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The limestone islands of Malta face high levels of water stress due to low rainfall over a small land area and a high population density. We investigate an innovative, cost-effective approach to groundwater monitoring in an island environment by computing auto- and cross-correlations of ambient seismic noise recorded on short-period and broadband seismic stations. While borehole readings give accurate site-specific water level data of the groundwater across the islands, this technique provides a more regional approach to quantitative groundwater monitoring. We perform the moving window cross-spectral method to determine temporal changes in seismic velocity (δv/v). Comparison of the δv/v with groundwater levels from boreholes and precipitation shows comparable patterns. We find that the variations of the δv/v from auto-correlations are more pronounced than the cross-correlation, and that short-period seismic stations are also sensitive. The δv/v signal deteriorates at longer interstation distances, presumably because paths traverse complex geology. We conclude that changes in the groundwater level found beneath very small islands, even as small as 3 km2, can be detected seismically. Low-cost, easy-to-deploy seismic stations can thus act as an additional tool for groundwater monitoring, especially in places with limited natural water reservoirs, like rivers and lakes, and infrastructure.

The Maltese archipelago lies in the centre of the Sicily Channel (Central Mediterranean), a crustal domain subject to extension since the Miocene and characterised by the presence of several active graben systems in the Plio-Quaternary. This region is affected by diffuse seismicity, in the form of isolated swarms interspersed with long pauses of quiescence and events of modest magnitude, in general not exceeding 4.0. The key features of seismicity are poorly constrained by unfavourable geographical conditions and the scarcity of seismic stations in the area between southern Sicily and Tunisia. Until a few years ago, the only Maltese station operating (WDD of the MedNet network) was the only source of information to characterise local Maltese seismicity. On the other hand, the localisations of major events (M > 4) made by the other monitoring agencies (e.g. Istituto nazionale di Geofisica e Vulcanologia - INGV) are based on traveltime readings from seismograms recorded at great distances from Malta (> 100 km) and are therefore affected by considerable formal errors. Moreover, the only published catalogue of offshore seismicity around the Maltese islands (Seismic Monitoring and Research Group, University of Malta) is based on single station location at WDD up to 2014. This is characterised by considerable uncertainties in hypocentral parameters, in particular a lack of depth information. In consequence, therefore, it is very difficult to obtain information on the geometry and kinematics of active tectonic structures, both offshore and possibly on.In recent years, there has been a considerable increase in the number of seismic stations on the Maltese archipelago: there are currently eight active stations, that make up the Malta Seismic Network (MSN). The study we present is an example of the application of 3-D localisation techniques of Maltese seismicity, which benefits from the recent implementation of the local seismic network. In particular, we focus on the swarm occurring offshore in the period September-October 2020, and characterised by an unusual number of events (> 100), including a main event (M > 4.0) that was strongly felt over the archipelago. We have handpicked P- and S-wave traveltimes for all the events using recordings from the MSN and a selected number of Italian stations according to quality criteria, and inverted them using the Hypoellipse code. The precise earthquake localisations allow us to obtain details of the structures activated during the swarm, together with a more detailed insight into the time evolution of the sequence. Accompanying these analyses, we calculated the moment tensor solutions for some of the largest events (M > 3) and performed a spectral analysis to distinguish the waveform characteristics of events occurring at shallow depths (< 10 km) from those nucleating in the mid-crust (> 15 km depth), as a function of different wavepaths through the crust. The prospect is that in the future it will be possible to better constrain the seismotectonics of the Maltese archipelago and have a more accurate picture of the seismogenic potential of active faults in this sector of the Mediterranean.

Special issue EPOS a Research Infrastructure in solid Earth: open science and innovation .-- 21 pages, 8 figures, Tsunamis constitute a significant hazard for European coastal populations, and the impact of tsunami events worldwide can extend well beyond the coastal regions directly affected. Understanding the complex mechanisms of tsunami generation, propagation, and inundation, as well as managing the tsunami risk, requires multidisciplinary research and infrastructures that cross national boundaries. Recent decades have seen both great advances in tsunami science and consolidation of the European tsunami research community. A recurring theme has been the need for a sustainable platform for coordinated tsunami community activities and a hub for tsunami services. Following about three years of preparation, in July 2021, the European tsunami community attained the status of Candidate Thematic Core Service (cTCS) within the European Plate Observing System (EPOS) Research Infrastructure. Within a transition period of three years, the Tsunami candidate TCS is anticipated to develop into a fully operational EPOS TCS. We here outline the path taken to reach this point, and the envisaged form of the future EPOS TCS Tsunami. Our cTCS is planned to be organised within four thematic pillars: (1) Support to Tsunami Service Providers, (2) Tsunami Data, (3) Numerical Models, and (4) Hazard and Risk Products. We outline how identified needs in tsunami science and tsunami risk mitigation will be addressed within this structure and how participation within EPOS will become an integration point for community development, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

