Inversion of surface wave data for subsurface shear wave velocity profiles characterized by a thick buried low-velocity layer

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-reviewe

Inversion of surface wave data for shear-wave velocity profiles : case studies of a thick buried low-velocity layer in Malta

The islands composing the Maltese archipelago (Central Mediterranean) are characterised by a four layer sequence of limestones and clays, with the Lower Coralline Limestone being the oldest exposed layer. The hard Globigerina Limestone (GL) overlies this layer and is found outcropping in the eastern part of Malta and western part of Gozo. The rest of the islands are characterised by Upper Coralline Limestone (UCL) plateaus and hillcaps covering a soft Blue Clay (BC) layer which can be up to 75 m thick. Thus the BC layer introduces a velocity inversion in the stratigraphy, and makes the Vs30 parameter not always suitable for seismic microzonation purposes. Such a layer may still produce amplification effects, however would not contribute to the numerical mean of Vs in the upper 30m. Moreover, buildings are being increasingly constructed on this type of geological foundation. Obtaining the shear wave (Vs) profiles of the different layers around the islands is the first step needed for a detailed study of local seismic site response. A survey of Vs in each type of lithology and around the islands has never been undertaken.N/

Dynamic characteristics of an active coastal spreading area using ambient noise measurements - Anchor Bay, Malta

Anchor Bay and surrounding regions are located on the northwest coast of the island of Malta, Central Mediterranean. The area is characterized by a coastal cliff environment having an outcropping layer of hard coralline limestone (UCL) resting on a thick (up to 50 m) layer of clays and marls (Blue Clay, BC). This configuration gives rise to coastal instability effects, in particular lateral spreading phenomena and rock falls. Previous and ongoing studies have identified both lateral spreading rates and vertical motions of several millimetres per year. The area is an interesting natural laboratory as coastal detachment processes in a number of different stages can be identified and are easily accessible. We investigate the site dynamic characteristics of this study area by recording ambient noise time-series at more than 30 points, over an area of 0.07 km2, using a portable three-component seismograph. The time-series are processed to give both horizontal-to-vertical spectral ratio graphs (H/V) as well as frequency-dependent polarisation analysis. The H/V graphs illustrate and quantify aspects of site resonance effects due both to underlying geology as well as to mechanical resonance of partly or wholly detached blocks. The polarization diagrams indicate the degree of linearity and predominant directions of vibrational effects. H/V curves closer to the cliff edge show complex responses at higher frequencies, characteristic of the dynamic behaviour of individual detached blocks. Particle motion associated with the higher frequencies shows strongly directional polarization and a high degree of linearity at well-defined frequencies, indicative of normal-mode vibration. The stable plateau areas, on the other hand, show simple, single-peak H/V curves representative of the underlying stratification and no predominant polarization direction. These results, which will be compared with those from other experiments in the area, have important implications for the understanding of ongoing processes in geologically active and unstable coastal environments.peer-reviewe

Georisks in the Mediterranean and their mitigation

An international scientific conference organised by the Seismic Monitoring and Research Unit, Department of Geoscience, Faculty of Science, Department of Civil and Structural Engineering and Department of Construction and Property Management, Faculty of the Built Environment, University of Malta.Part of the SIMIT project: Integrated civil protection system for the Italo-Maltese cross-border area. Italia-Malta Programme – Cohesion Policy 2007-2013This conference is one of the activities organised within the SIMIT strategic project (Integrated Cross-Border Italo-Maltese System of Civil Protection), Italia-Malta Operational Programme 2007 – 2013. SIMIT aims to establish a system of collaboration in Civil Protection procedures and data management between Sicilian and Maltese partners, so as to guarantee the safety and protection of the citizens and infrastructure of the cross-border area. It is led by the Department of Civil Protection of the Sicilian region, and has as other partners the Department of Civil Protection of Malta and the Universities of Palermo, Catania and Malta. SIMIT was launched in March 2013, and will come to a close in October 2015. Ever since the initial formulation of the project, it has been recognised that a state of national preparedness and correct strategies in the face of natural hazards cannot be truly effective without a sound scientific knowledge of the hazards and related risks. The University of Malta, together with colleagues from other Universities in the project, has been contributing mostly to the gathering and application of scientific knowledge, both in earthquake hazard as well as in building vulnerability. The issue of seismic hazard in the cross-border region has been identified as deserving foremost importance. South-East Sicily in particular has suffered on more than one occasion the effects of large devastating earthquakes. Malta, although fortunately more removed from the sources of such large earthquakes, has not been completely spared of their damaging effects. The drastic increase in the building density over recent decades has raised the level of awareness and concern of citizens and authorities about our vulnerability. These considerations have spurred scientists from the cross-border region to work together towards a deeper understanding of the underlying causes and nature of seismic and associated hazards, such as landslide and tsunami. The SIMIT project has provided us with the means of improving earthquake surveillance and analysis in the Sicily Channel and further afield in the Mediterranean, as well as with facilities to study the behaviour of our rocks and buildings during earthquake shaking. The role of the civil engineering community in this endeavour cannot be overstated, and this is reflected in the incorporation, from the beginning, of the civil engineering component in the SIMIT project. Constructing safer buildings is now accepted to be the major option towards human loss mitigation during strong earthquakes, and this project has provided us with a welcome opportunity for interaction between the two disciplines. Finally the role of the Civil Protection authorities must occupy a central position, as we recognize the importance of their prevention, coordination and intervention efforts, aided by the input of the scientific community. This conference brings together a diversity of geoscientists and engineers whose collaboration is the only way forward to tackling issues and strategies for risk mitigation. Moreover we welcome the contribution of participants from farther afield than the Central Mediterranean, so that their varied experience may enhance our efforts. We are proud to host the conference in the historic city of Valletta, in the heart of the Mediterranean, which also serves as a constant reminder of the responsibility of all regions to protect and conserve our collective heritage.peer-reviewe

