Paroxysmal activity at Stromboli: lessons from the past

The persistent normal activity of Stromboli is occasionally interrupted by sudden and highly energetic explosive events called Strombolian paroxysms. These phenomena together with landslide-generated tsunamis represent the most hazardous manifestations of present-day volcanic activity at Stromboli. The most recent paroxysms, on 5 April 2003 and 15 March 2007, have drawn attention to these energetic events because they significantly threatened inhabitants and tourists. Historical accounts and field evidence indicate, however, that even larger paroxysms, in terms of volume, dispersal of products and intensity of explosive phenomena, occurred in the recent past. During these paroxysms incipiently welded spatter deposits mantled the north and south rims of the Sciara del Fuoco down to low elevations, extending much farther than the similar deposits from recent observed events (5 April 2003 and 15 March 2007). In order to identify, characterize and discriminate among products of these outstanding spatter-forming eruptions, more than 50 stratigraphic sections were measured and sampled. Stratigraphic, sedimentological and radiometric (¹⁴C) data indicate that only two paroxysms produced spatter that reached very low elevations and inhabited areas: the first occurred in the 16th century and the last in AD 1930. Analysis of texture and deposit components reveals that the early phases of the two eruptions were driven by distinctly different eruptive dynamics. Both identified paroxysms are at least one order of magnitude greater than any similar event observed by monitoring systems at Stromboli. These two large paroxysms were the most powerful volcanic events at Stromboli in the last eighteen centuries.     

Seafloor mapping for geohazard assessment: state of the art

During the last two decades, increasing use of full-coverage sonic mapping of the seafloor has made us more aware of the large and different number of seafloor processes and events bearing significant geohazard potential. This awareness combines with the increasing use of the seafloor for infrastructure and with the high density of population and settlement on the coast. Seafloor mapping is the first step in making a census of the geohazard-bearing features present in a given offshore area. It often provides the only tool for a comprehensive, although non-specific, seafloor geohazard assessment over large areas that are scarcely groundtruthed by acoustic prospection and seafloor sampling. However, the characterization of geohazard features on a morphological basis alone is limited, and more detailed investigations are needed to define the character and state of activity of potentially hazardous features. Such investigations include the use of deep-tow or autonomous platforms designed to acquire high-resolution data at depth as well as in situ measurements, both being very expensive activities not applicable over large areas. Thus seafloor mapping is often not only the first and the main but also the only tool for a comprehensive seafloor geohazard assessment. This special issue represents an example of the diversity of approaches to seafloor geohazard assessment and summarizes the present state of this discipline. Both the diverse technologies applied and the specific aims of offshore geohazard assessment brought different communities to deal with the study of seafloor processes and events from remarkably distinct viewpoints. We identified three end members in offshore geohazard assessment: (1) geohazard assessment ??sensu stricto??, (2) ??engineering?? geohazard assessment, (3) ??non-specific?? geohazard assessment. These are being conducted by industry, academia and public agencies in charge of civil protection and land-use planning and management. Understanding the needs and geohazard perception of the different groups is a necessary step for a profitable collaboration in such an interesting and rapidly developing field of marine geology.     

Vegetative reproduction by multicellular propagules in Rhodophyta: an overview

This paper deals with vegetative reproduction by multicellular propagules in Rhodophyta. An extensive examination of the relevant literature shows that this phenomenon in Rhodophyta is not well known. A propagule is here defined as a vegetative multicellular structure which spontaneously detaches from the parent thallus and gives rise to a new individual. The origin and morphological features of multicellular propagules are examined in the various known propagule‐forming species. The importance of multicellular propagules as both overwintering and resting organs and as a taxonomic feature is also shown. Their role in increasing local populations and/or for long‐distance dispersal is discussed. The relative abundance of vegetative reproduction by multicellular propagules versus sexual reproduction, as well as the advantages of this additional mode of reproduction, are shown and discussed. The production of multicellular propagules may contribute to the capacity of species to increase populations, to weather unpredictable environmental changes, to survive in conditions that would be lethal for entire thalli, and to reach new habitats. As resting organs, they may also be responsible for long‐distance dispersal and may account for the introduction of some alien species. Accordingly, propagule‐forming species are probably more competitive than taxa that do not produce such propagules. These considerations should be taken into account in future studies of the biology, ecology and demography of Rhodophyta.     

Improved seismic velocity reference model from local earthquake data in Northwestern Italy

Resolution and reliability estimates of results obtained by seismic tomography strongly depend on the reference model. Inadequate initial reference models may severely distort tomographic images or introduce artefacts that lead to misinterpretations of the results. Reference models are usually obtained by means of a priori near-surface geological information or by geophysical information derived by controlled-source seismology.
Starting from the idea that a reference model must approximate the weighted average of data selected for the three-dimensional (3D) inversion, one-dimensional (1D) model for Northwestern Italy is derived that is able to minimize mean of RMS of a set of well-locatable earthquakes, by computing a solution of the coupled hypocentre 1D velocity problem.
Such a model, termed the Minimum 1D model, can be used both as an initial reference model for 3D inversion and as a reference velocity model for high-quality routine earthquake location.     

