The central part of the Mediterranean Sea often experiences strong winds and high waves related to marine storms, which are common during both the winter and autumn seasons. In recent years they have been linked to an increase in the occurrence of violent “tropical-like cyclone” events (Emanuel, 2005; Fita et al., 2007; Lionello et al., 2006). Such a type of storm waves may represent a severe geo-hazard for inshore facilities and related land use development, as evidenced by the recent impact of severe storms on the coasts of Apulia (southern Italy) which caused extensive flooding both on Ionian and Adriatic sides. In historical and recent times, tsunamis of impressive heights have been recorded to have hit some parts of the Mediterranean coasts. The 1908 earthquake-generated tsunami that struck the coasts of Calabria and Sicily in southern Italy developed waves up to 13 m above sea level, contributing to heavy destruction and 1500 casualties (Bertolaso et al., 2008). More recently, in 2006, the collapse of Sciara del Fuoco along the flanks of the Stromboli volcano island generated waves that destroyed harbour structures and other facilities on the island and along the Tyrrhenian coasts of Calabria and Sicily (Mastronuzzi et al., 2013a).

In the Maltese archipelago the recorded tsunamis (Tinti et al., 2004) are the ones of AD 1169, 1693 (described locally also by Agius de Soldanis in 1746) and 1908. The latter is well known to have affected the eastern coast of Malta (Pino et al., 2008) and the southern coast of Gozo with a wave height ranging between 0.72 and 1.50 m (Guidoboni and Mariotti (2008) and references therein). These events have been triggered by earthquakes which occurred in eastern Sicily. Moreover, according to the model provided by Tinti el al. (2005), a potential tsunami impacting on the Maltese coasts may be generated either by an earthquake of Mw = 7.4 along an offshore fault located parallel to the Malta Escarpment (the former considered as one of the possible sources for the AD 1693 earthquake) with a wave height of 0.15–1 m, or by an earthquake from the western Hellenic Arc (Mw = 8.3) with marginally higher wave heights (1–1.5 m).

One of the most impressive pieces of evidence of extreme wave impact on the Mediterranean coasts is represented by the occurrence of mega-boulders, sparse or in fields or berms whose accumulations have been attributed both to tsunamis and storm events (Mastronuzzi and Sansò, 2000, 2004; Mastronuzzi et al., 2007; Scicchitano et al., 2007, 2012; Barbano et al., 2010; Vacchi et al., 2012; Raji et al., 2015). Notwithstanding the impressive growth in the last 50 years of studies aimed to develop an appropriate methodology, which may (in the absence of field witnesses) link these boulder deposits to a well-defined process (Williams and Hall, 2004; Hall et al., 2006; Mastronuzzi et al., 2006; Scheffers and Scheffers, 2006; Pignatelli et al., 2009; Goto et al., 2010), no undisputed consensus has yet been reached on how to differentiate between the boulders accumulated by a sea storm from those deposited by a tsunami. Some studies point to the presence of boulders and their size, to evaluate the characteristics of the impacting waves (i.e. Mastronuzzi and Pignatelli (2012); Mastronuzzi et al. (2013b) and references therein). An important degree of uncertainty lies in this methodology due to the definition of the origin of the wave responsible for the deposition of the boulders. The hydrodynamics of boulder emplacement and transport to the shore platform have been dealt with, among others, by Nott (1997, 2003) and Noormets et al. (2004). Nott attributes the force required to transport boulders to wave height and proposes a straightforward method to determine if storm or tsunami waves are responsible for their emplacement. In the equation developed by Noormets, hydrodynamic forces at the low submerged shoreline cliff are computed using design wave characteristics, based on linear wave theory and experimental results, which include also the local wave climate, nearshore bottom topography and initial fracturing of cliff rocks. More recently, research attention has shifted its focus to the role of impacting wave height compared to the wave length and to the wave period. Different theories have been proposed (Goto et al., 2007, 2009, 2010; Hansom et al., 2008; Imamura et al., 2008; Pignatelli et al., 2009; Nandasena et al., 2011), suggesting that in order to evaluate the wave impact on a rocky coast, these parameters should be considered all together.

The eastern coast of the island of Malta is characterised by the occurrence of deposits of anomalous calcareous boulders (Furlani et al., 2011; Mottershead et al., 2014; Causon Deguara, 2015). Their surface is frequently covered by biogenic encrustations, which indicate without any doubt that they were detached from the mid- or sublittoral zone.