The Sicily Channel, situated on the leading edge of the African plate as it collides with Europe, presents a range of interesting and complex tectonic processes that have developed in response to various regional stress fields. The characterization and interpretation of the seismic activity, however, still presents a challenge. The Maltese islands, lying approximately 100 km to the south of Sicily, are known to have been affected by a number of earthquakes in the Channel, with some of these events estimated to be very close to the islands. Yet, in the absence of nearby seismic instruments, an accurate evaluation and mapping of small magnitude seismicity, and, hence, the identification of unmapped active faults in the region, remains a challenge. This situation is being partially addressed through the deployment of more seismic stations on the Maltese archipelago. The Malta Seismic Network (MSN; International Federation of Digital Seismograph Networks code ML, see Data and Resources), managed by the Seismic Monitoring and Research Group, within the Department of Geosciences, University of Malta, currently comprises eight broadband, three-component stations covering an area of, approximately, 315 km2. Continuous seismic monitoring is possible following upgrades to real-time data transmission and automated epicenter location, coupled with a virtual seismic network established through SeisComP3, and focused mainly on the Mediterranean region. Such a dense national network, besides improving epicentral location in the Sicily Channel, will provide valuable information on microearthquake activity known to occur in close proximity to the islands, which has been very difficult to study in the past. It will also provide opportunities to study shallow crustal structure, site response on different geological substrates, microseismic noise propagation, and effects of anthropogenic activities. Here, we give a technical description of the MSN and an appraisal of its potential.

It is known that local geology can alter the ground motion characteristics at a particular site. Whereas some sites are characterised by outcropping low-velocity layers, others might have buried ones of considerable thickness which might still be a cause of ground motion amplification. The Maltese islands (Central Mediterranean) provide a good case study for profiles with a buried low-velocity layer. Parts of the islands are characterised by Upper Coralline Limestone covering a soft Blue Clay layer of considerable thickness (up to 75 m). This gives rise to a velocity inversion and makes the travel-time average shear-wave velocity (VS) in the upper 30 m not suitable for seismic microzonation purposes. This study presents a comprehensive site response analysis for such areas. Previously obtained shear-wave velocity (VS) profiles were used as an input to the equivalent linear programme SHAKE2000. Maps of amplification factors and 5% damped elastic response spectra confirm that the clay, even when buried under a hard outcropping layer, can still produce significant amplifications at frequencies which are of engineering interest when considering the recent urbanisation patterns. Moreover, the suitability of the VS30 and the travel-time average shear-wave velocity till bedrock parameters as proxies for site response was investigated, with the results highlighting the inadequacy of these parameters for sites characterised by buried low-velocity layers and the necessity for site-specific response studies.

In the past decade, studies to determine amplification effects due to the local geology have been conducted in the Maltese islands (Central Mediterranean) by means of ambient noise techniques. Particular areas of interest include the north and north-western areas of the islands which are characterised by clay, that can reach a thickness of 75 m, buried beneath limestone. This introduces a velocity inversion in the stratigraphy and consistent, characteristic peaks in the H/V spectral ratios. With the expansion of the Malta Seismic Network (MSN) to these geological areas of concern, the possibility of confirming and further investigating the results using empirical data arises. Here we present results, mainly in terms of Standard Spectral Ratio (SSR) and earthquake H/V, using 3 years of earthquake data at three stations of the MSN. In particular we note that the amplifications obtained using the SSR technique are significantly higher than those obtained using both noise and earthquake H/V techniques. While the peaks observed in the H/V spectra are also reproduced in the SSR curves using earthquake data, the latter exhibit important additional peaks at frequencies below 1 Hz, whose amplitude may be as high as 30. By separating the earthquake data set on the basis of distance from the islands, we show that the amplification is source-dependent, and that the high amplification values originate from larger, more distant earthquakes in the Hellenic arc. This has important implications for seismic hazard determination.

