The use of scenarios to evaluate the tsunami impact in southern Italy

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Marine Geology 199 (2003) 221^243 www.elsevier.com/locate/margeo

The use of scenarios to evaluate the tsunami impact in southern Italy Stefano Tinti  , Alberto Armigliato Universita' di Bologna, Dipartimento di Fisica, Settore di Geo¢sica, Viale Berti Pichat, 8, 40127 Bologna, Italy Received 29 November 2002; received in revised form 2 June 2003; accepted 17 June 2003

Abstract This paper outlines the main contributions to the definition and evaluation of tsunami hazard and risk resulting from studies undertaken in Italy in recent years and emphasises that adopting characteristic cases or scenarios is a very useful and advantageous technique. Three main cases are given as valuable examples, that is the 1627 Gargano tsunami, the 1693 eastern Sicily tsunami and the 1908 Messina Straits tsunami, since: (1) they characterise three distinct tsunamigenic regions; (2) they are instances of destructive events; and (3) they have been extensively studied in the last decade. The paper elucidates the state-of-the-art of the research on these events, clarifies the chief points of agreement and disagreement among scientists, and illustrates the main issues that are to be addressed by future research to provide reliable assessment of tsunami risk and to implement efficient countermeasures to defend the life of people, coastal structures and the coastal environment against the attacks of tsunamis. 1 2003 Elsevier B.V. All rights reserved. Keywords: natural disasters; submarine faults; tsunamis; tsunami risk

1. Introduction The main purposes of the present paper are: (1) to review the research performed in recent years to reconstruct the largest tsunami events that occurred in southern Italy, i.e. the Italian region most prone to tsunami attacks; (2) to emphasise the importance of the approach based on the adoption of scenarios to evaluate the impact of

* Corresponding author. Fax: +39-051-2095058. E-mail addresses: [email protected] (S. Tinti), [email protected] (A. Armigliato).

tsunamis and to contribute to the assessment of tsunami hazard and risk; and (3) to delineate the problems that are still open and need to be addressed in the next years mostly in order to suggest e⁄cient actions of prevention and of preparedness in the frame of policies of natural disaster mitigation and sustainable development of the coastal zone. This section touches the topics of the tsunami catalogues and the statistical means that can be used to evaluate the tsunami potential. The critical re-examination of the major tsunami cases and the utilisation of proper scenarios to analyse the impact of future events will be discussed in the body of the paper, whereas the perspective for future research will be delineated in its ¢nal part.

0025-3227 / 03 / $ ^ see front matter 1 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0025-3227(03)00192-0

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1.1. Observations of historical tsunamis in Italy The past history of Italy tells us that tsunami attacks occurred several times. Great e¡orts in the past 15 years has been put in collecting and examining documents and data on historic events a¡ecting Italy and more generally the Mediterranean Sea and the European coasts. A number of catalogues have been published either in the form of traditional papers and books (Soloviev, 1990; Tinti and Maramai, 1996; Soloviev et al., 2000) or in the form of digital databases or both (Tinti et al., 1999a ; see also the catalogue on Mediterranean events prepared by Gusiakov and collaborators, HTDB/MED, 1999). Owing to the very long period of civilisation and the corresponding amount of documentation that characterises Italy together with few other countries in the world, the catalogues cover about 2000 years of history, though with various degree of completeness in di¡erent periods. From the data available it results that Italy and Greece are the Mediterranean countries with the highest known number of tsunamis, and that southern Italy is one of the regions with the most relevant tsunamigenic sources of Europe. The Italian events are related to the volcanic activity of Vesuvius and of the Aeolian Islands volcanoes, Stromboli and Vulcano, but mostly to seismic activity, although some examples can also be found of sizeable to large tsunamis caused by landslides and slumping. The ¢rst tsunami report for Italy is associated with the large Plinian eruption of Vesuvius in AD 79 that destroyed Pompei, Herculaneum and some other Roman villages (Tinti and Saraceno, 1993). The most recent ones occurred in the Aeolian Islands and o¡shore eastern Sicily. One of them was due to a landslide from the eastern £ank of Vulcano on 20 April 1988 (Tinti et al., 1999b). Another small event is related to the M = 5.4 earthquake of 13 December 1990 o¡shore Augusta, and was probably generated by small submarine slides set in motion by the quake. Finally, the most recent tsunami attacked the coasts of Stromboli on 30 December 2002: it was a complex and destructive event generated by a series of landslides occurred along the NW £ank of the volcano, called Sciara