The aims of this paper are to identify the physical processes responsible for the accumulation of the coastal boulders and to understand whether these boulders can be a potential geo-hazard for the low-lying rocky coasts of Malta.

The Maltese archipelago is located in the Sicily–Malta channel (central Mediterranean Sea, Fig. 1a), 90 km south of Sicily and 290 km north of Libyan coasts, and consists of three main islands, namely Malta (245.7 km2), Gozo (67.1 km2) and Comino (2.8 km2). From a geo-tectonic point of view, together with the Hyblean Plateau (SE Sicily), the archipelago belongs to the Pelagian Block (Grasso and Pedley, 1985), the northernmost sector of the African plate, mostly composed of foreland Neogene carbonate successions (Patacca et al., 1979). Eastwards, the Pelagian Block is bounded by the Malta Escarpment, a Mesozoic passive margin separating the continental domain from the oceanic crust of the Ionian Basin (Scandone et al., 1981; Makris et al., 1986). Since the middle Pleistocene, it has been locally reactivated by normal faulting (Argnani and Bonazzi, 2005), related to a regional WNW–ESE-oriented extension (Monaco et al., 1997; Bianca et al., 1999; Palano et al., 2012) (Fig. 1a). It is marked by a high level of crustal seismicity, producing earthquakes with intensities of up to XI–XII MCS and M ∼ 7, such as the 1169, 1693 and 1908 events (Baratta, 1901; Postpischl, 1985; Boschi et al., 1995).

The Sicily–Malta channel underwent transtensional processes during the Neogene–Quaternary times, which led to the development of the Pantelleria, Linosa and Malta grabens, partially filled by Pliocene–Pleistocene sediments (Finetti, 1984). These structures are mostly bounded by NW–SE-trending subvertical conjugate normal faults (Fig. 1a) whose activity would have reached the acme at approximately 5 Ma. Reactivation of the fault systems accommodated SW–NE extension in the late Quaternary (Corti et al., 2006; Catalano et al., 2009). As revealed by an available seismic database (INGV, http://emidius.mi.ingv.it/DBMI11), the above-cited structural features are a source of moderate seismicity, mostly located in the Linosa graben, with shallow events (h < 25 km) and a magnitude usually from 2 to 4 (Civile et al., 2008).

Several earthquake-generated tsunamis struck the Ionian coast of south-eastern Sicily and the Maltese archipelago in historical times such as in AD 1169, 1693 and 1908 (Tinti et al., 2004) (Fig. 1b).

According to published geological data and numerical modelling, the seismogenic source of these events should be located in the Strait of Messina and in the Ionian offshore (the Malta Escarpment) between the towns of Catania and Siracusa (e.g. Piatanesi and Tinti, 1998; Bianca et al., 1999; Monaco and Tortorici, 2000; Azzaro and Barbano, 2000). On the other hand, we must consider that many tsunamis that have occurred in the Mediterranean Sea have been generated in the Hellenic Arc area (i.e. Vött et al. (2010), Mastronuzzi et al. (2014) and references therein).

From a geomorphological point of view, the southern and western sectors of the Malta island are characterised by subvertical cliffs increasing in height northwards (mean heights of 100–120 m in the southern tract, and of 200–225 m along the western coast of the island, close to the Great Fault). Low-lying coasts are dominant on the eastern and north-eastern parts of Malta, showing a system of surf bench and wave-cut platforms, some of which host boulder accumulations. This difference in the coastal morphology is linked to the development of a northeastwards tilting in response to the fault system activity, also responsible for the forcing of the surface waters in a WSW–ENE direction and the formation of NE-oriented fluvial valleys (Alexander, 1988; Biolchi et al., 2016). Wide sectors of NW coast of Malta are characterised by the presence of extensive landslides, mainly rock spreads and block slides (Devoto et al., 2012, 2013; Piacentini et al., 2015). These slow-moving landslides detach and move hundreds of limestone blocks from the karst plateaus towards the sea, forming peculiar coastal landforms named “rdum” by locals. Conversely, slow-moving landslides and related slope-failure accumulations are not common on the NE coast, although rock spreads and block slides have been recognised and investigated on the northern side of Xemxija Bay by Panzera et al. (2012).