Malta, a small island nation in the centre of the Mediterranean, is deemed as the European country facing the highest stress on its water resources. Malta has a semi-arid climate with approximately 550 mm of annual rainfall over an area of ∼315 km2 and a very high population density. Consequently, 80% of the water used in the Maltese agricultural sector is directly abstracted from groundwater resources via boreholes or underground galleries. To improve the groundwater monitoring in Malta, which currently depends on a network of in situ borehole readings, we analyse ambient seismic noise data recorded on the Malta Seismic Network (MSN) as part of the project SIGMA (Seismic Imaging of Groundwater for Maltese Aquifers). We investigate temporal changes in seismic velocity as an indication of the variability of water in underground rocks. Water-saturated rocks have an increased pore pressure, which, in turn, leads to the opening of cracks in the rock that reduces the contact area between different grains of rock leading to a decrease in seismic velocity. We compiled the seismic data from each station of the MSN (Galea et al., 2021, SRL) and the FASTMIT experiment (Bozionelos et al., 2019, Xjenza) consisting of a combination of eight broadband and six short-period, three-component seismic stations for the years 2017-2020. The data was pre-processed by demeaning, tapering and merging into a 1-day long trace, which were then band-pass filtered and decimated or downsampled. Power Spectral Density (PSD) charts for the data show that most microseisms energy has a period range of 1-10s. We therefore test different filtering bands encompassing this frequency range. We perform auto and cross-correlation of noise data from 78 station pairs. We perform stacking for 1, 5 and 10 days for smoother cross-correlation functions. We then compute the time delays using the Moving-Window Cross-Spectral analysis (Clarke et al., 2011, JGI). Finally, the change in velocity (dv/v) is determined from the calculated time delays. The algorithm was run via the software package MSNoise (Lecocq et al., 2014, SRL).We find that the changes in the dv/v time series (~±0.01%) have seasonal patterns, where a negative dv/v in the winter period and a positive dv/v in summer is observed. We compare the auto and cross-correlations with the time series of groundwater measurements from nearby boreholes (ranging from 0.25-3.3m above mean sea level) to investigate the correlation between them. We also take into consideration the NW-SE geology of the island, distinguished by an impermeable layer between the geological strata (Blue Clay) in the north. We find that long paths traversing across different geological layers show weak correlations. We present the tests performed and the results showing the extended spatial coverage for groundwater monitoring in conjunction with the borehole data for the Maltese Islands. Project SIGMA is financed by the Energy and Water Agency under the National Strategy for Research and Innovation in Energy and Water (2021-2030).

The Cathedral of St John in Valetta, Malta, represents a unique monument of historical importance. In this paper, we present the results of a detailed ground penetrating radar campaign performed in this Cathedral. The campaign was aimed at investigating the distribution of buried tombs under the mosaic floor of the main nave and of the lateral chapels of the Co‐Cathedral. The floor of the church shows a continuity of grave inscriptions in the main nave as well as in all lateral chapels. It was suspected, based on available historical documents, that only a part of the present‐day inscriptions corresponds to the original sepultures or tombs. The ground penetrating radar results presented here further highlight this conjecture and offer additional information regarding this important monument.

In recent years, we have been witnessing the widespread use of low-cost, increasingly high-performance Unmanned Aerial Vehicles, or UAVs, equipped with a large number of sensors capable of extracting detailed information on several scales and in an immediate manner. This study was motivated by the need to perform a geological survey in an area with difficult physical access, and to compare the results with those from conventional surveys. Here we used a Multirotor UAV equipped with a high definition RGB camera and the digital photogrammetry technique to reconstruct a three-dimensional model of the Selmun promontory, located in the northern part of the island of Malta (central Mediterranean Sea). In this area, the evident cliff retreat is linked to landslide processes involving the outcropping geological succession, characterized by the over position of stiff limestones on ductile clays. Such an instability process consists of a lateral spreading associated with toppling and fall of different-size rock blocks. Starting from the 3D model obtained from the UAV-photogrammetry, a digital geological-structural survey was performed in which we identified the spatial geometry of the fractures that characterize the area of the Selmun promontory by measuring strike, dip and dip direction of the fractures with semi-automatic digital tools. Furthermore, we were able to measure the size and volume of singularized rock masses as well as cracks, and their sizes were mapped in a GIS environment that contains a large number of digital structural measures. It is the first application of this type for the Maltese islands and the results obtained with this innovative digital methodology were then compared with those of the traditional field survey of the same area acquired during a previous campaign. This study demonstrated how the innovation of digital geological surveying lies in the possibility of mapping areas and geological features not detectable with traditional methods, mainly due to the high risk associated with the stability of the cliff or, more generally, the inaccessibility of some sites, therefore allowing the user to operate in safety and to detect in detail the most remote rocky outcrops.