del Fuoco. The most severe tsunamis occurred in southern Italy and the most catastrophic one is certainly that generated by the devastating earthquake of 28 December 1908, involving the Messina Straits. This event was very important and was and still is object of intensive research, as will be explained below. 1.2. Assessing tsunami hazard through statistical means Tsunamis occur much more rarely than earthquakes. The number of tsunamis in the Mediterranean known in the last 2000 years amounts to some hundreds, whereas the number of earthquakes recorded by the seismic network only in the last few decades is as large as tens of thousands. Therefore, tsunami databases are not adequate for the direct application of those statistical analyses that normally are applied to seismic catalogues. However, some indirect applications can be attempted. One possibility is to restrict the attention to tsunami generation induced by earthquakes, which is the most frequent genetic mechanism. In this way statistical analysis can be applied to seismic databases in order to estimate the occurrence probabilities of tsunamigenic earthquakes having epicentres close to the coast or in the open sea. Then, on using some statistical or theoretical relationships linking earthquake source parameters and tsunamis, earthquake occurrence probabilities can be converted to occurrence probabilities of tsunamis. This general scheme was applied ¢rst to evaluate the tsunamigenic potential for all Italian coasts by Tinti (1991a), and was then re¢ned and focussed on southern Italy (Tinti, 1991b; Tinti et al., 1995). These studies show that the areas where most tsunami activity related to earthquakes was and is expected to be concentrated in the future are the Messina Straits and the coasts of eastern Sicily, and that a further zone with a relevant tsunami potential is the region of Gargano in the Adriatic Sea. This conclusion is important since it helps to orient researches and to focus e¡orts in understanding tectonics, in identifying seismic sources and in studying the possibly associated tsunamis, within an area that is rather limited and well de¢ned.

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1.3. Deterministic approach to tsunami hazard: scenarios Statistical approaches cannot be applied to tsunamis induced by slides or related to volcanic activities because of the dramatic de¢ciency of data. Furthermore, even their application to tsunamis of seismic origin could be hampered by the need of adopting relationships linking earthquakes to tsunamis that may not be empirically well grounded. This means that in many cases alternative approaches to evaluate tsunami hazard are to be invoked. Most often, a solution to the problem is searched for in terms of a scenario. The philosophy is very simple and ¢ts the need of engineers that routinely adopt it in their practical applications. Let us consider the question of how to protect a segment of coastline, or a given industrial plant, or private edi¢ces and houses, etc. from a tsunami attack within a given period of time in the next years. The only reasonable action that can be taken is that of considering the largest event known to have hit the area of interest in the past history, to reconstruct that event from the historical source at best, to simulate that event through numerical modelling, and to compute the wave e¡ects on structures that have to be protected. Possible outcomes of this approach are maximum wave heights and impact forces, maximum wave penetration distances in any given localities, i.e. data that serve coastal and seismic engineers to plan adequate countermeasures. Maybe this is a too simple scheme and some adjustments are required to improve the overall reliability of the results, but the essential idea is that of considering in great detail a single characteristic case, namely the scenario that for some reasons is believed to be well representative of what is expected to occur. This approach entails the need of developing and using adequate numerical models for tsunami propagation and impact on the coast, and stresses the importance of studying single events in a deterministic way. In this paper we shall consider three examples of such an approach concerning the most important earthquake-induced tsunamis that a¡ected the coasts of southern Italy, i.e., in chronological order, the 30 July 1627 tsunami in

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Gargano, the 11 January 1693 tsunami in eastern Sicily, and the 28 December 1908 Messina Straits tsunami. All of the tsunami simulations presented below were performed by means of a numerical code (Tinti et al., 1994) solving hydrodynamical shallow-water equations via a ¢nite-element technique. This implies that the numerical domain where waves are to be computed is discretised in cells or elements of irregular shape and that water elevation and water velocity ¢elds are calculated in characteristic points, the nodes, belonging to these elements. In all applications, elements of triangular shape are used and the nodes are the vertices of the triangles. The advantage of this approach on ¢nite-di¡erence technique is that it permits to use grids ¢tting much better domains with irregular curvilinear boundaries, which is an important requirement to study basins bounded by irregular natural coastlines. The initial sea surface displacement is assumed to coincide with the vertical co-seismic movement induced by the earthquake, computed through the analytical formulas by Okada (1992), valid for rectangular faults embedded in homogeneous elastic halfspaces bounded by £at free surfaces.