The submerged landscape is mainly composed of flat to gently sloping terrain and comprises coastal landforms, such as fault-related scarps, palaeo shore platforms, palaeo shoreline deposits and slope-failure deposits, as well as terrestrial landforms, such as river valleys, alluvial plains, karstified limestone plateaus and sinkholes (Micallef et al., 2013).

The Maltese sedimentary succession mainly consists of pelagic limestones, clayey terrains and marls (Pedley et al., 1976, 1978). As illustrated in Fig. 2, it includes five formations: (1) Lower Coralline Limestone formation (LCL), consisting of late Oligocene (Brandano et al., 2009) bioclastic limestones; (2) Globigerina Limestone formation (GLO), late Oligocene to middle Miocene in age (Baldassini et al. (2013), Baldassini and Di Stefano (2015) and reference therein), consisting of pelagic marly limestones. It is subdivided, based on the occurrence of phosphoritic conglomerate beds (Baldassini and Di Stefano, 2015), into three members; (3) Blue Clay formation (BC), middle to late Miocene in age (Giannelli and Salvatorini, 1975; Hilgen et al., 2009), formed by silty marlstones; (4) Greensand formation, late Miocene (Tortonian) in age (Giannelli and Salvatorini, 1975), consisting of greenish marly/clayey glauconite sands and arenites; (5) Upper Coralline Limestone formation (UCL), late Miocene in age (Giannelli and Salvatorini, 1975), consisting of shallow-water bioclastic limestone deposits.

To identify and map the boulder accumulations, field surveys were carried out along the eastern low-lying coasts of Malta (Fig. 2). Some of these sites had already been recognised and categorised by various authors: Armier Bay by Furlani et al. (2011) and Biolchi et al. (2016); Ahrax Point, Pembroke and Xghajra by Mottershead et al. (2014); and Żonqor by Causon Deguara and Gauci (2014) and Causon Deguara (2015).

The most representative boulders, in terms of size, shape, distance from the coastline and presence of encrusted marine bioforms, were chosen for further analysis. The 77 selected boulders included the largest observed blocks, slab-like, roughly cubic and rectangular, as well as assembled and isolated ones.

To verify whether the detachment of the boulders is due to the storm wave regime of the area or a tsunami-related event, a hydrodynamic approach was applied in this study. In particular, we applied the well-known and accepted equations developed by Pignatelli et al. (2009), Nandasena et al. (2011) and Engel and May (2012), in order to calculate the minimum tsunami and storm wave heights required to detach a boulder from the coast edge or the nearshore environment (Table 1).

Direct measurements on each boulder were carried out to determine size, imbrication direction and distance from the shoreline, whilst the density was determined by means of the N-type Schmidt hammer (SH). The latter is a field instrument (Viles et al., 2011) to determine rock physical properties (intact rock strength and densities) by means of non-destructive testing (Yilmaz and Sendir, 2002). Katz et al. (2000) correlated an index named Hammer Rebound (HR), which is a function of the resistance of surface material hit by the SH to the density of the rocks. The boulder density was associated to averaged HR value assigned to each block by means of Eq. (1): ρ=1308.2ln⁡(HR)-2873.9, where ρ is the density unit expressed in kg m-3.

To avoid interferences due to the occurrence of discontinuities, fossils and weathering processes, we followed recommendations by the International Society for Rock Mechanics (ISRM). We took at least 10 single-impact readings for each block and averaged only the upper 50 % for the determination of the boulder HR value, as suggested by the ISRM (1978). Moreover, in order to minimise deviations that would arise from an oblique impact (Aydin and Basu, 2005), we performed field tests, keeping the hammer axis perpendicular to the boulder surface (Table 2).

Given that the hydrodynamic approach also depends on the pre-transport environment, the most likely setting (submerged, subaerial, etc.) prior to transportation has been determined. Moreover, detailed submerged profiles of the five coastal sites have been constructed by carrying out direct underwater surveys, in order to verify a correlation between the submerged limestone features in the nearshore zones (such as fractures, rupture surfaces, detachment scarps, holes) and the quantity and shape of the deposited boulders.

The onshore boulders at each site were inspected to identify the presence of any biological encrustations, mainly of calcareous marine bioform type, which may remain attached to the boulders after emergence above sea level. When present on a boulder, such bioforms may serve as a strong indicator of the original location of the boulder in a submerged environment and which would die once the boulder is removed from its underwater environment. The taxa of these bioforms were identified in order to create an identity list of biota. The identity and ecology of the species served as a basis to draw inferences on the origin and history of the boulders and thus be of help to further corroborate the results obtained from hydrodynamic modelling.

