Earthquake ground-motion simulations for the Maltese Archipelago

The main goal of this paper is to provide earthquake ground motion simulations for particular earthquake scenarios, in terms of ground motion parameters for the Maltese islands. We used a stochastic approach to simulate high-frequency strong-ground motions, using an extended-source model code. This code was developed for earthquake simulations using stochastic finite-fault modelling and a dynamic corner frequency approach. The extended-source model code is a reliable and practical method to simulate ground motion records of moderate and large earthquakes especially in regions where structural damage is expected, but sparse ground motion recordings are available. In this paper, we show that in the Maltese archipelago, the ground motion from the repeat occurrence of historically recorded earthquakes, or from other potential sources, coupled with existing geological conditions and building typologies has the potential to cause significant structural damage in the area.peer-reviewe

Seismic history of the Maltese Islands and considerations on seismic risk

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-reviewe

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

Prediction of High-Frequency Ground Motion Parameters Based on Weak Motion Data

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. Building earthquake-resistant structures and retrofitting old buildings on a national scale may be extremely costly and may represent an economic challenge even for developed western countries, but it is still a very important issue (Rapolla et al., 2008). Planning and design should be based on available national hazard maps, which, in turn, must be produced after a careful calibration of ground motion predictive relationships (Kramer, 1996) for the region. Consequently, the assessment of seismic hazard is probably the most important contribution of seismology to society. The prediction of the earthquake ground motion has always been of primary interest for seismologists and structural engineers. For engineering purposes it is necessary to describe the ground motion according to certain number of ground motion parameters such as: amplitude, frequency content and duration of the motion. However it is necessary to use more than one of these parameters to adequately characterize a particular ground motion. Updating existing hazard maps represents one of the highest priorities for seismologists, who contribute by recomputing the ground motion and reducing the related uncertainties. The quantitative estimate of the ground motion is usually obtained through the use of the so-called predictive relationships (Kramer, 1996), which allow the computation of specific ground-motion parameter as a function of magnitude, distance from the source, and frequency and they should be calibrated in the region of interest. However this is only possible if seismic records of large earthquakes are available for the specific region in order to derive a valid attenuation relationship regressing a large number of strong-motion data (e.g. Campbell and Bozorgnia, 1994; Boore et al., 1993; Ambraseys et al., 1996, Ambraseys and Simpson, 1996; Sabetta and Pugliese, 1987, 1996; Akkar and Bommer 2010). For the Italian region the most used attenuation relationships are those obtained by Sabetta and Pugliese (1987, 1996) regressing a few data recorded for earthquakes in different tectonic and geological environments. It has been shown in several cases that it is often not adequate to reproduce the ground motion in each region of the country using a single model. Furthermore the different crustal properties from region to region play a key role in this kind of studies. However, the attenuation properties of the crust can be evaluated using the background seismicity as suggested by Chouet et al. (1978) and later demonstrated by Raoff et al. (1999) and Malagnini et al (2000a, 2007). In other words, it becomes possible to develop regionallycalibrated attenuation relationships even where strong-motion data are not available. One of the purposes of this work is to describe quantitatively the regional attenuation and source characteristics for constraining the amplitude of strong motion expected from future earthquakes in the area. In this work we describe how to use the background seismicity to perform the analysis (details in Malagnini et. 2000a, 2007). In particular, this chapter describes the procedures and techniques to study the ground motion and will focus on describing both strong motion attenuation relationships and the techniques used to derive the ground motion parameters even when strong ground motion data are not available. We will present the results obtained for different regions of the Italian peninsula, showing that the attenuation property of the crust and of the source can significantly influence the ground motion. In addition, we will show that stochastic finite-fault modeling based on a dynamic frequency approach, coupled with field investigations, confirms to be a reliable and practical method to simulate ground motion records of moderate and large earthquakes especially in regions prone to widespread structural damage

Dynamic properties of buildings evaluated through ambient noise measurements

It is well known that the damage level and its distribution during an earthquake is due to the combined effects of seismic hazard in the investigated area, the features of the local site response, based on the near–surface and subsurface ground conditions, as well as on the dynamic features of the erected buildings. The extent of building dam- age and its distribution is indeed tightly linked to the combined effect of local site response and the dynamic features of the human-made structures. The dynamic properties of a building are usually described through its natural frequency (or period T) and the damping ratio (ζ ), the latter representing the energy loss of an oscillating system. The damping ratio is important in seismic design since it allows to evaluate the ability of a structure to dissipate the vibration energy during an earthquake. Such energy causes a structure to have the highest amplitude of response at its fundamental period, which depends on the structure’s mass and stiffness. The knowledge of damping level and fundamental period of the building is therefore particularly important for estimating the seismic base shear force F in designing earthquake resistant structures.peer-reviewe