2. The tsunamigenic zone of Gargano and the 1627 tsunami 2.1. Tectonic setting The local tectonic setting of the Gargano, a promontory protruding into the southern Adriatic Sea, is mainly characterised by a system of strikeslip and normal faults with predominant W^E and NW^SE orientation (Argnani et al., 1993; Favali et al., 1993; Salvi et al., 1999). Fig. 1 illustrates some of the main structures that have been recognised through geological and geophysical studies. The most evident one is the Mattinata Fault (MF), which cuts across the southern part of the promontory. It is a sub-vertical fault, striking approximately E^W, and with a signi¢cant continuation o¡shore that was revealed by seismic re£ection pro¢le analyses (Argnani et al., 1993). The fault is characterised mainly by strike-slip movements, although a complex kinematic history

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Fig. 1. Shaded relief map of the Gargano promontory, southern Italy, showing the main known faults and tectonic lineaments discussed in the text and the epicentral position of the most destructive historic earthquakes (black stars). The codes denoting the di¡erent structures are taken from Salvi et al. (1999): MF, Mattinata Fault; RF, Rignano Fault; CF, Candelaro Fault; AP^SN, Apricena^Sannicandro. Macroseismic data are derived from Boschi et al. (2000), except for those relating to the 1889 event which are taken from Monachesi and Stucchi (2000).

and deformation pattern is hypothesised by several authors (Argnani et al., 1993; Favali et al., 1993; Anzidei et al., 1996; Salvi et al., 1999; Ridente and Trincardi, 2002). The direction of the strike-slip motion itself on MF is rather controversial: some authors (e.g. Favali et al., 1993) individuated a predominant left-lateral motion, while others (e.g. Anzidei et al., 1996) favour a right-lateral mechanism. Two more tectonic lineaments are found in the southern Gargano sector, namely the Candelaro Fault (CF) and the Rignano Fault (RF), striking NW^SE and W^E, respectively. The main characteristics of these structures as well as of other faults identi¢ed in the prom-

ontory are controversial and subject to open discussion, in particular regarding their faulting mechanism. Several authors (e.g. Salvi et al., 1999) tend to assign strike-slip mechanisms based on the assumption of a regional left-lateral strikeslip tectonics for southern Gargano. Other studies (Bertotti et al., 1999) privilege thrust mechanisms, introduced as a consequence of substantial NE^ SW-directed shortening inferred from the analysis of Tertiary sediments. Moving toward northwest, a complex ENE striking 26-km-long morphological lineament, called the Apricena^Sannicandro escarpment (AP^SN in Fig. 1), has recently been observed south of Lesina Lake on the LANDSAT

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satellite images (Salvi et al., 1999). Moreover, airphoto analysis revealed that the northern side of the AP^SN structure underwent uplift and also possible horizontal movements. One last active structure, revealed by seismicity and seismic pro¢les, is found o¡shore the northern coast of Gargano and corresponds to the Tremiti Islands Deformation Belt, which shows a south-vergent monoclinal setting possibly resulting from transpressive deformation (Argnani et al., 1993; Ridente and Trincardi, 2002).

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rier, dividing the lake of Lesina from the Adriatic Sea (see Figs. 1 and 2). Their main conclusion is that the three washover fans were produced by three distinct tsunamis, whose chronology was determined through radiocarbon techniques: the oldest one should be dated at about 2430 years BP (pre-Roman age), the second could date back to the end of the ¢fth century, which is compatible with the sacred legends reporting a strong earthquake hitting the Gargano promontory in AD 493, and the last one can probably be traced back to the 1627 tsunami.

2.2. Seismic and tsunami history 2.3. The reconstruction of the 1627 tsunami The Gargano area is known to have been struck by several violent earthquakes in historic times. Earthquake catalogues (Boschi et al., 2000; Monachesi and Stucchi, 2000) report several events with maximum macroseismic intensity exceeding degree VII on the MCS scale. We recall here the shocks which occurred in 1223 (maximum intensity IX), 1627 (XI), 1646 (IX^X), 1731 (IX), 1889 (VII) and 1893 (VIII^IX). The epicentral positions for these historic earthquakes, proposed in the aforementioned catalogues on the basis of a macroseimic approach, are plotted as black stars on the map of Fig. 1, the magnitude of the stars being proportional to the maximum MCS intensity. It must be stressed that the plotted locations should be treated just as an indication, especially for the older events: it is indeed plausible that at least some of the events have been generated by o¡shore structures, and as a consequence the macroseimic approach could lead to unreliable results. Some of the most energetic earthquakes that occurred in the area were accompanied by relevant tsunamis. A critical review of the historic accounts, available only after AD 1600, on tsunami e¡ects observed along the Gargano coasts was undertaken by Guidoboni and Tinti (1988) and Tinti et al. (1995) : the largest tsunami was generated by the 30 July 1627 earthquake, but trustworthy reports indicate that signi¢cant tsunami waves accompanied at least also the 1646 and 1889 seismic events. Recently, Gianfreda et al. (2001) carried out an interesting geomorphological study on three wide washover fans which are found on the Lesina coastal bar-