Radiocarbon age dating of marine bioconcrections helped to reveal the deposition time frame. They were performed by the CeDaD Laboratory (Centro di Datazione e Diagnostica of the University of Salento, Italy). The calibration is based on the data set available on the website http://calib.qub.ac.uk/marine/index.html and provides a choice between different procedures. We adopted the MMHM (i.e. mixed marine Northern Hemisphere) equation, with a δR = 59 ± 40 and δR = 71 ± 50, respectively obtained in the Tyrrhenian Sea on Arca tetragona species and Cerastoderma genus. We preferred to use the first value because of the greater ecological similarity, in particular for nourishment, with Vermetidae and Chthamalus. Indeed, the genus Arca lives fixed on the rocky bottoms and is characterised by suspension feeding behaviour. Conversely the genus Cerastoderma, although it shows suspension feeding behaviour, occupies the infaunal niches. Furthermore, it was considered that the percentage of carbonate origin besides continental starts from the value of δ13C.

Finally, the collected field data were compared to the Maltese wave data and to historical catalogues of earthquakes and tsunamis (Tinti et al., 2004; Fago et al., 2014; Papadopoulos et al., 2014) in order to make a possible correlation with known events. With regards to local wave height parameters, the Malta Environment and Planning Authority (Malta Maritime Authority, 2003; Malta Environment and Planning Authority, 2007) provided a statistical study of two different areas of Malta (close to Bahar iċ-Ċaghaq on the NE coast and close to Żonqor on the SE coast) from data measured during 2007, through which the inshore wave extremes have been estimated as 5.1–5.6 and 5.3–5.9 m respectively for a return period of 50 years and as 5.3–5.8 and 5.4–6.0 for a return period of 100 years at 20 m depth. A wave height of almost 7 m was recorded on 15 October 2007 during a very strong storm.

Drago et al. (2013) provided an analysis of the Maltese waves by taking into account wave data over a span of 5 years (2007–2011), measured by a buoy located 2 km offshore from the NW coast of Gozo. The highest wave was registered on 6 January 2012 with a height of 7.46 m.

This value, associated with a wave period of 9 s, has been used to apply the Sunamura and Horikawa (1974) equation which calculates the wave height at breaking point (Hb) of a coastal area: HbH0=(tan⁡β)0.2⋅H0L0-0.25, where Hb is the breaking wave height, H0 is the wave height in deep water, β is the slope of the sea bottom in the coastal area and L0 the wave length in deep water (L0 = gT22π; Sarpkaya and Isaacson, 1981).

The resultant breaking wave height Hb for the five study areas were as follows: 6.6 m at Armier Bay, 9.4 m at Ahrax Point, 8.7 m at Buġibba, 7.1 m at Qawra peninsula, 8.0 m at Bahar iċ-Ċaghaq, 10.2 m at Pembroke and 8.4 m at Żonqor. These values have been considered as thresholds to distinguish storm waves from tsunami waves.

From a morphological point of view, the occurrence of low-lying rocky coasts makes the eastern coast of Malta better predisposed to the accumulation of large boulder deposits derived from the impact of extreme waves. Moreover, the horizontal bedding, the presence of subvertical discontinuities and the poor geo-mechanical properties of the rocks play a crucial role in the rupture and detachment of large blocks from the coastline.

Concerning the pre-dislodgement setting of the boulders, a joint bounded submerged scenario is the most frequent, while for the boulders at Ahrax Point and locally at Żonqor, Buġibba and Bahar iċ-Ċaghaq, a subaerial scenario is suggested. These settings were validated by underwater surveys carried out in all the investigated sites; in all locations, the shapes of the boulders correspond to the shapes of the pluck holes and detachment scarps. Originally, most of the large boulders investigated in this study must have been part of the coastline edge, since they comprise rock pools from the most seaward surface, as well as vermetid colonies.

Furthermore, according to local eyewitness accounts, several boulders recently deposited by swell waves were dislodged, moved and transported landward. Mechanical quarrying of the boulders requires the presence of initial cracks. As a matter of fact, most of the measured boulder c and b axes correspond respectively to bed thickness and bed planes, which are smooth at the base and karstified at the top. These discontinuities favoured the detachment of regular slabs. Especially at Żonqor, the quarrying of regularly shaped boulders is encouraged by the presence of subvertical faults and fractures, which are clearly visible also underwater.

