Classifying Major Explosions and Paroxysms at Stromboli Volcano (Italy) from Space

Stromboli volcano has a persistent activity that is almost exclusively explosive. Predominated by low intensity events, this activity is occasionally interspersed with more powerful episodes, known as major explosions and paroxysms, which represent the main hazards for the inhabitants of the island. Here, we propose a machine learning approach to distinguish between paroxysms and major explosions by using satellite-derived measurements. We investigated the high energy explosive events occurring in the period January 2018–April 2021. Three distinguishing features are taken into account, namely (i) the temporal variations of surface temperature over the summit area, (ii) the magnitude of the explosive volcanic deposits emplaced during each explosion, and (iii) the height of the volcanic ash plume produced by the explosive events. We use optical satellite imagery to compute the land surface temperature (LST) and the ash plume height (PH). The magnitude of the explosive volcanic deposits (EVD) is estimated by using multi-temporal Synthetic Aperture Radar (SAR) intensity images. Once the input feature vectors were identified, we designed a k-means unsupervised classifier to group the explosive events at Stromboli volcano based on their similarities in two clusters: (1) paroxysms and (2) major explosions. The major explosions are identified by low/medium thermal content, i.e., LSTI around 1.4 °C, low plume height, i.e., PH around 420 m, and low production of explosive deposits, i.e., EVD around 2.5. The paroxysms are extreme events mainly characterized by medium/high thermal content, i.e., LSTI around 2.3 °C, medium/high plume height, i.e., PH around 3330 m, and high production of explosive deposits, i.e., EVD around 10.17. The centroids with coordinates (PH, EVD, LSTI) are: Cp (3330, 10.7, 2.3) for the paroxysms, and Cme (420, 2.5, 1.4) for the major explosions.

High Crystal Number Densities From Mechanical Damage

Laboratory experiments investigating syn-eruptive crystallization are fundamental for interpreting crystal and vesicle textures in pyroclasts. Previous experiments have advanced our understanding by varying decompression and cooling pathways, volatile components, and melt composition. However, they have largely failed to produce the high crystal number densities seen in many cryptodome and dome samples. This is feasibly due to the relatively simple decompression pathways employed in experimental studies. In this study, we approach the problem by exploring non-linear decompression pathways. We present two series of experiments: (1) decompression from low initial starting pressure and (2) a compression-and-release step after the initial decompression. The purpose of each series was to simulate (1) decompression of magma that stalls during ascent and (2) pressure cycling that occurs in non-erupted magma during episodic explosive activity. The experiments were carried out on a synthetic rhyodacite (SiO2 = 69 wt%) held initially at 50 MPa and 885°C then decompressed at rates of 0.026 and 0.05 MPa s−1 to 10 MPa A subset of experiments was then subjected to a compression step to 110 MPa followed by near-instantaneous release back to 10 MPa. A substantial volume fraction of dendritic microlites (ϕxtl = 0.27–0.32, Na = 4.79 × 103 mm–2) formed during the initial hold at 50 MPa; additional crystallization during subsequent decompression to ≥ 10 MPa was minimal, as evidenced by only small increases in crystallinity (ϕxtl = 0.28–0.33) and comparable crystal number densities (4.11–7.81 × 103 mm–2). Samples that underwent recompression followed by a second decompression showed no increase in crystal volume fraction but did show extensive disruption of the initial dendritic, box-work microlite structures that produced high number densities (Na = 43.5–87.2 × 103 mm–2) of small individual crystals. The disruption was driven by a combination of rapid vesiculation, expansion and resulting shear along the capsule walls. From these results, we suggest that high crystal number densities may be a signature of rapid deformation occurring after magma stalling in the subsurface, perhaps related to pressure cycling and accompanying rapid changes in vesicularity during repeated small and shallow-sourced explosions. We compare our experiments to pyroclasts from shallow intrusions that preceded the 18 May 1980 eruption of Mount St Helens. These pyroclasts were erupted both prior to 18 May, during episodic precursory explosive activity, and by the 18 May initial lateral blast. The pattern of precursory activity indicates multiple episodes of pressurization (prior to explosive events) and rapid decompression (during explosive events) that we use to illustrate the significance of our experimental results.