The general picture resulting from all of these studies shows that, among the Italian coastal zones, Gargano must be regarded as one of the most exposed to tsunami attacks. Indeed, an essential step toward a correct assessment of tsunami hazard is the identi¢cation of the active seismic structures that could have been responsible for the most devastating known tsunamigenic earthquakes hitting the area. In this sense, the 30 July 1627 event is the one that has received the greatest attention in the published literature, and it is the most suitable to be adopted as a scenario of future events since it is the most energetic earthquake known to have occurred in Gargano. Moreover, su⁄ciently detailed historic sources are available on both the earthquake and tsunami e¡ects (see Molin and Margottini (1985) and Guidoboni and Tinti (1988) for the historic reconstruction). The earthquake occurred around noon and was followed by four aftershocks. The isoseismal lines plotted in Fig. 2, taken from Molin and Margottini (1985), show that the area where the most severe damage was produced is found between Lesina and S. Severo, where the maximum intensity (XI MCS) was reached (see also Guidoboni and Tinti (1988) and Tinti et al. (1995)). The macroseismic studies show that all the isoseismal lines between VIII and X are open, with an ideal continuation into the Adriatic Sea north of Gargano; moreover, no privileged direction of elongation is shown by the isoseismal ¢eld. Concerning the tsunami, the most evident e¡ects were observed along the northern Gargano

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Fig. 2. Isoseismal lines and epicentral position (black star) for the 1627 seismic event. The isoseismal lines have been digitised from Molin and Margottini (1985). Segments denoted as F1 and F2 are the surface traces of the vertical dip-slip faults studied by Tinti et al. (1997): in their paper, they were denoted with the codes S1 and S4, respectively.

coast. In particular, in the proximity of the Fortore River mouth (see Fig. 2) the sea water ¢rst retreated for about 3.5 km and then violently £ooded the coast, submerging the village of Lesina. Other places where tsunami e¡ects were reported are the coastal town of Termoli (NW of Gargano), the mouth of the river Faro (60 km north of Termoli) and the town of Manfredonia on the southern coast of the promontory, where the waves invested the town walls up to the half of their height, which could correspond to a runup in the order of 2^3 m (Guidoboni and Tinti, 1988; Tinti and Maramai, 1996). No clear information is retrievable from the coeval accounts regarding the number of victims caused by the tsunami. This could be attributed to the fact that at those times the northern coasts of Gargano were very scarcely inhabited, since villages

were located on higher land (Guidoboni and Tinti, 1988). Nowadays, the situation has completely changed: the tourism industry has recently led to a growing urbanisation of the coastal area close to Lesina Lake, which highly increases the risk connected to possible tsunami occurrence in the future. Numerical simulations of the 1627 tsunami have been performed by Tinti and Piatanesi (1996a) and Tinti et al. (1997), with the main goal of formulating some hypotheses on the possible parent fault. Their study led to privilege faults placed in the area involving the Fortore River and Lesina Lake, though tsunami data were found insu⁄cient to put constraints on the fault strike. The two particular solutions indicated by Tinti et al. (1997) as equivalently plausible (F1 and F2 in Fig. 2) are to be considered as two

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extreme possibilities: hence, in principle, any fault with intermediate strike could produce waves consistent with the coeval observations. On the other hand, a stronger constraint is put on the dip-slip mechanism : the uplifting block should be the northern one in the case of fault F1 and the eastern one for fault F2. It is interesting to notice that the spectrum of possible solutions determined on the basis of the tsunami data analysis is compatible with two hypotheses formulated in the literature by using different approaches. The ¢rst was introduced by Panza et al. (1991) : by comparing synthetic and experimental isoseismals, they proposed that the genetic fault could be a 310‡ striking, 45‡ dipping thrust placed approximately in the Fortore River mouth area. The second hypothesis (Salvi et al., 1999) proposes the Apricena^Sannicandro structure (see Fig. 1) as a plausible candidate for the parent fault of the 1627 earthquake: its focal mechanism should be mainly dip-slip but with a relevant strike-slip component. It is worth noting that, among the discussed sources, this is the only one with clear geomorphological evidence.