Overview of the seismic hazard in the Sicily channel archipelagos

A joint Italo–Maltese research project (Costituzione di un Sistema Integrato di Protezione Civile Transfrontaliero Italo–Maltese, SIMIT) was financially supported by the European community. One of the aims of SIMIT was to improve the geological and geophysical information in Lampedusa and in Malta and ultimately to mitigate natural hazards. Although this region lies on the Sicily Channel Rift Zone, a seismically active domain of Central Mediterranean, the knowledge about seismotectonic and seismic hazard is not satisfactory. At present, seismic hazard assessment (SHA) for Italy (MPS Working Group, 2004), Tunisia (Ksentini and Romdhane, 2014) and more generally for whole European areas (Giardini et al., 2013) do exist, whereas no specific SHA for the Sicily channel archipelagos are available. The Sicily Channel appears to be a region of moderate seismic activity, with the seismicity mainly located in the surrounding areas (Fig. 1). For the Malta archipelago a first catalogue, listing historical and felt earthquakes, was made by Galea (2007), whereas the Database Macrosismico Italiano (DBMI11; Locatiet al., 2011) does not list any data as regards earthquakes felt in Lampedusa. For this reason, in the present study, a theoretical seismic history was derived (Fig. 2) for Lampedusa and Malta, using the European–Mediterranean Earthquake Catalogue (EMEC) (Grünthal and Wahlström, 2012) and the attenuation relationship for macroseismic intensity data by Pasolini et al. (2008). The two study areas do not appear to have been affected by strong earthquakes occurring in the Sicily channel, but they were somehow struck by major earthquakes occurring in the surrounding area.peer-reviewe

Seismic site response of unstable steep slope using noise measurements : the case study of Xemxija Bay area, Malta

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-reviewe

Seismic site response in Siracusa

In the frame of the Italo–Maltese research project (Costituzione di un Sistema Integrato di Protezione Civile Transfrontaliero Italo–Maltese, SIMIT), researches financially supported by the European Community were performed in the area between the south–eastern Sicilian coast and the islands of Lampedusa and Malta. Aim of these stud- ies is to mitigate natural hazards and to develop the geological and geophysical information in the investigated region. The damage to buildings further to a seismic input is tightly linked, besides their vulnerability, to both the characteristics of the maximum acceleration and frequency of the ground motion, as well as to the features of surface geology. From this point of view, the geophysical and geotechnical characterization of the soil conditions, down to the bedrock, is very important in order to identify the site effects, in terms of fundamental frequencies, for a correct planning of earthquake resistant structures.peer-reviewe

The role of slope instability on directional site effects observed at Fekruna Bay, Malta

The Maltese Archipelago is situated in the Mediterranean Sea, about 290 km NE of Tunisia and 90 km South of Sicily. It consists of three major islands: Malta and Gozo, the southerly and northerly islands respectively, and Comino which lies in the Comino straits separating the two largest islands. In order to better preserve the historical heritage, landscapes, and coastal areas and to promote tourism activities it has been proposed that the archipelago might be considered as an open air laboratory. In this context multidisciplinary studies integrating geology, geotechnical earthquake engineering, geomorphology as well as history and archeology were undertaken in order to develop and test methodologies for the assessment of the relationship between physical environment and cultural heritage (e.g., Soldati et al., 2008). The paper focuses on an integrated study about geomorphology and seismic site response in the Fekruna bay, in the area of Xemxija (northen-eastern part of the Malta Island, Fig. 1).N/

Seismic site response in Lampedusa

In the frame of a joint Italo–Maltese research project (Costituzione di un Sistema Integrato di Protezione Civile Transfrontaliero Italo– Maltese, SIMIT), financially supported by the European Community, a research plan was developed. Its final purpose is to mitigate natural hazards and to improve the geological and geophysical information in the area between the south–eastern Sicilian coast and the islands of Lampedusa and Malta. Although this region lies on the Sicily channel rift zone, a seismically active domain of the Central Mediterranean, knowledge about seismotectonic, seismic hazard and local seismic response is at present quite poor. In order to improve the awareness of problems linked to natural hazards and with the aim of toning down them, we investigated the island of Lampedusa (Pelagian archipelago). A multidisciplinary approach concerning tectonic, structural, morphologic and lithologic analyses was performed trying to contribute to fill up the information gap on the seismic features of this territory. The results of the geological–structural surveys were used to standardize the evaluation of the seismic hazard and, in particular, to understand the local seismic response of the distinct outcropping terrains and its influence on the dynamic behavior of existing buildings.peer-reviewe