On the other hand, at Ahrax Point, the boulders seem to have been detached from the top part of the cliff face, as they have not been colonised by marine organisms and the geomorphological setting includes a steep cliff very close to the deposits. These boulders are referred to as cliff-top storm deposits.

The application of the hydrodynamic equations (Table 3) has highlighted the lack of correlation between density and volume values and the obtained results, meaning larger boulders do not necessarily require higher waves to be detached from the coastline edge. When comparing results, it can be observed that the results of Nandasena et al. (2011) and Pignatelli et al. (2009) are very similar: the highest values obtained reach 14 and 13.35 m for the equations of Nandasena and 12.8 and 12.7 m for Pignatelli, thus recording a marginal difference of slightly more than 1 m. For all the other values, the decrease of the storm wave height values also decreases the difference between the obtained results. Out of the 77 selected boulders, 22 boulders recorded storm wave heights exceeding the estimated breaking wave heights. Conversely, the calculated tsunami wave heights are very low and range between 3.5 m (3.2 for Pignatelli) and 0.55 m (0.51 for Pignatelli). Engel and May's (2012) equations obtained values much lower relatively, with the storm wave heights ranging from 0.9 to 6.5 m and tsunami wave height from 0.2 to 1.6 m.

Equally, the obtained values for tsunami wave heights are comparable to those observed during the 1908 earthquake (Pino et al., 2008; Guidoboni and Mariotti, 2008), as well with those obtained by Tinti et al. (2005) for tsunamis generated by earthquakes both in eastern Sicily and in the Western Hellenic Arc.

In comparing their results, these equations provide values which are too different from each other, even though they take into account different parameters and sometimes consider scenarios which are distant from the real geomorphological setting. It was noticed that bulk and volume values do not influence the results in the same way when using different equations. Moreover, parameters such as the distance from the coastline, the elevation position of a boulder and the local topographical characteristics of the sea bottom are not taken into consideration. These are the reasons why it can be concluded that the hydrodynamic approach as a stand-alone method is not sufficient to distinguish between storm and tsunami waves.

With regards to the position of the boulders, we are not able to establish whether the current position of the boulders is the original one when the first depositional event occurred. There is the possibility that some boulders may have been deposited on the coast by a tsunami event and later transported and deposited by storm waves to their present position. As a matter of fact, comparing the distance from the coastline with the mass of each boulder for each site (Fig. 7), two different distributions can be observed (the Ahrax Point case is not representative because of the poor number of data).

At Armier Bay and Żonqor, the distribution is regular, with the lighter boulders as the ones further way from the coastline. This could be indicative of a storm event or of a “perfect tsunami”, but the radiocarbon dating results on different boulders at Armier Bay are varied. In this case, even if the diagram in Fig. 7 is typical of a storm, we suggest the combined action of storms (which may occasionally be severe) and one or more tsunamis. For the Żonqor site, the hypothesis of extreme storms is confirmed by the hydrodynamic approach as well as by radiocarbon dating and the geomorphological and biological characteristics.

At Bahar iċ-Ċaghaq, Buġibba and Pembroke, the boulders are scattered and their distribution is highly irregular, indicating a chaotic event or the succession of several events. However, such a distribution of the boulders and their dating (post 1954 for Buġibba, AD 1672 ± 45 for Bahar iċ-Ċaghaq and AD 1723 ± 40 for Pembroke) may indicate the succession of several events (storm and storms, or storm and tsunami or tsunami and tsunami).

Radiocarbon dating was performed on marine organisms sampled from 10 representative boulders in all the sites. Moreover, three dating samples provided by Biolchi et al. (2016) were recalibrated (Table 4; Fig. 8). Four samples support the hypothesis of recent strong storm events, dating back to post AD 1954. This occurred in particular at Żonqor Point, which is exposed to storms blowing from more than one direction. The north-easterly storms batter this stretch of coastline just as they do in areas such as Qawra. However, Żonqor Point is also exposed to storms that originate from the south-east – known locally as “Xlokk”. Such storms can blow both in winter and in summer, and their strength, though not as powerful as storms from the north-east, can create very rough conditions in the area.

Additional proof of recent extreme waves is provided by tracks on freshly damaged karst surfaces, generated by rolling/saltating boulder transport, which lead directly from the fresh scarp at the terrace edge to the boulder's current position.