Seasonal changes of microbial populations in the sediments of the basins of Lugano and Agno

This research was carried out with the aim to explore the heterotrophic microbial population of two sediments in different oxic conditions of the Lake of Lugano (Lago di Lugano). The values of the viable bacterial counts found in our sediment samples were typical for an eutrophic lake.The increase in the proportion of anaerobic to aerobic bacteria at 30 °C observed in the sediment samples of Agno may reflect the depletion of oxygen concentrations in the water column.The generaAeromonas, Pseudomonas, Bacillus andClostridium, as well as strains belonging to the Coryneform-group, represented the major taxonomic groups of heterotrophic bacterial communities in the water-sediment interface.     

Small-scale coexistence of island-arc- and enriched-MORB-type basalts in the central Vanuatu arc

We report here major, trace element and Sr–Nd–Pb isotopic data for a new set of basaltic lavas and melt inclusions hosted in Mg-rich olivines (Fo_86–91) from Mota Lava, in the Banks islands of the Vanuatu island arc. The results reveal the small-scale coexistence of typical island-arc basalts (IAB) and a distinct type of Nb-enriched basalts (NEB) characterized by primitive mantle-normalized trace element patterns without high-field-strength element (HFSE) depletion. The IAB show trace element patterns with prominent negative HFSE anomalies acquired during melting of mantle sources enriched with slab-derived, H₂O-rich components during subduction. In contrast, the NEB display trace element features that compare favourably with enriched-mid-ocean ridge basalt (MORB) and the most enriched basalts from the Vanuatu back-arc troughs. Both their trace element and Nd–Sr isotopic compositions require partial melting of an enriched-MORB-type mantle source, almost negligibly contaminated by slab-derived fluids (~0.2 wt%). The coexistence of these two distinct types of primitive magma, at the scale of one volcanic island and within a relatively short span of time, would reflect a heterogeneous mantle source and/or tapping of distinct mantle sources. Direct ascent of such distinct magmas could be favoured by the extensive tectonic setting of Mota Lava Island, allowing decompression melting and sampling of variable mantle sources. Significantly, this island is located at the junction of the N–S back-arc troughs and the E–W Hazel Home extensional zone, where the plate motion diverges in both direction and rate. More broadly, this study indicates that crustal faulting in arc contexts would permit basaltic magmas to reach Earth’s surface, while preserving the geochemical heterogeneity of their mantle sources.     

Analysis of Italian Earthquake catalogs in the context of intermediate-term prediction problem

We perform a comparative analysis of regional and global earthquake catalogs currently available for the territory of Italy. We consider: (a) instrumental seismic catalogs provided by the Istituto Nazionale di Geofisica e Vulcanologia, Roma (INGV) for earthquake forecasting experiment in Italy within the Collaboratory for the Study of Earthquake Predictability (CSEP); (b) Global Hypocenters’ Data provided by the USGS/NEIC, which is currently used in the real-time earthquake prediction experiment by CN and M8S algorithms in Italy, and (c) seismological Bulletin provided by the International Seismological Centre (ISC). We discuss advantages and shortcomings of these catalogs in the context of intermediate-term middle-range earthquake prediction problem in Italy, including the possibility of the catalog’s combined or integrated use. Magnitude errors in the catalog can distort statistics of success-to-failure scoring and eventually falsify testing results. Therefore, the analysis of systematic and random errors in magnitude presented in Appendixes can be of significance in its own right.     

The impact of internal waves on upper continental slopes: insights from the Mozambican margin (southwest Indian Ocean)

Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high-resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex-shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. However, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small-scale sedimentary features generated by internal waves are superimposed on large-scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

A Waveform Similarity Approach to Investigate Seismicity Patterns

the temporal and spatial evolution of seismicity, a simple cross-correlation technique was applied to a waveform data set of western Alps earthquakes. A selection of events recorded by a reference station of the regional seismic network of North-Western Italy (IGG network) within a particular S-P range was used. The adopted method was tested on 380 events recorded in 1995, allowing for the definition of the critical parameters for an optimal identification of earthquake families.The comparison of the individuated families definition with the relevant epicentres map confirms that, on small scales, the epicentre positions of microearthquakes located by a regional network could be strongly influenced by location errors. The analysis was extended to a more complete data set of 2653 events; 1171 of them were grouped in families. The results demonstrated that our single station method is able to discriminate between closely spaced families on the basis of the position and of the radiation properties.