CO<Subscript>2</Subscript>-driven large mafic explosive eruptions: the Pozzolane Rosse case study from the Colli Albani Volcanic District (Italy)

Generally, the intensity and magnitude of explosive volcanic activity increase in parallel with SiO₂ content. Pyroclastic-flow-forming eruptions in the Colli Albani ultrapotassic volcanic district (Italy) represent the most striking exception on a global scale, with volumes on the order of tens of cubic kilometres and K-foiditic compositions (SiO₂ even <42 wt.%). Here, we reconstruct the pre-eruptive scenario and event dynamics of the ~456 ka Pozzolane Rosse (PR) eruption, the largest mafic explosive event of the Colli Albani district. In particular, we focus on the driving mechanisms for the unusually explosive eruption of a low-viscosity, mafic magma. Geologic, petrographic and geochemical data with mass balance calculations, supported by experimental data for Colli Albani magma compositions, provide evidence for significant ingestion of carbonate wall rocks by the Pozzolane Rosse K-foiditic magma. Moreover, the scattered occurrence of cored bombs in Pozzolane Rosse pyroclastic-flow deposits records carbonate entrainment even at the eruptive time scale, as also tested quantitatively by thermal modelling of magma–carbonate interaction and carbonate assimilation experiments. We suggest that the addition of free CO₂ from decarbonation of country rocks was the major factor controlling magma explosivity. High CO₂ activity in the volatile component, coupled with magma depressurisation, produced extensive leucite crystallisation at short time scales, resulting in a dramatic increase in magma viscosity and volatile pressurisation, which was manifested a change of eruptive dynamics from early effusion to the Pozzolane Rosse's highly explosive eruption climax.     

Dynamic properties of low velocity alluvial deposits influencing seismically-induced shear strains: the Grottaperfetta valley test-site (Rome, Italy)

The presence of peats and high compressibility inorganic clays within alluvial fills on the left-side tributaries of the Tiber River, close to Rome’s historic center in Italy, is well documented in literature. Nevertheless, few literature data exist until now on the dynamic properties of these deposits by undisturbed bore-hole samples. The Galba test-site was set up to characterise dynamic properties of the alluvial deposits by using lab-tests as well as to derive velocity profiles by seismic noise measurements. These were performed in the Giustiniano Imperatore area located in the Grottaperfetta valley, about 2?km south of Rome’s historic centre. The alluvial deposits filled a paleovalley excavated in the bedrock during the Würm glacial (18–20 ky). The stratigraphic setting of the alluvial body was reconstructed along three geological cross-sections by means of the available logs; seven lithotecnical horizons can be distinguished within the alluvial body, some tens of meters thick, based on both log-stratigraphic data and in-site geotechnical tests. These horizons include peaty layers (T) and high compressibility inorganic clays (AGI), which characterise the alluvial deposits in the Grottaperfetta valley. They do not have direct correlation with the alluvial horizons which constitute the alluvial body of the main Tiber valley in Rome’s historical centre. These alluvial horizons which are distinguished and characterised at the Galba test-site can be regarded as typical of other lateral valleys of the Tiber River in Southern Rome. They are characterised by the presence of similar high compressibility clayey deposits as well as peaty layers up to some meters thick. Undisturbed samples were also obtained at the Galba test-site for dynamic testing via resonant column and cyclic torsional shear tests. In order to attribute dynamic properties to the alluvial body at the Grottaperfetta valley, an extrapolation process was performed based on a detailed engineering-geology model of the alluvial body which was reconstructed along three transversal geological sections of the valley using bore-hole data. Three subsoil profiles, considered representative of the geological setting along the three reconstructed transversal cross sections, were analysed by seismic noise measurements performed specifically to derive S-waves velocity profiles. The results obtained show a very low velocity (<180 m/s) for the layers T and AGI. 1D modelling of seismic shaking was performed by the code Shake91, in order to evaluate the influence of the low-velocity strata on maximum shear strains induced within the alluvial deposits under the maximum expected seismic action. The results of the numerical modelling indicate that the AGI and the T layers play a key role in: (i) concentrating the maximum shear strain along the subsoil profiles, even though the volumetric threshold is never exceeded; (ii) increasing the maximum shear strain along the subsoil profiles; (iii) causing the resonance frequency of the alluvial fill to assume an almost constant value (about 1Hz) which is quite similar to that measured in the main Tiber River valley, despite a significant change in thickness of the alluvial body along the Grottaperfetta valley.     