3. Southern Calabria and eastern Sicily 3.1. Tectonics Southern Calabria and eastern Sicily are among the most seismically active areas in Italy and the Mediterranean as a whole. This area is dominated by a prominent belt of Quaternary normal faults, which are interpreted as the expression of a regional ESE^WNW extensional regime (e.g. Monaco and Tortorici, 2000). In all, this regional fault belt is about 370 km long and is made up of distinct fault segments with typical lengths ranging from 10 to 45 km. It runs more or less continuously along the inner side of the Calabrian arc and crosses the Straits of Messina (Fig. 3); then it continues into Sicily where, according to several authors (e.g. Westaway, 1993; Monaco and Tortorici, 2000), its expression has to be found mainly o¡shore the Ionian coasts as far as the Hyblaean Plateau. The most impressive structure characterising the southeastern portion of the

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fault belt is the Hyblaean^Malta Escarpment (HME in Fig. 3), which separates the continental Hyblaean domain from the oceanic crust of the Ionian Basin (Bianca et al., 1999). 3.2. Seismic and tsunami activity The region is characterised by a high level of crustal seismicity, with earthquakes reaching intensities of up to XI MCS and maximum magnitudes in the order of MV7 (e.g. Boschi et al., 2000). Several of the largest seismic events recorded in the area were likely generated by structures placed partly or completely o¡shore, and were accompanied by relevant tsunamis. Fig. 3 shows possible epicentre locations of the largest historic tsunamigenic earthquakes, derived from the CPTI catalogue (Boschi et al., 1999). The event with the northernmost epicentre was the 8 September 1905 earthquake, with MCS intensity IX^X and magnitude MW7.0 (Boschi et al., 2000). The earthquake caused destruction in many villages of the Tyrrhenian side of Calabria and about 550 casualties. A large tsunami was induced by the earthquake, whose e¡ects were observed both in the open sea and along the coasts : in particular, inundation of some tens of metres was reported in several coastal villages in the epicentral area. The tsunami was recently studied numerically by Piatanesi and Tinti (2002) with the main aim of deriving information on the genetic fault. Their main conclusions were that, among the sources proposed in the literature, the Capo Vaticano and Vibo Valentia faults (CV and VV in Fig. 3) are the most plausible candidates based on the available tsunami reports, which are anyhow insu⁄cient to clearly discriminate between the two. In 1783 the most persistent and violent earthquake sequence of Italian history took place in southern Calabria, with ¢ve large events (5.6 9 M 9 7.1) occurring between February 5 and March 28. A large area embracing the whole southern Calabria and the Messina Straits suffered extensive destruction and a very high number of victims. At least two of the major events were followed by very relevant tsunamis. The ¢rst occurred on 5 February 1783, and was produced

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Fig. 3. Seismotectonic sketch of the Calabrian Arc and of eastern Sicily. Fault lineaments from Bianca et al. (1999) and Monaco and Tortorici (2000). The epicentres of the most important tsunamigenic earthquakes which occurred in the region, indicated as black stars, are derived from Boschi et al. (1999). Bathymetric labels in metres. The code HME stands for the Hyblaean^Malta Escarpment.

by a XI MCS earthquake, causing almost total destruction in about 200 villages along the Tyrrhenian side of the Calabrian Arc, and in particular in the foothills of the Aspromonte mountain range. The greatest e¡ects of the tsunami, which was strong but not disastrous, were observed along the northern portion of the Messina Straits (Tinti and Maramai, 1996). Tinti and Gavagni

(1995) and Tinti and Piatanesi (1996b) performed ¢nite-element simulations of this event and concluded that most of the empirical tsunami observations are consistent with a genetic large-angle, 30^50-km-long normal fault placed parallel or sub-parallel to the Apennine Chain, with WNW dipping direction. The second disastrous tsunami followed the IX^X MCS earthquake which took

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place on 6 February 1783. The earthquake e¡ects were su¡ered in particular by Messina in Sicily and by Scilla, placed on the upper end of the Calabrian side of the Messina Straits. It is very plausible that the tsunami was not triggered directly by the earthquake, but by a huge earthquake-induced rockfall coinciding with the collapse of a portion of Mt. Pac|' (close to Scilla) into the sea (Tinti and Maramai, 1996). The effects of the tsunami were tremendous in particular at Scilla where many people, frightened by the earthquake sequence, tried to take shelter on the beach. The tsunami waves, with estimated heights in the range 6^9 m, caused more than 1500 victims. The other tsunamigenic events indicated in Fig. 3 are concentrated in correspondence with the Messina Straits and with southeast Sicily. Here the two most destructive tsunamis of Italian history took place, generated by the earthquakes of 11 January 1693 (eastern Sicily) and of 28 December 1908 (Messina Straits), respectively.

