Suchergebnisse

The study formed part of the SIMIT project (Integrated Italy-Malta Cross-Border System of Civil Protection) (B1-2.19/11) part-financed by the European Union under the ItaliaMalta Cross-Border Cooperation Programme, 2007–2013. ; The islands composing the Maltese archipelago (Central Mediterranean) are characterized by a four-layer sequence of limestones and clays. A common feature found in the western half of the archipelago is Upper Coralline Limestone (UCL) plateaus and hillcaps covering a soft Blue Clay (BC) layer which can be up to 75 m thick. The BC layer introduces a velocity inversion in the stratigraphy, implying that the VS30 (traveltime average sear wave velocity (VS) in the upper 30 m) parameter is not always suitable for seismic microzonation purposes. Such a layer may produce amplification effects, however might not be included in the VS30 calculations. In this investigation, VS profiles at seven sites characterized by such a lithological sequence are obtained by a joint inversion of the single-station Horizontal-to-Vertical Spectral Ratios (H/V or HVSR) and effective dispersion curves from array measurements analysed using the Extended Spatial Auto-Correlation technique. The lithological sequence gives rise to a ubiquitous H/V peak between 1 and 2 Hz. All the effective dispersion curves obtained exhibit a 'normal' dispersive trend at low frequencies, followed by an inverse dispersive trend at higher frequencies. This shape is tentatively explained in terms of the presence of higher mode Rayleigh waves, which are commonly present in such scenarios. Comparisons made with the results obtained at the only site in Malta where the BC is missing below the UCL suggest that the characteristics observed at the other seven sites are due to the presence of the soft layer. The final profiles reveal a variation in the VS of the clay layer with respect to the depth of burial and some regional variations in the UCL layer. This study presents a step towards a holistic seismic risk assessment that includes the implications on the site effects induced by the buried clay layer. Such assessments have not yet been done for Malta. ; peer-reviewed

In this paper we report the preliminary results of the measurements of ambient noise on a dense network of measurement sites in and around the cities of Valletta and Mdina; two important historical heritage sites in Malta. The city of Valletta is the present capital of Malta and it is inextricably linked to the history of the military and charitable Order of St John of Jerusalem. ; N/A

The Sicily Channel, located on the north-central African plate foreland between Sicily, Tunis and Libya, is characterised by a seismically and volcanically active rift zone. This rift extends for more than 600 km in length offshore from the south of Sardinia to the south-east of Malta. Much of the observations we have today are either limited to the surface and the upper crust, or are broader and deeper from regional seismic tomography, missing important details about the lithospheric structure and dynamics. The project GEOMED (https://geomed-msca.eu) addresses this issue by processing all the seismic data available in the region in order to understand better the geodynamics of the Central Mediterranean. A recently compiled earthquake catalogue for the eastern part of the Sicily Channel Rift Zone (SCRZ) will be presented highlighting offshore active faults in the region, and new results from seismic tomography give an insight of what lies beneath. We measure seismic velocities from across the region using ambient seismic noise recorded on more than 50 stations located on Algeria, Italy (Lampedusa, Linosa, Pantelleria, Sardinia, Sicily), Libya, Malta, and Tunisia. The phase-velocity dispersion curves have periods ranging from 5 to 100 seconds and sample through the entire lithosphere. We find that slow and fast seismic velocities coincide with regional tectonic and topographic features. At short periods, Africa and Italy have slower velocities indicating these areas have thick continental crust in contrast to areas beneath Tyrrhenian and Ionian basins. The central area of the SCRZ has relatively faster velocities suggesting that there is a thinner crust. At longer periods the central area of SCRZ is characterised by slower velocities indicative of warmer temperatures than the surrounding. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 843696. ; peer-reviewed