Nishimura, Koichi

The mild activity of basaltic volcanoes is punctuated by violent explosive eruptions that occur without obvious precursors. Modelling the source processes of these sudden blasts is challenging. Here, we use two decades of ground deformation (tilt) records from Stromboli volcano to shed light, with unprecedented detail, on the short-term (minute-scale) conduit processes that drive such violent volcanic eruptions. We find that explosive eruptions, with source parameters spanning seven orders of magnitude, all share a common pre-blast ground inflation trend. We explain this exponential inflation using a model in which pressure build-up is caused by the rapid expansion of volatile-rich magma rising from depth into a shallow (<400 m) resident magma conduit. We show that the duration and amplitude of this inflation trend scales with the eruption magnitude, indicating that the explosive dynamics obey the same (scale-invariant) conduit process. This scale-invariance of pre-explosion ground deformation may usher in a new era of short-term eruption forecasting.

The Lautaret full-scale avalanche test site in the southern French Alps has been used by IRSTEA (Cemagref) Research Institute since 1973. Over the recent years two avalanche paths are used to release small to medium avalanches 3 or 4 times each winter. Avalanche flows are generally dense, whether wet or dry, sometimes with a powder part. Main path n°2 (track length 800 m) is dedicated to avalanche dynamics. Within the flow of the avalanche, flow height and vertical profiles of pressure and velocity are measured along a 3.5 m tripod. The snow volume released in the starting zone is quantified by a differential analysis of laser scanning measurements set before and after triggering. A high rate positioning of the avalanche along the track is determined from terrestrial oblique photogrammetry. Above the dense layer, the saltation layer and the powder part are characterized by particles and air fluxes measurements. In path n°1 smaller in size, medium-size avalanches (track length 500 m) make this track of particular interest for experiments on structures. A macroscopic sensor-structure is set nearly 150 m downhill from the starting zone, that is, in the area where avalanches generally reach their maximum velocity. It consists is a one square-meter plate supported by a 3.5 m high steel cantilever fixed in the ground, facing the avalanche. Impact pressures are reconstructed from the cantilever deformations, while avalanche velocity is measured from optical sensors. Seismic signals generated by avalanches of those 2 paths are recorded by a 3-axial broadband seismometer. Around those experimental devices dedicated to the understanding of avalanche physics, a national and international partnership has been developed from years to years, including INSA de Lyon, CNRS and Université Joseph Fourier (France), Aalto University (Finland), Nagoya University (Japan), Boku University (Austria), IGEMA (Bolivia), OGS (Italy)