Large amplification of ground motion at rock sites within a fault zone in Nocera Umbra (central Italy)

During the two mainshocks of September 26, 1997 inthe Umbria-Marche border a strong-motion accelerographrecorded peak ground accelerations as large as 0.6 g,approximately, in the town of Nocera Umbra, atdistances of 10 to 15 km from the epicentres. Thisvalue is significantly larger than expected on thebasis of the usual regressions with magnitude anddistance. A broad-band amplification up to a factor of10 was consistently estimated in previous papers,using both weak and strong motion data recorded at theaccelerograph site during local moderate earthquakes.To study the cause of this amplification we deployedsix seismologic stations across the tectonic contactbetween the Ceno-Mesozoic limestone and the Mesozoicmarly sandstone where the accelerograph is installed.Seismograms of 21 shallow aftershocks in the magnituderange from 2.2 to 4.0 and a subcrustal M_w = 5.3event are analysed. Regardless of epicentre location,waveforms show a large complexity in an approximately200 m wide band adjacent to the tectonic contact. Thisis interpreted as the effect of trapped waves in thehighly fractured, lower velocity materials within thefault zone.     

Influence of surface/groundwater interaction on pollution by pesticides in farmlands of the Fucino Plain,central Italy

This paper analyses flow and transport of pesticides from the unsaturated zone to groundwater so as to predict concentration of those contaminants in the Fucino Plain’s groundwater, by site investigations and numerical simulations. Pesticides were detected in surface water (peaks of 13 μg L⁻¹) and groundwater (peaks of 0.37 μg L⁻¹). Modelling tools made it possible to identify that pattern of precipitation, organic matter content, and root thickness are the key factors involved in vertical seepage of pesticides. Numerical simulations indicated that a significant fraction of contaminants is leached from the most surficial soil layers through runoff, while only a secondary fraction is mobilised towards groundwater. Likelihood of contaminating deep groundwater is fairly low, whereas surface waters show higher susceptibility. Results of the proposed conceptual hydrogeological model show that pesticides are more likely to be entrained by mixing of stream water with shallow groundwater in periods of high water exploitation from shallow wells.     

Late Pleistocene to Holocene record of changing uplift rates in southern Calabria and northeastern Sicily (southern Italy, Central Mediterranean Sea)

A combination of published and new radiometric dates on uplifted Holocene fossil beaches from northeastern Sicily and southern Calabria (southern Italy) is compared with the altitude of the inner margin of the Last Interglacial (LIg) (Late Pleistocene, 124 ka) and older marine terraces in order to gain a regional-scale outline of uplift rates and their temporal changes in a region which is one of the fastest uplifting sectors of the Central Mediterranean Sea. Late Holocene radiocarbon dates from Ioppolo (southern Calabria) and Ganzirri (northeast Sicily), two newly discovered sites are here presented for the first time. The Holocene uplift rates are highest at St. Alessio and Taormina in eastern Sicily (2.4 mm/y) and at Scilla in southwestern Calabria (2.1 mm/y), two sites located across the Messina Straits and which separate the island of Sicily from mainland Italy. Uplift rates decrease towards the south and north from this centre of uplift. Late Holocene uplift rates show an apparent increase of between 64 and 124% when compared with the longer-term uplift rates calculated from the LIg highstand terraces. Furthermore, we discovered that the locations of fastest Late Pleistocene and Late Holocene uplift rates spatially coincide. To what extent the Holocene increase in uplift rates results from incomplete elastic strain release along the major extensional faults which frame the seismotectonic of the area, or indicate a true change in regional tectonic processes, is not resolved. Nonetheless, the heterogeneity of uplift, with a well-defined centre that crosses the Messina Straits, and its persistence at different time-scales indicates a tight connection between wider regional processes and fault-related displacement in controlling crustal instability in this area.     