4. The 1693 tsunami in eastern Sicily In January 1693 eastern Sicily was hit by a disastrous sequence of earthquakes, starting on January 9 with a strong foreshock (VIII^IX MCS according to Boschi et al., 2000) and culminating in the disastrous mainshock of January 11, whose estimated maximum intensity and magnitude are XI MCS and 7^7.3. Even though in general the discrimination of the macroseismic e¡ects produced by the two shocks is di⁄cult, some hypotheses have been formulated regarding the epicentral locations (drawn as stars in Fig. 4, derived from Boschi et al., 2000) and the areas where the earthquakes produced the highest damage. The 9 January foreshock is believed to have caused relevant destruction in the town of Augusta and in a number of villages (Lentini, Carlentini, Melilli, Floridia, Avola Vecchia, Noto Antica) placed in a narrow belt along the eastern border of the Hyblaean Plateau (Fig. 4). It is plausible that no relevant tsunami was induced by the 9 January event, even though some authors report an anomalous sea movement in the Augusta har-

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bour (see Tinti and Maramai (1996) for further details). 4.1. The 11 January 1693 tsunami The 11 January mainshock was by far more energetic and its e¡ects were catastrophic, also because they partly added to those produced by the foreshock. A tremendously high number of casualties were claimed (probably more than 54 000) and almost complete destruction was suffered within a 14 000 km2 area (Boschi et al., 2000), going from Catania in the north to Noto and Siracusa in the south (Fig. 4). Earthquake e¡ects were felt as far as southern Calabria to the north, Palermo to the west and the Isle of Malta to the south. The 11 January earthquake produced a tsunami that was observed along the entire coast of eastern Sicily from Messina to Siracusa, along the southern coast of Calabria, and even at the Aeolian Islands. The ¢rst observed sea movement was a withdrawal almost everywhere along the Ionian coast of Sicily. The most evident tsunami e¡ects were experienced by the town of Augusta, where the initial sea recession drained the harbour completely, causing severe damage to ships, especially to two large vessels coming from Malta. Then the sea inundated the town, killing many people and submerging the district close to the port as far as the San Domenico monastery: the waves were likely as high as 15 m (Tinti and Maramai, 1996). At Catania remarkable sea-level withdrawal and rise was observed. The sea water inundated the town penetrating San Filippo Square. In the Siracusa harbour three withdrawals and £ooding were observed, as well as in the Messina harbour to the north. Initial sea withdrawal followed by substantial £ooding was also reported in small villages like Agnone, Mascali and Taormina (Tinti et al., 2002a). 4.2. The 1693 source identi¢ed by means of seismological and geological data Initial macroseismic studies of the 11 January event resulted in epicentre location in the sea between the towns of Augusta and Catania (Barba-

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Fig. 4. Close-up view of the region hit by the 9 and 11 January 1693 earthquakes.

no, 1985), but more sophisticated later studies stressed the idea of an epicentre inshore (Boschi et al., 1999, 2000 ; see Fig. 4) and of a responsible fault located almost completely inland and associated with the Scicli^Ragusa^Monte Lauro Fault system (SRL in Fig. 5). This is interpreted as a transfer zone between the two adjacent Simeto and Scordia^Lentini grabens to the north and the Sicily Channel to the south. In particular, Sirovich et al. (1998) and Sirovich and Pettenati (1999, 2001) proposed the fault they called

EBT78 (Fig. 5) as the possible source for both the 9 January foreshock and the 11 January mainshock: it is a strike-slip sub-vertical transfer structure, about 60 km long and striking approximately NNE. A di¡erent inland source, the Avola Fault (AF in Fig. 5) has been suggested by Bianca et al. (1999) for the 9 January foreshock on the basis of geological and morphological data: it is a 20-km-long east-dipping normal fault separating the Avola Mountains from the coast.

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Fig. 5. Shaded relief map of the topography of southeastern Sicily and position of the faults discussed in the text.

Geomorphological investigations have led some authors (e.g. D’Addezio and Valensise, 1993) to introduce faults associated with the Scordia^Lentini extensional structure, striking NE^SW. Sirovich et al. (1998) and Sirovich and Pettenati (1999, 2001) studied the fault indicated with code SL1 in Fig. 5 and tested it against the macroseismic dataset. On the other hand, fault SL2 was suggested by D’Addezio and Valensise (1993), who considered it as the best candidate for the 11 January earthquake.

