Catching geomorphological response to volcanic activity on steep slope volcanoes using multi-platform remote sensing

The geomorphological evolution of the volcanic Island of Stromboli (Italy) between July 2010 and June 2019 has been reconstructed by using multi-temporal, multi-platform remote sensing data. Digital elevation models (DEMs) from PLÉIADES-1 tri-stereo images and from Light Detection and Ranging (LiDAR) acquisitions allowed for topographic changes estimation. Data were comprised of high-spatial-resolution (QUICKBIRD) and moderate spatial resolution (SENTINEL-2) satellite images that allowed for the mapping of areas that were affected by major lithological and morphological changes. PLÉIADES tri-stereo and LiDAR DEMs have been quantitatively and qualitatively compared and, although there are artefacts in the smaller structures (e.g., ridges and valleys), there is still a clear consistency between the two DEMs for the larger structures (as the main valleys and ridges). The period between July 2010 and May 2012 showed only minor changes consisting of volcanoclastic sedimentation and some overflows outside the crater. Otherwise, between May 2012 and May 2017, large topographic changes occurred that were related to the emplacement of the 2014 lava flow in the NE part of the Sciara del Fuoco and to the accumulation of a volcaniclastic wedge in the central part of the Sciara del Fuoco. Between 2017 and 2019, minor changes were again detected due to small accumulation next to the crater terrace and the erosion in lower Sciara del Fuoco.Publishedid 4385V. Processi eruttivi e post-eruttiviJCR Journa

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

Relative seismic and tsunami risk assessment for Stromboli Island (Italy)

An innovative method of estimating the relative risk of buildings exposed to seismic and tsunami hazards in volcanic islands is applied to Stromboli (Italy), a well-known stratovolcano affected by moderate earthquakes and mass-flow-induced tsunamis. The method uses a pre-existing quali-quantitative analysis to assess the relative risk indices of buildings, which provide comparative results useful for prioritisation purposes, in combination with a historical-geographical settlement analysis consistent with the ‘territorialist’ approach to the urban and regional planning and design. The quali-quantitative analysis is based on a new proposed survey-sheet model, useful to collect building information necessary for the relative risk estimation, whereas the historical-geographical investigation is based on the multi-temporal comparison of aerial and satellite images. The proposal to combine two consolidated methods represents an innovation in estimating relative risk. Considering that Stromboli Island had never been subjected to similar analyses, the results of the relative seismic risk assessment are novel and moreover identify buildings with a fairly-low and spatially-uniform relative risk. The results of the relative tsunami risk assessment are consistent with results of similar past studies, identifying buildings with a higher relative risk index on the northern coast of the island. The combined use of a building-by-building survey with a multi-temporal analysis of settlements allows obtaining a higher detail than previously available for the region. If adequately modified, the proposed combination of methods allows assessing relative risk also considering other geo-environmental hazards and their cascading effects, in a multi-hazard risk assessment perspective

Use of PSInSAR data to map highly compressible soil layers

A new approach to the use of Persistent Scatterers (PS) Interferometry data in the reconstruction of the extension of compressible geological bodies is presented. The methodology was applied in the test area of the Tiber River delta (Italy), characterized by the presence of two large marshy zones, known as the Maccarese and Ostia Antica ponds. PSInSARâ\u84¢ data, derived from ERS1/2, ENVISAT and RADARSAT-1 images, and spanning a time interval between 1992 and 2006 were used to verify the possibility to reconstruct the spatial distribution of the peat levels inside the Maccarese and Ostia Antica ponds. Borehole information was analyzed to calibrate the InSAR data and the deformation rates were used to hypothesize the presence of a thick compressible layer where geological information is lacking. Variations in deformation rates registered by the single PS were assumed to be representative of a variation in the stratigraphic asset. The obtained results demonstrate that this approach could be satisfactorily used to investigate wide areas in a short time, reducing the number of boreholes to drill, and it could be a complementary technique to obtain information about the 2D geometry of specific geological levels

UAV-based multitemporal remote sensing surveys of volcano unstable flanks: a case study from Stromboli