Earthquake ground motion is dependent on various factors, including local ground conditions. Whilst many studies have characterized the effect of having outcropping “soft” geological layers which have the ability to amplify ground motion, there is minimal literature on the effect of having such layers embedded between two harder layers. This situation creates a seismic wave velocity inversion. The Maltese islands (Central Mediterranean) present a good opportunity for the study of velocity inversion as almost half of the islands are characterized by a thick buried layer of clay. The results presented in this chapter are a combination of studies which have been conducted on the Maltese islands, using non-invasive geophysical prospecting techniques in areas characterized by a thick buried low-velocity layer, to characterize the response of earthquake ground shaking in such geological situations., peer-reviewed

We present the first instrumental earthquake catalogue for Malta using 20 years of broadband seismic data recording. For about two decades Malta had only one station (WDD) which formed part of regional networks. Its location in the eastern part of the Sicily Channel puts the station at the periphery of these networks with the result that weak, off-shore earthquakes that occur between Malta and Libya, are in many cases recorded on WDD only and are undetected or unlisted by the regional networks. We adopt the single-station earthquake location method to process the continuously recorded seismic data of station WDD from 1995 to 2014. We combine our earthquake list with the bulletins of INGV and IRIS to catalogue 550 earthquakes. We statistically quantify the uncertainties of the earthquake epicentres and establish that many earthquake locations differ from INGV/IRIS locations by < 20 km at local epicentral distances from WDD and that earthquake magnitudes determined from single station are overestimated by 0.2. We find that the Malta and Linosa grabens are seismically active, and a high concentration of seismic activity is located 80–120 km SSE of Malta at around 35° N latitude. Closer to land, clusters of epicentres are also located, within 40 km to the east and south of Malta. This new earthquake catalogue shows that the regional seismicity is higher than previously observed and that a number of submarine structures in the area are active as part of the ongoing extension in the Sicily Channel., peer-reviewed

The Victoria Lines Fault (island of Malta) is a >15 km-long and N260°-striking segmented normal fault-system, which is probably inactive since the late Pliocene. In the westernmost part, the Fomm Ir-Rih segment displays comparable geologic throw and escarpment height (~150–170 m), moreover its hangingwall hosts thin patches of Middle Pleistocene clastic continental deposits (red beds), which are poorly preserved elsewhere. We acquired two seismic transects, by collecting ambient vibration recordings, processed by using horizontal-to-vertical spectral ratios, complemented by one high-resolution 2-D refraction tomography survey crossing this fault where it is locally covered by red beds and recent colluvial deposits. We found a resonance peak at ~1.0 Hz in the hangingwall block, whereas clear peaks in the range ~5.0–10.0 Hz appear when approaching the subsurface fault, and we relate them to the fractured bedrock within the fault zone. The best-fit tomographic model shows a relatively high-Vp shallow body (Vp 2200–2400 m/s) that we relate to the weathered top of the Miocene Upper Coralline Limestone Fm., bounded on both sides by low-Vp regions ( 230 m/s above the weathered top-bedrock. Our results depict a clear seismic signature of the Victoria Lines Fault, characterized by low seismic velocity and high amplification of ground motion. We hypothesize that, during the Middle Pleistocene, faulting may have affected the basal part of the red beds, so that this part of the investigated complex fault-system may be considered inactive since ~0.6 Myr ago.

Seismic site response analysis is an important procedure used to obtain parameters such as peak ground motion values. These are needed by the engineering community for reliable seismic design, analysis and retrofitting of structures. One of the inputs for such analysis is the shear-wave velocity profile ( V S ) of the site. Surface wave methods are increasingly being used to obtain these profiles. However, these are characterised by uncertainties related to the measurements and the model used for interpretation (e.g. the inadequacy of 1D assumption). Moreover, the inversion procedure is also highly non-unique because numerous profiles are able to fit the experimental dispersion curve/s. Even though the uncertainties are not commonly included in site response analysis, their propagation from the dispersion curves to the inversion procedure can lead to significant differences in the site response analysis results. In a previous study, V S profiles were obtained at 20 sites on the Maltese islands (Central Mediterranean) using the Horizontal-to-Vertical Spectral Ratio (H/V), the Extended Spatial Auto-Correlation (ESAC) technique and the Genetic Algorithm. The geology of all the sites is characterised by a layer of clay, which can be up to 75 m thick, buried under the Upper Coralline Limestone, a fossiliferous coarse grained limestone. The effect of a thick buried low V S layer, which gives rise to a velocity inversion, in ground response is not well understood and/or documented. The above concerns raised the need for carrying out tests with the aim of better clarifying and understanding which parameters have the most influence on site response analysis and to assess the consequences of inversion non-uniqueness on ground motion amplification and response. From the first part of the study, it was observed that the thickness of the clay layer has the greatest effect on the ground response results. By obtaining ground motion parameters from equivalent profiles, the consequences of the uncertainty in the inversion process, which can be serious at times, are revealed.