4.3. The 1693 source identi¢ed through o¡shore seismic data and tsunami analyses An alternative view based on considerations about the regional tectonic setting and on interpretation of seismic re£ection pro¢les (e.g. Hirn et al., 1997) suggests di¡erent possible faults and fault systems associated with the o¡shore Hyblaean^Malta Escarpment, which is the most prominent topographic feature in the whole area. Three main faults are drawn in Fig. 5, in-

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dicated with the code OF (standing for ‘O¡shore Fault’). Fault OF1 has been introduced by Sirovich and Pettenati (1999) on the basis of macroseismic data inversion: it runs for about 60 km along a direction mainly tangent to the Augusta and Siracusa promontories. Fault OF2 is formed by two separate o¡shore sub-faults, running parallel to the Hyblaean^Malta Escarpment for a total length of about 70 km from Catania to Siracusa. This fault is important since it was assumed as the basis to envisage a scenario of seismic damage evaluation for the town of Catania (see e.g. Pessina, 1999; Priolo, 1999; Zollo et al., 1999). Fault OF3 coincides with the so-called ‘Western Fault’, proposed by Bianca et al. (1999) based on the analysis of seismic re£ection pro¢les: it runs about 12 km o¡shore, parallel to the coastline from Catania to the promontory north of Siracusa. Analysis of tsunami data was somewhat overlooked by the initial studies on the 11 January earthquake, but it should be realised that it can give a relevant contribution. The ¢rst contribution to the modelling of the tsunami waves induced by the 11 January mainshock was provided by Piatanesi and Tinti (1998), whose work was recently further extended by Tinti et al. (2001). Tsunami modelling shows quite clearly that faults placed almost completely inland, like the SRL system, produce negligible displacement of the sea£oor and cannot therefore explain the generation of the observed tsunami. Tinti et al. (2001) simulated the tsunami produced by both the Scordia^Lentini Faults (SL1 and SL2) and by the OF1 and OF2 sources associated with the Hyblaean^Malta Escarpment. Tide-gauge records produced by each of the selected faults were computed in some of the major localities for which tsunami observations are available : then they were checked against the two main criteria that can be extracted from the coeval sources, namely: (1) the ¢rst water movement was a withdrawal, and (2) the largest waves were observed at Augusta, smaller waves in Catania and even smaller waves in the

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localities to the north of Catania and to the south of Augusta, including Siracusa. The main result of the simulations performed by Tinti et al. (2001) is that none of the studied faults is able to fully satisfy both the posed conditions. For example, taking into consideration fault OF2, numerical modelling shows that the tsunami produced by this fault has some accord with historic observations, but there are important data that are not matched. The most signi¢cant agreement concerns the negative ¢rst arrivals of the waves, that are reproduced rather well by simulations, whereas the most relevant mis¢t regards wave height in Siracusa. Computed waves are exceedingly higher than those reported in the accounts, from which it turns out that Siracusa did not su¡er great damage from the tsunami, and only a series of sea recessions and progressions were observed. This is mainly due to the fact that fault OF2 extends southward well beyond Siracusa (see Fig. 5), that is therefore on the main direction of propagation of tsunami energy. Anyhow, Tinti et al. (2001) showed that assuming a variant of fault OF2, that is obtained by cutting its southernmost part by about 20 km, leads to the production of a tsunami respecting better the observations. Fig. 6 shows the water elevation ¢elds at di¡erent tsunami evolution times computed for the modi¢ed version of fault OF2: the main tsunami front approaching the coasts hits predominantly the towns of Augusta and Catania, while Siracusa is mainly a¡ected by the trapped waves that become clearly visible after the main tsunami fronts have left the basin (see panels for t = 800 s and t = 1000 s in Fig. 6). Concerning the faults placed in correspondence with the Scordia^ Lentini graben, they are the ones that best reproduce the historic data in general, but the matching they provide with the wave amplitude inside the Augusta harbour, which should be the most important aspect according to the historic accounts, is not completely satisfactory. Indeed, the studies performed by Piatanesi and Tinti (1998) and by Tinti et al. (2001) do not

Fig. 6. Snapshots of the simulated propagation of the tsunami waves induced by the modi¢ed version of fault OF2 discussed in Tinti et al. (2001).

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exhaust the analysis, since it cannot be ruled out that other faults might exist that explain the data even better (for example, fault OF3). Nonetheless, it should be emphasised that only by means of tsunami analysis it has been possible to prove that fault SRL has to be rejected and fault OF2 is rather problematic. 4.4. The impact of the scenario tsunami In addition to the problem of the identi¢cation of the genetic fault, tsunami modelling can provide very interesting and useful information on the interaction of the water waves with the local coastal morphology and with the man-made works existing today, especially with the harbour structures. Clearly, this kind of study has a relevant impact on the correct assessment of the tsunami hazard for a given region and/or coastal town. From this point of view, the case of eastern Sicily is particularly interesting, since both very irregular coastlines (see Figs. 4 and 5) and impor-

tant harbours works (Augusta, Catania, Siracusa, Messina) can be found here. Tinti et al. (2001) showed that the peninsulas on which the historic centres of Augusta and Siracusa are built play a remarkable screening e¡ect on the incoming tsunami waves, reducing sensibly their amplitude within the inner basins, regardless of the particular source fault adopted. Recently, a very similar e¡ect has been found by Tinti et al. (2002b) to characterise the interaction of the tsunami waves with the set of three breakwaters that delimit the harbour of Augusta (Fig. 7). Investigation on the tsunami behaviour immediately outside and inside the harbour leads to the main conclusion that the three major breakwaters are able to reduce the impact of the incoming tsunami waves. One possible direction for future research will be a more in-depth analysis of the Augusta harbour characteristics: more precisely, much ¢ner details will have to be represented in the computational ¢nite-element grid both as regards the harbour structures and the bathymetry inside the harbour