Variable Magnitude and Intensity of Strombolian Explosions: Focus on the Eruptive Processes for a First Classification Scheme for Stromboli Volcano (Italy)

Strombolian activity varies in magnitude and intensity and may evolve into a threat for the local populations living on volcanoes with persistent or semi-persistent activity. A key example comes from the activity of Stromboli volcano (Italy). The “ordinary” Strombolian activity, consisting in intermittent ejection of bombs and lapilli around the eruptive vents, is sometimes interrupted by high-energy explosive events (locally called major or paroxysmal explosions), which can affect very large areas. Recently, the 3 July 2019 explosive paroxysm at Stromboli volcano caused serious concerns in the local population and media, having killed one tourist while hiking on the volcano. Major explosions, albeit not endangering inhabited areas, often produce a fallout of bombs and lapilli in zones frequented by tourists. Despite this, the classification of Strombolian explosions on the basis of their intensity derives from measurements that are not always replicable (i.e., field surveys). Hence the need for a fast, objective and quantitative classification of explosive activity. Here, we use images of the monitoring camera network, seismicity and ground deformation data, to characterize and distinguish paroxysms, impacting the whole island, from major explosions, that affect the summit of the volcano above 500 m elevation, and from the persistent, mild explosive activity that normally has no impact on the local population. This analysis comprises 12 explosive events occurring at Stromboli after 25 June 2019 and is updated to 6 December 2020.

A UK local seismic magnitude scale, MLP, using P-wave amplitudes

SUMMARY A local seismic magnitude scale, MLP, has been developed for the United Kingdom (UK) using automated measurements of 8902 half peak-to-peak vertical component seismic P-wave displacement amplitudes from 630 earthquakes. The measurement time window increases with source-to-receiver range such that MLP is sensitive to the dominant phase within the P-wavetrain at a given distance. To avoid contamination due to low-frequency noise, the P-wave amplitude measurements are made in the 1.5–30 Hz passband. A least-squares inversion was undertaken to estimate source size, distance and station effects. The distance effect values suggest that P-wave amplitude attenuation across the UK is low when compared to other tectonically stable regions. The station effects are broadly consistent with UK geology, with signal amplification observed within the sediments towards the south-east of the country. MLP has been tied to the UK local magnitude scale routinely estimated by the British Geological Survey (BGS, determined using S waves, and here denoted MLBGS). For earthquakes with MLBGS > 3, MLP exhibits a closer correspondence to the moment magnitude than MLBGS (i.e. MLP≈Mw). It is tentatively suggested that this reduction in bias is caused by the P-wave scale being less affected by along-path attenuation. The difference with respect to physical source scaling helps explain the divergence of the MLBGS and MLP scales at ML > 3. MLP allows a robust estimate of event size to be made for small events which predominantly generate P waves, for example, near-surface explosions. MLP values have been calculated for 239 explosive events, mostly mining blasts and munitions disposal. Although there is significant scatter, explosive events exhibit elevated MLP values compared to MLBGS, consistent with explosions generating proportionally more compressional wave energy than earthquakes. For example, 33 explosions at sea exhibit a median MLP–MLBGS value of 0.50 mag units. Despite its sensitivity to P-wave amplitude, MLP is not a more consistent estimator of explosive source size than MLBGS; the magnitude residuals (station estimate − event estimate) are slightly less for MLBGS compared to MLP. This is primarily due to variability of the P-wave amplitudes that cannot be explained by a 1-D distance correction. MLP should be considered as an additional tool for characterizing small seismic events within the UK.

Production and Separation of Radioactive Ion Beams

Radioactive nuclei play an important role in many astrophysical scenarios,from the Big-Bang Nucleo-synthesis to the standard solar model, from quiescent burning to the most explosive events that can occur in our universe. A huge effort has been made for more than thirty years to construct facilities able to deliver beams of radioactive nuclei with increasing intensity and better quality. This contribution revises the different mechanisms and the separation techniques employed for the production of Radioactive Ion Beams.