Patrice Boissier

Automated identification, location, and volume estimation of rockfalls at Piton de la Fournaise volcano

Since the collapse of the Dolomieu crater floor at Piton de la Fournaise Volcano (la Reunion) 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.

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.

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.

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.

New perspectives on volcano monitoring in a tropical environment: Continuous measurements of soil CO2 flux at Piton de la Fournaise (La Réunion Island, France)

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.

Implementation of a Multistation Approach for Automated Event Classification at Piton de la Fournaise Volcano

ABSTRACT 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.