UAV-based photogrammetry is becoming increasingly popular even in application fields that, until recently, were deemed unsuitable for this technique. Depending on the characteristics of the investigated scenario, the generation of three-dimensional (3D) topographic models may in fact be affected by significant inaccuracies unless site-specific adaptations are implemented into the data collection and processing routines. In this paper, an ad hoc procedure to exploit high-resolution aerial photogrammetry for the multitemporal analysis of the unstable Sciara del Fuoco (SdF) slope at Stromboli Island (Italy) is presented. Use of the technique is inherently problematic because of the homogeneous aspect of the gray ash slope, which prevents a straightforward identification of match points in continuous frames. Moreover, due to site accessibility restrictions enforced by local authorities after the volcanic paroxysm in July 2019, Ground Control Points (GCPs) cannot be positioned to constrain georeferencing. Therefore, all 3D point clouds were georeferenced using GCPs acquired in a 2019 (pre-paroxysm) survey, together with stable Virtual Ground Control Points (VGCPs) belonging to a LiDAR survey carried out in 2012. Alignment refinement was then performed by means of an iterative algorithm based on the closest points. The procedure succeeded in correctly georeferencing six high-resolution point clouds acquired from April 2017 to July 2021, whose time-focused analysis made it possible to track several geomorphological structures associated with the continued volcanic activity. The procedure can be further extended to smaller-scale analyses such as the estimation of locally eroded/accumulated volumes and pave the way for rapid UAV-based georeferenced surveys in emergency conditions at the SdF

Tracking morphological changes and slope instability using spaceborne and ground-based SAR data

Stromboli (Aeolian Archipelago, Italy) is an active volcano that is frequently affected by moderate to large mass wasting, which has occasionally triggered tsunamis. With the aim of understanding the relationship between the geomorphologic evolution and slope instability of Stromboli, remote sensing information from space-born Synthetic Aperture Radar (SAR) change detection and interferometry (InSAR) and Ground Based InSAR (GBInSAR) was compared with field observations and morphological analyses. Ground reflectivity and SqueeSAR⢠(an InSAR algorithm for surface deformation monitoring) displacement measurements from X-band COSMO-SkyMed satellites (CSK) were analysed together with displacement measurements from a permanent-sited, Ku-band GBInSAR system. Remote sensing results were compared with a preliminary morphological analysis of the Sciara del Fuoco (SdF) steep volcanic flank, which was carried out using a high-resolution Digital Elevation Model (DEM). Finally, field observations, supported by infrared thermographic surveys (IRT), allowed the interpretation and validation of remote sensing data. The analysis of the entire dataset (collected between January 2010 and December 2014) covers a period characterized by a low intensity of Strombolian activity. This period was punctuated by the occurrence of lava overflows, occurring from the crater terrace evolving downslope toward SdF, and flank eruptions, such as the 2014 event. The amplitude of the CSK images collected between February 22nd, 2010, and December 18th, 2014, highlights that during periods characterized by low-intensity Strombolian activity, the production of materials ejected from the crater terrace towards the SdF is generally low, and erosion is the prevailing process mainly affecting the central sector of the SdF. CSK-SqueeSAR⢠and GBInSAR data allowed the identification of low displacements in the SdF, except for high displacement rates (up to 1.5 mm/h) that were measured following both lava delta formation after the 2007 eruption and the lava overflows of 2010 and 2011. After the emplacement of the 2014 lava field, high displacements in the central and northern portions of the SdF were recorded by the GBInSAR device, whereas the spaceborne data were unable to detect these rapid movements. A comparison between IRT images and GBInSAR-derived displacement maps acquired during the same time interval revealed that the observed displacements along the SdF were related to the crumbling of newly emplaced 2014 lava and of its external breccia. Detected slope instability after the 2014 flank eruption was related to lava accumulation on the SdF and to the difference in the material underlying the 2014 lava flow: i) lava flows and breccia layers related to the 2002â03 and 2007 lava flow fields in the northern SdF sector and ii) loose volcaniclastic deposits in the central part of the SdF. This work emphasizes the importance of smart integration of spaceborne, SAR-derived hazard information with permanent-sited, operational monitoring by GBInSAR devices to detect areas impacted by mass wasting and volcanic activity