Experimental models of extension of continental lithosphere weakened by percolation of asthenospheric melts

Analogue models are used to investigate extension of a continental lithosphere weakened by asthenospheric melts percolating through the upper mantle, a process that has been hypothesised to control the opening of the Ligurian Tethys. Models were performed in a centrifuge apparatus and reproduced, by using materials such as sand and viscous mixtures, extension of 60-km thick, three-layer continental lithosphere floating above the asthenosphere. The percolated lithospheric mantle was assumed to be characterised by a rheological behaviour similar to that of the asthenosphere. Two sets of experiments investigated the influence on deformation of (1) the thickness of the percolated mantle and the associated strength contrast between the normal and weakened lithosphere, and (2) the lateral width of the weakened zone. Model results suggest that mantle percolation by asthenospheric melts is able to promote strong localised thinning of the continental lithosphere, provided that a significant thickness of the lithospheric mantle is weakened by migrating melts within a narrow region. Strain localisation is maximised for percolation of the whole lithospheric mantle and strong strength contrast between the normal and weakened lithosphere. Under these conditions, the thickness of the lithosphere may be reduced to less than 12 km in 3 Ma of extension. Conversely, localised thinning is strongly reduced if the thickness of the percolated zone is ≤1/3 of the thickness of the whole lithospheric mantle and/or the lithosphere is weakened over wide regions. Overall, model results support the working hypothesis that mantle percolation by asthenospheric melts is a controlling factor in the transition from distributed continental deformation to localised oceanic spreading.     

Benthic foraminifera as bio-indicators of trace element pollution in the heavily contaminated Santa Gilla lagoon (Cagliari, Italy)

In order to assess the response of benthic foraminifera to trace element pollution, a study of benthic foraminiferal assemblages was carried out into sediment samples collected from the Santa Gilla lagoon (Sardinia, Italy). The lagoon has been contaminated by industrial waste, mainly trace elements, as well as by agricultural and domestic effluent. The analysis of surficial sediment shows enrichment in trace elements, including Cr, Cu, Hg, Ni, Pb and Zn. Biotic and abiotic data, analyzed with multivariate techniques of statistical analysis, reveal a distinct separation of both the highly polluted and less polluted sampling sites. The innermost part of the lagoon, comprising the industrial complex at Macchiareddu, is exposed to a high load of trace elements which are probably enhanced by their accumulation in the finer sediment fraction. This area reveals lower diversity and higher percentages of abnormalities when compared to the outermost part of the lagoon.     

A Tsunami Detection and Warning-focused Sea Level Station Metadata Web Service

Currently information used to describe sea-level stations (such as location, collection and transmission capabilities, operator identification, etc.) is distributed among databases held by multiple agencies, institutions and organizations. Such information could be used to support detection and warning. However, the Indian Ocean Tsunami of 26 December, 2004 made it clear that such information is not readily accessible, is difficult to use, and is often incomplete. In addressing this issue, agencies within the Pacific region are collaborating to develop a web service to expose station metadata enabling various types of real-time data mining client applications that support decision-making and strategic planning at Tsunami Warning Centers. Because information about sea levels has a broad range of applications, integration of this information in a way that is comprehensive, and enhances its access and use, would have a tremendous impact on lives and livelihoods.     

A comparison of seismic risk maps for Italy

National seismic risk maps are an important risk mitigation tool as they can be used for the prioritization of regions within a country where retrofitting of the building stock or other risk mitigation measures should take place. The production of a seismic risk map involves the convolution of seismic hazard data, vulnerability predictions for the building stock and exposure data. The seismic risk maps produced in Italy over the past 10 years are compared in this paper with recent proposals for seismic risk maps based on state-of-the-art seismic hazard data and mechanics-based vulnerability assessment procedures. The aim of the paper is to open the discussion for the way in which future seismic risk maps could be produced, making use of the most up-to-date information in the fields of seismic hazard evaluation and vulnerability assessment.