Other radiocarbon dating seems to be related to events (extreme storms and/or tsunamis) occurring in a time span ranging from 514 ± 104 BC to AD 1723 ± 40. They have been compared with historical events (Papadopoulos et al., 2014; Tinti et al., 2004). Our results could be tentatively linked to some of these tsunami events which have occurred in the Mediterranean Sea and the Aegean Sea (Table 4). Amongst them, the most ancient are the 373 BC (west Corinthian Gulf) and the 426 BC (island of Crete) events for the boulder AB1 at Armier Bay; but this boulder, despite its significant size (4.2 × 2.8 × 0.5 m), is located very close to the the coast (15 m) and is placed above other boulders.

The event of AD 963, which occurred in eastern Sicily, which is reported in the tsunami catalogue provided by Tinti et al. (2004) as a “false event”, could instead be tentatively proven by two nearby boulders at Armier Bay (AB4 and C82). The more recent event of AD 1303 (Crete Island) and, more probably, the AD 1329 (eastern Sicily) event, could be related to three nearby boulders at Armier Bay (AB2, AB6 and Q2). Finally, two other events, which occurred in the early modern period and have been reported in historical accounts of Malta (De Soldanis, 1746; Galea, 2007) are the AD 1693 (eastern Sicily) and the AD 1743 (Apulia, Lower Ionian Sea) events. These strong earthquakes triggered tsunamis which, according to Scicchitano et al. (2007) and Mastronuzzi et al. (2007), are responsible for similar boulder deposits in eastern Sicily and the Apulia region. These two tsunamis could be related to the boulders located at Bahar iċ-Ċaghaq (B1) and Pembroke (16). Gozitan historian Agius de Soldanis writes the following description of the 1693 event in his 1746 account. “On 11 January of this year, the earth trembled and everyone was scared. The earthquake damaged the Collegiate Church and many other churches. The sea at Xlendi receded instantly and returned back with great fury like a tidal wave and with a thundering sound. At Sannat a part of the land measuring round a wejba crumbled down into the sea” (De Soldanis, 1746, p. 149).

Unfortunately, these claims could not be further verified since a.

the radiocarbon age is limited due to the limited number of samples and to the calibration;

Along the eastern and north-eastern Maltese coasts, about 20 boulder deposits occurred. Reconstructing the history of these blocks and distinguishing events, such as storm waves or tsunamis, play a crucial role in assessing this type of coastal geo-hazard. A detailed field survey has been carried out along the Maltese coasts in order to identify and map all the sites in which these kinds of deposits occur, to analyse their characteristics in detail, to determine their provenance and study the processes responsible for their transport from the sea to the coast.

Data suggest that these boulders testify to the existence of a real hazard for the eastern and north-eastern coasts, considering the high land use development and coastal infrastructures present in proximity to the Maltese coastline on this part of the island. The frequent storms affecting the Maltese coasts are able to detach large boulders both from the coast edge and the sea bottom, and to transport them onshore. Very high waves are common. They can detach and move blocks whose volume can exceed 10 m3. These blocks can be detached and moved inshore, or boulders can be initially overwhelmed and brought inshore only at a later time. The occurrence of recent extreme storm waves is supported by radiocarbon dating performed on marine organisms. Such events are likely to increase in frequency and intensity due to climate change, whilst sea level rise, even of a temporary nature, such as that brought about by a storm surge, could shift coastal processes landward and impinge on urban areas.

However, the possibility that one or more tsunami events may have affected these coasts cannot be ruled out, since radiocarbon dating of some marine organisms encrusted on the boulders surfaces has revealed ages that can be related to historical known tsunamis. In particular, at the Armier Bay site (north-eastern coast), there could be geomorphological evidence of the AD 963 and 1329 tsunami events, which occurred in eastern Sicily (southern Italy). Conversely, at Bahar iċ-Ċaghaq and Pembroke (eastern coast), two boulders could be related to one of the two most important tsunamis (AD 1693, eastern Sicily, or AD 1743, Apulia), which have also been reported in the historical accounts of Malta.

Thus a national risk assessment of extreme wave events will need to consider both an ongoing monitoring system of storm wave events and related impacts on the low-lying urban coasts, as well as the inclusion of the Maltese islands in a Mediterranean-based tsunami early warning system, as part of a long-term strategic hazard management plan.