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 characterized 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. We also highlight the importance of performing seismic velocity measurements even at rock sites. As an example, the Golden Bay site would be classified as class “A” following the EuroCode EC8 when considering only the outcropping lithology represented by limestone rocks. Conversely the subsoil characterization provided by this study has revealed that this site falls in the EC8 class “B”, stressing the importance of direct geophysical measurements since the a-priori assignment to A-class could lead to wrong estimates in evaluating the site response.

Għajn Ħadid Tower represents an important cultural heritage of Malta (Central Mediterranean Sea). This tower, built in 1658 on the Selmun Promontory on the northeast coast of the island, was severely damaged by the 1856 Crete earthquake (MW 7.7). The area where the tower ruins stand is involved in a significant landslide process of lateral spreading. During 2015 and 2016, engineering geological surveys and passive seismic measurements were carried out to evaluate the main resonance frequencies of the promontory and monitor the gravity-induced instability process. Seismic ambient noise measurements in the area of the tower show significant horizontal-to-vertical spectral ratio (HVSR) resonance peaks at 3.3–3.5Hz characterized by linearity and polarization of the particle motion. These features are not present in the measurements carried out on the stable zone and can be related to the vibrational behaviour of the unstable rock block. Additionally, a shear wave velocity profile for the area was obtained by using a seismic array. At the same time, two different approaches were tested for monitoring the landslide process: (i) a Seismic Navigating System (SNS) array detected 20 natural microseismic events caused by the ongoing landslide process; (ii) few-days continuous seismic noise measurements on the unstable rock block were used to carry out a preliminary study of the variation of specific parameters over time.

In this contribution the results of a Ground Penetrating Radar (GPR) prospecting survey in the chapel of Aragon, within the Co-cathedral of St. John in Valletta, Malta, are presented. These results are part of a wider campaign carried out with the final goal of prospecting the subsurface in order to identify voids and/or graves beneath the tomb slabs composing the complex flooring of the entire Co-Cathedral. In particular, in the chapel of Aragon results were compared with historical documents in order to identify possible burial sites. In addition, a 3D digital model of the Chapel has been obtained through the use of photogrammetric techniques with the ultimate goal of integrating the GPR results within the digital model.

The cliff slope of the Selmun Promontory, located in the Northern part of the island of Malta (Central Mediterranean Sea) close to the coastline, is involved in a landslide process as exhibited by the large block-size talus at its bottom. The landslide process is related to the geological succession outcropping in the Selmun area, characterized by the overposition of a grained limestone on a plastic clay, that induces a lateral spreading phenomenon associated with detachment and collapse of different-size rock blocks. The landslide process shapes a typical landscape with a stable plateau of stiff limestone bordered by an unstable cliff slope. The ruins of Ghajn Hadid Tower, the first of the 13 watchtowers built in 1658 by the Grand Master Martin de Redin, stand out on the Selmun Promontory. The conservation of this important heritage site, already damaged by an earthquake which struck the Maltese Archipelago on 1856 October 12, is currently threatened by a progressive retreat of the landslide process towards the inland plateau area. During 2015 and 2016, field surveys were carried out to derive an engineering geological model of the Selmun Promontory. After a highresolution geomechanical survey, the spatial distribution of the joints affecting the limestone was obtained. At the same time, 116 single-station noise measurements were carried out to cover inland and edge of the limestone plateau as well as the slope where the clays outcrop. The obtained 1-hour time histories were analysed through the horizontal to vertical spectral ratio technique, as well as polarization and ellipticity analysis of particle motion to define the local seismic response in zones having different stability conditions, that is, related to the presence of unstable rock blocks characterized by different vibrational modes. The results obtained demonstrate the suitability of passive seismic geophysical techniques for zoning landslide hazard in case of rock slopes and prove the relevance of anisotropies in conditioning the polarization of vibrational modes for dislodged rock masses. © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society.