Evaluation of seismic site response in the Maltese archipelago

The investigation of local ground conditions is an important part of seismic hazard assessment (Fäh et al., 2003). It is now well–established that earthquake ground shaking is not only a function of the earth- quake magnitude and epicentral distance, but also of the site conditions, including soft layers in the sub–soil stratification and topographical features. Local geology can greatly alter the seismic waves from earthquakes by amplifying their amplitude, changing the frequency content and increasing the shaking duration during an earthquake (Kramer, 1996). In fact, several unconsolidated soft sites have suffered significantly greater damage than rock sites. One case was the 1985 Michoacán earthquake which showed low peak ground acceleration near the epicenter, yet caused severe damage in Mexico City, which is found more than 350 km away and is characterized by soft shallow sediments (Campillo et al., 1989).peer-reviewe

An earthquake swarm on the Malta graben, Central Mediterranean, September – November 2020

The seafloor of the Sicily Channel is characterised by an extensional regime, governed by a network of normal and strike-slip fault systems. These faults generate a background level of seismicity that rarely exceeds magnitude 5.0. A number of these faults pass close to the Maltese islands. In particular the Malta graben lies less than 15 km to the south of Malta at its closest point, and the islands have been shaken a number of times by earthquakes originating on this, and other fault systems. In this study we describe the occurrence of a seismic sequence, that started in September 2020 and lasted for several weeks, the largest event having a local magnitude of 4.5 and being strongly felt throughout the archipelago. The sequence was located at a distance of around 23 km south of the eastern tip of Malta. We have used single station polarization analysis at seismic station WDD to estimate epicentral location for all events, down to magnitude 1.2. For the largest events, we have also used conventional network location, utilising phase picks at stations of the Malta Seismic Network and in Southern Sicily. We describe the time evolution of the sequence, felt effects, the public response and the implications for seismic hazard.peer-reviewe

The specific chemical profile of Mediterranean propolis from Malta

Seventeen Maltese propolis samples were studied by GC–MS after silylation. They exhibited the typical Mediterranean chemical profile, rich in diterpene compounds (18–92% of TIC, GC–MS): 32 individual diterpenes were identified; 22 of them were present in each specimen. The other abundant compound group was that of sugars and sugar derivatives. In some samples, however, another compound group was observed (0–12% of TIC, GC–MS); the corresponding mass spectra were consistent with monoand sesquiterpenyl esters of substituted benzoic acids. Two new propolis constituents of this group, daucane diterpene esters of hydroxybenzoic acids, were isolated. Their origin is suggested to be Ferula communis, as they are taxonomic markers for this species. All propolis samples were active against Staphylococcus aureus but only those with high concentrations of terpenyl esters showed antifungal activity against Candida albicans. The present results confirm that Mediterranean propolis is a valuable natural product with potential to improve human health.peer-reviewe

Results from shallow geophysical investigations in the northwestern sector of the island of Malta

We performed geophysical investigations in the northwestern sector of the island of Malta to reconstruct velocity-depth models and provide shear-wave velocity profiles. We have chosen two sites, one located in Rabat (Malta) and another in the Golden Bay area. We used both active (seismic and electrical 2D-tomography, Multichanel Analysis of Surface Waves – MASW) and passive (2D arrays and single-station measurements using ambient noise) geophysical methods. Consistently with previous studies performed in this part of Malta, we have found that both sites are characterised by site resonance in the frequency range 1-2 Hz as an effect of the local lithostratigraphic succession that shows an impedance contrast at about 60-90 m depth. This resonance effect can have important implications on both seismic hazard as well as seismic risk evaluation of the region since the amplified frequency range coincides with the resonance frequencies typical of 5–10 storey buildings which are very diffuse in the Maltese Islands, especially after intense recent urbanization.Published41-484T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

Integrated geophysical and geomatics study at Xrobb l-Ghagin archaeological site : preliminary results

This study reports the results obtained by combining geophysical methods and geomatis techniques to study the Xrobb l-Ghagin archaeological site. We use unmanned aerial vehicle equipped with different sensors in order to reconstruct the 3D digital model of the area with the main goal of obtaining quantitative information. In particular, we used optical and Lidar sensors mounted on our drone and we perform also ground-based topographic survey in order to properly georeferenced the obtained 3D digital model. Geophysics data (e.g. ambient noise vibration, electrical resistivity tomography and ground penetrating radar) have been collected to study potential buried features present at the site. The 3D model and geophysical investigations helped in identifying potential buried archeological structures as well as the mapping of shallow geological features as fractures, faults and caves.peer-reviewe

An affordance-based requirements approach for developing therapeutic artefacts - a case study of speech and language toys

An effort to solve real-world problems through the creation of new or improved products, such as rehabilitation or therapeutic devices, requires a human-centred design approach. Lack of domain knowledge about the use context and accessibility to key experts or end users pose significant challenges to the designer during the task clarification stage in understanding the end-user requirements. This article presents a computer-based design support tool, ACQUAINT-SALTT, based on a prescriptive computer architecture that allows the generation of affordance-based requirements (ABRs) for an emerging family of products known as speech and language therapeutic toys (SALTTs). Considering affordances, the end-user requirements can be detailed as a relationship between the product and the user within a context while keeping the problem as abstract as possible without restricting creativity. A prototype therapeutic toy, Olly Speaks, was developed and evaluated through usability studies carried out with clinicians, caregivers, and pre-schoolers to assess its therapeutic impact both within and outside the clinic.peer-reviewe