices: Implications for edifice strength, magma pressure, and eruptive processes

Monitoring of large basaltic volcanoes, such as Piton de la Fournaise (La Reunion 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.

The interplay between collapse structures, hydrothermal systems, and magma intrusions: the case of the central area of Piton de la Fournaise volcano

We explore the possible relationships between a structural heterogeneity, the hydrothermal system, and the intrusive activity at Piton de la Fournaise volcano. Geological and geophysical data show that as the result of repeated collapses (the last one in 2007), a cylinder of faulted, fractured, and crumbled rocks must exist between the surface and the top of a magma reservoir at about sea level. This structure constitutes a major geological heterogeneity. An obvious spatial correlation exists between this column of fractured and brecciated rock and the location of (1) most of the seismic activity, (2) a low-resistivity dome, (3) a huge self-potential anomaly, (4) thermal evidence of hydrothermal activity, and (5) the root of magma intrusions. The dominant factors that make this structural heterogeneity a trap for the activity are probably its higher permeability and its weaker mechanical strength. Evidence exists for the presence of an active hydrothermal system confined in this permeable zone. The long-term stability of the activated zone above sea level and the similarity of the pre-eruptive crises, in spite of the inferred large perturbation of the magmatic system in 1998, suggest a common triggering mechanism for all the eruptions since at least the first data recorded by the observatory in 1980. This mechanism can be purely magmatic, resulting from the pressurization of a reservoir, but we also propose that the hydrothermal system may play a role in the development of volcanic instabilities. A qualitative model is proposed to explain the triggering of magma intrusions by hydrothermal processes, and its speculative aspects are discussed. This work represents a first attempt to integrate the structural and dynamic information in a unified framework at Piton de la Fournaise.

Automated recognition of spikes in 1 Hz data recorded at the Easter Island magnetic observatory

In the present paper we apply a recently developed pattern recognition algorithm SPs to the problem of automated detection of artificial disturbances in one-second magnetic observatory data. The SPs algorithm relies on the theory of discrete mathematical analysis, which has been developed by some of the authors for more than 10 years. It continues the authors’ research in the morphological analysis of time series using fuzzy logic techniques. We show that, after a learning phase, this algorithm is able to recognize artificial spikes uniformly with low probabilities of target miss and false alarm. In particular, a 94% spike recognition rate and a 6% false alarm rate were achieved as a result of the algorithm application to raw one-second data acquired at the Easter Island magnetic observatory. This capability is critical and opens the possibility to use the SPs algorithm in an operational environment.

Long‐term mass transfer at Piton de la Fournaise volcano evidenced by strain distribution derived from GNSS network

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.

Toward continuous quantification of lava extrusion rate: Results from the multidisciplinary analysis of the 2 January 2010 eruption of Piton de la Fournaise volcano, La Réunion

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.

Seismic anisotropy and its precursory change before eruptions at Piton de la Fournaise volcano, La Réunion

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.

Fifteen Years of Intense Eruptive Activity (1998–2013) at Piton de la Fournaise Volcano: A Review

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.

Retrieving 65 years of volcano summit deformation from multitemporal structure from motion: The case of Piton de la Fournaise (La Réunion Island)

The structure-from-Motion photogrammetry technique enables use of historical airborne photography to achieve high-resolution 3D terrain models. We apply this method on Piton de la Fournaise volcano (La Reunion), which allows a unique opportunity to retrieve high-resolution (1.5-0.11 m) Digital Elevation Models and precise deformation maps of the volcano since 1950. Our results provide evidence that the summit volume increased throughout the study period, at a stable rate of 2.2 Mm3/yr between 1950 and 2000, increasing by a factor of four (to 8.0Mm3/yr) prior to the major 2007 eruption which was accompanied by summit caldera collapse. At the same time, summit deformation was asymmetric, with 9.2±2.5 m of eastward seaward displacement, and 1.3±2.5 m to the west during 1950-2015. Our results reveal a temporal evolution in the volcano magma influx rate and deformation. Tracking these fluxes and the long-lived preferential eastern motion is crucial to mitigate risks associated to flank destabilization.