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-reviewed

The Maltese islands in the Central Mediterranean are composed of a simple 4-layer sedimentary sequence of Oligocene–Miocene limestones and clays, highly disturbed by synsedimentary to recent tectonic activity, so that the surface outcrops present a variety of lithologies, as well as complex geomorphology. The seismic history of the islands includes a number of damaging earthquakes, the largest intensity since 1500 being EMS-98 VII–VIII in 1693. In this study a first seismic site response survey was undertaken over the whole archipelago, by recording ambient noise and applying the H/V (Horizontal-to-Vertical Spectral Ratio) technique. The results show a clear correlation between site response and outcrop geology. In general, where the two oldest layers, the Lower Coralline Limestone and the Globigerina Limestone outcrop, the H/V is flat above 0.5 Hz. A thick layer of clays and marls (Blue Clay) deposited originally on top of the Globigerina Limestone is preserved in the western half of the archipelago, and this strongly influences the site response. Where the clay outcrops at the surface, it produces the highest amplitudes, and peak frequencies ranging between 2 and 5 Hz, whereas all sites on the hard Upper Coralline Limestone, which overlies the clay in many regions, still exhibit H/V peaks at a consistent frequency between 1 and 2 Hz, which we attribute to the presence of the underlying clay. Results were validated by a second measurement survey and numerical modelling. The site effect induced by the clay layer has implications for the assessment of seismic risk, as well as for the validity of using VS30 as a proxy for site class and site amplification.

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

Landslide phenomena involve the northern coast of Malta, affecting in particular the urban area of Xemxija. Limestones overlying a clayey formation represent the shallower lithotypes that characterize the surficial geology of this area, where lateral spreading phenomena and rockfalls take place. Ambient noise records, processed through spectral ratio techniques, were analysed in order to characterize the dynamic behavior of the rock masses affected by the presence of fractures linked to the landslide body existing in the area. Experimental spectral ratios were also calculated after rotating the horizontal components of the seismic signal, and a direct estimate of the polarization angle was also performed in order to investigate the existence of directional effects in the ground motion. The results of the morphologic survey confirmed the existence of large cliff-parallel fractures that cause cliff-edge and unstable boulder collapses. Such phenomena appear connected to the presence, inside the clay formation, of a sliding surface that was identified through the interpretation of the noise measurement data. The boundaries of the landslide area appear quite well defined by the pronounced polarization effects, trending in the northeastern direction, observed in the fractured zone and in the landslide body in particular., peer-reviewed

Bidirectional phase-stabilized optical fiber links allow state-of-the-art optical clocks to be compared on a continental scale. However, intercontinental comparisons are still based on satellite techniques, preventing optical clocks from being compared without degradation. We performed the first optical frequency transfer experiment over submarine links, measuring levels of environmentally induced noise substantially lower than for on-land links. From these measurements, we predict the transfer stability that can be achieved over transoceanic links. We also performed the first optical frequency transfer over fibers installed in power cables, observing optical perturbations caused by the high-voltage field. Finally, we show that the low background noise of fibers on the seafloor allows applications in geophysical sensing.

In the last forty years, Malta has only experienced a few occasional tremors from local or regional earthquakes, with only some being reported briefly in the local newspapers. These reports gave limited qualitative and quantitative information about the shaking experience felt across the islands. The Seismic Monitoring and Research Unit at the University of Malta has put in place a ‘Did you feel an earthquake?’ online questionnaire in order to start gathering information from locally felt earthquake related shaking. On Easter Sunday 24th of April 2011 windows rattled, doors shook open, and furniture shifted in many homes across Malta when a series of earthquakes occurred 38 km off the eastern coast. In total, the SMRU located 15 earthquakes with magnitudes ranging from M L 1.8 to 4.1 over a period of 4 days. A total of 489 felt reports were submitted through the online questionnaire. The compilation of the data is a first of its kind for the Maltese islands. Here we present a summary of the reports following the main shock. A maximum intensity value IV on the European Macroseismic Scale was assigned. No structural damage was reported. The data reflects the demographics as well as the different types of buildings found across the archipelago., peer-reviewed

On December 28, 1908 at 5:20 a.m. local time, a devastating earthquake (Mw = 7.2) struck Southern Italy along the Messina Strait (Fig. 1). This event caused severe ground shaking throughout the region and triggered a local tsunami. As result the cities of Messina along Sicily’s coast and Reggio di Calabria were completely destroyed (Baratta, 1910) causing more than 120,000 fatalities and with many left without shelter. The effects of the earthquake were felt within a 300-kilometer radius. Rescuers searched through the rubble for weeks, and people were still being pulled out alive days later, but thousands remained buried there. The 1908 earthquake had a significant impact on buildings and people and local communities which were displaced. The Maltese experience of the Messina 1908 earthquake relied on communication which reached Malta after the event. The assessment of the Maltese experience of the Messina Earthquake has so far been carried out with reference to published newspaper reports and other brief accounts including Herbert Ganado’s Rajt Malta Tinbidel. Alfons Maria Galea a Maltese author and filanthropist published a book in Maltese on the earthquake and its devastating effects in the popular educational series il-Kotba tal-Mogħdija taż-Żmien just a few weeks after the event. The book is a vivid account of the destruction caused by the earthquake, the suffering of the survivors and the reaction of the population in reviving the city. The document presents first-hand accounts of the events in sufficient detail to give a clear picture of the severity of the event, extents of the damage and impact on the population. It is mostly based on accounts received by Galea from persons in institutions including religious orders in Sicily who he knew. Newspaper reports in Malta and other countries together with Galea’s book present clear first-hand accounts of this event and provide information on the building deficiencies and damage, limitations of communication infrastructure during that period, limits to timely emergency response to support the population and emergency action at the beginning of the 20th century., peer-reviewed

The ground motion that can be recorded at the free surface of a terrain is the final result of a series of phenomena that can be grouped into three fundamental typologies: the source mechanism, the seismic wave propagation till the bedrock interface below the investigated site and the site effects (Fig. 1). The first two features define the kind of seismic input whereas the third represents all modifications that can occur as a consequence of the interaction between seismic waves and local characteristics of the investigated site. The physical and mechanical properties of terrains as well as their morphologic and stratigraphic features appreciably affect the characteristics of the ground motion observed at the surface. The whole process of modifications undergone by a given seismic input in terms of amplitude, frequency content and duration, as a consequence of local characteristics, is generally termed the “local seismic response”. It is indeed well known that the spectral composition of a seismic event is modified first during the source-bedrock path (attenuation function), and second, when the seismic input interacts with the soft terrains layered between the bedrock and the free surface (Fig. 1a). This latter effect, significantly changes the spectral content so that it is extremely important for estimating the final input to which all structures built in the study area will be subjected., peer-reviewed

This research was carried out using computational facilities procured through the European Regional Development Fund, Project ERDF-080 ‘A supercomputing laboratory for the University of Malta (http://www.um.edu.mt/research/scienceeng/erdf_080)., Large earthquakes that have occurred in recent years in densely populated areas of the world (e.g. Izmit, Turkey, 17 August 1999; Duzce, Turkey, 12 November 1999; Chi-Chi, Taiwan 20 September 1999, Bhuj, India, 26 January 2001; Sumatra 26 December 2004; Wenchuan, China, May 12, 2008; L’Aquila, Italy, April 6, 2009; Haiti, January 2010 Turkey 2011) have dramatically highlighted the inadequacy of a massive portion of the buildings erected in and around the epicentral areas. For example, the Izmit event was particularly destructive because a large number of buildings were unable to withstand even moderate levels of ground shaking, demonstrating poor construction criteria and, more generally, the inadequacy of the application of building codes for the region. During the L’Aquila earthquake (April, 06, 2009; Mw=6.3) about 300 persons were killed and over 65,000 were left homeless (Akinci and Malagnini, 2009). It was the deadliest Italian earthquake since the 1980, Irpinia earthquake, and initial estimates place the total economic loss at over several billion Euros. Many studies have already been carried out describing the rupture process and the characteristics of local site effects for this earthquake (e.g. D’Amico et al., 2010a; Akinci et al., 2010). It has been observed that many houses were unable to withstand the ground shaking., peer-reviewed

SUMMARY A set of high quality aftershock data recorded on a closely spaced network of seismographs at Wellington, New Zealand, has enabled the determination of apparent wavefront speeds and azimuths controlled by the subducting Pacific plate. The former were very high (8.7 f 0.2 km s-') and the latter were shifted by as much as 10" eastward of the source-receiver line, which made an angle of about 12" downdip with the strike of the subducted slab beneath the North Island. The wavefront characteristics are modelled in terms of head-wave propagation along a curved refractor with a cylindrical geometry, using both a simple geometrical approach as well as a 3-D ray tracing program. With this structure, it is not possible to explain both the high apparent velocity as well as the large azimuthal anomaly, without the presence of an unusually high P-wave velocity of around 9.0 km s-' within the upper mantle of the subducted Pacific plate. Such high velocities, although reported previously in the North Island, are very hard to reconcile with known upper mantle rock velocities. Other alternatives were sought to explain the apparent slowness. In particular, modifications in the slab curvature along strike, according to proposed models, were found to contribute to the high wavefront speeds, but very high intrinsic P-velocities (8.8 km s-l) are still required. From slowness vector data, it is not possible to constrain the depth at which such velocities have to exist; however, traveltime considerations limit them to depths between the base of the subducted oceanic crust to about 12 km deeper.

A historical catalogue of felt earthquakes in the Maltese islands has been compiled dating back to 1530. Al- though no fatalities were officially recorded during this time as a direct consequence of earthquake effects, serious damage to buildings occurred several times. In the catalogue time period, the islands experienced EMS-98 intensity VII-VIII once (11 January 1693) and intensity VII, or VI-VII five times. The northern segment of the Hyblean-Malta plateau is the source region which appears to pose the greatest threat, although large Greek events and lower magnitude Sicily Channel events also produced damage. Estimates of return periods for intensity ≥V are presented, and it is shown that expected peak ground accelerations justify the implementation of, at least, minimum anti-seismic provisions. The rapid and continual increase in the local building stock on the densely-populated islands warrants the implementation of an appropriate seismic building code to be enforced., peer-reviewed

Summarization: In this article an overall procedure is proposed, for the numerical investigation of the structural response of ancient monuments made of megalithic stones. The procedure is implemented on the middle temple of the Mnajdra Megalithic structure, listed as UNESCO World Heritage and regarded as one of the oldest structures from early civilization dating back to about the 4th millennium BC. First, the geometry of the structure is obtained from a point cloud using terrestrial laser scanning. To investigate and estimate the material properties of the structural system, parametric eigenvalue analysis is conducted using the finite element method and results are compared with experimental data from ambient vibration sensors applied to the same structure. The structural response of the system is then evaluated, using nonlinear finite element analysis. Variation of unilateral, frictional contact conditions between megaliths in a preceding analysis step may influence the eigenvalue analysis which follows, indicating the importance of the proper consideration of contact conditions, on the response of the structure. An old collapse in a part of the structure is also reproduced by the numerical model. The proposed scheme, which combines numerical analysis and experimental vibration output to derive structural properties of the monument, can be used in similar structural systems, consisting of multi-body megalithic stones. Presented on: Journal of Cultural Heritage

Summarization: The Maltese Megalithic Temples, are among the oldest free-standing structures of such complexity in the world. The Mnajdra megalithic structures, date to the 4th millennium BCE, are of international significance as a UNESCO World Heritage site. The appraisal of structural performance is necessary to safeguard these monuments. Towards this direction, assessment of the material properties of Mnajdra megalithic monument in Malta is considered by means of finite element analysis and comparison with experimental research. First, the geometry of the monument has been imported from laser-scanner data provided by the local authorities. Then, experimental microtremor measurements are compared with parametric eigenvalue finite element analysis, for the determination of the proper material properties. Παρουσιάστηκε στο: 7th International Conference on Structural Engineering, Mechanics and Computation

The Italian PLS-“Piano Lauree Scientifiche” project has among its objectives the improvement of school-university cooperation, with a special emphasis on in-service teacher professional development. In pursuit of such goal, growing resources and an increasing effort to strengthen the collaboration with schools have been employed in the past years. A working group within the physics section of the PLS project has developed a questionnaire to inquire about teacher formation activities organized during the past five years. After extensive revision and testing, the university network of PLS-Physics was asked to fill the questionnaire. The resulting data from 139 initiatives provides an interesting perspective on the kind of activities, organizational choices and educational priorities for teacher professional development within the PLS-physics project.

The last twenty years of research in Physics Education have underlined the important role of the so-called Phys-Math interplay. The lack of students’ ability, at different ages and levels, to construct the mathematical model of physical processes or to describe the physical meaning of mathematical constructs has been emphasized. The main research work is currently attributing this difficulty to the way in which these subjects are taught, with a direct connection to the teachers’ Pedagogical Content Knowledge (PCK). The purpose of the present investigation is to examine the role of the physics teachers’ PCK in this interplay at the early stage of the physics study. To this aim, teachers’ and classes’ activities in different high schools have been observed. We report here the results of such observations, focusing in particular to the integration of the two disciplines’ languages.