Fig. 7. Position and close-up view of Augusta’s peninsula and harbour. The three breakwaters cited in the text are evidenced.

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Fig. 8. Geographic sketch of the region a¡ected by the 28 December 1908 earthquake and tsunami. In panel (a) the solid circles represent some of the localities for which tsunami observations are available (Platania, 1909a,b; Tinti and Giuliani, 1983). The station numbering is the same as that used in the upper panel of Fig. 10. In panel (b) the open triangles are the benchmarks where pre- and post-event geodetic measurements were collected (Loper¢do, 1909). Note that two di¡erent paths were surveyed in Sicily, the ¢rst running from Messina (node 1) to Gesso, the second from node 15 to Castanea. The numbering of the geodetic benchmarks is the same as that used in the lower panel of Fig. 10.

basin. Moreover, a similar approach could be applied to other important harbours in the area, such as Catania, Messina and Reggio Calabria.

5. The 1908 tsunami in the Messina Straits The earthquake (MW 7.1) that occurred in the early morning of 28 December 1908 was catastrophic. It devastated many cities of Sicily and Calabria, destroying big towns such as Messina and Reggio Calabria and producing very severe damage in hundreds of smaller cities and villages in Sicily and Calabria. More than 60 000 people were killed. It generated a violent tsunami that hit all the coast of the Straits and was also observed and measured as far as the Malta archipelago,

more than 400 km to the south, and recorded by the mareograph in the harbour of Naples more than 350 km to the north of the source. The tsunami caused large damage in the Straits, and was particularly disastrous at S. Alessio (maximum observed run-up = 11.7 m) and Giardini Naxos (9.5 m) on Sicily, and at Lazzaro (10.6 m), Pellaro (8 m) and Reggio Calabria (9.7 m) on the opposite Calabrian coast (see Fig. 8a for the location of the cited stations). Data on this tsunami are abundant and comparable with data sets usually available for recent events, thanks to the great emotional impact and scienti¢c interest that the disaster excited in scientists in Italy as well as many other countries. Field surveys conducted in the weeks after the tsunami provide us run-up data along hundreds of kilometres of coasts in

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Fig. 9. Summary of some of the faults proposed in the literature as responsible for the 1908 earthquake. The rectangles are the surface projections of the normal fault planes, with the dipping direction indicated by the solid arrows.

Sicily and Calabria (see e.g. Platania, 1909a,b) and show that wave heights were 1^3 m in the northern part of the Straits and much larger (5^ 10 m) in the southern part, i.e. south of Galati Marina and Reggio Calabria, with extremes exceeding 11 m. Waves were very large even for a long coastal segment south of the Straits up to Aci Reale, a few km north of Catania. At Catania beach (called La Plaia) waves were less than 3 m, but penetrated about 700 m inland carrying a big vessel and several boats. To the south of Augusta

and Siracusa waves were about 1^2 m high. The distribution of experimental maximum wave elevations observed in the subregion depicted in Fig. 8a is shown in the upper panel of Fig. 10 as a solid line. 5.1. The 1908 source inferred from seismological and geodetic data Most studies on the earthquake concentrated on the macroseismic data, on records of seismom-

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eters that were operational in those early times of instrumental seismology in Italy and Europe, and on pre-event and post-event levelling data concerning Calabria and a small area close to Messina (Loper¢do, 1909). The surveyed levelling lines and the position of the 114 benchmarks for which measured vertical co-seismic displacements are available are shown in Fig. 8b: the corresponding measurements (Loper¢do, 1909) are plotted in the lower panel of Fig. 10. Based on the available data, as well as on geomorphological observations, a number of di¡erent faults have been proposed in the literature as the source of the earthquake ; some of them are summarised in Fig. 9. Shick (1977) retrieved his fault model from the analysis of P-wave polarities, while Bottari et al. (1986) model is mainly based on macroseismic data. It is interesting to notice that both these models, as well as the one proposed by Monaco and Tortorici (2000) based on tectonic and morphological considerations, indicate normal faulting on N^S to NE^SW striking faults with NW or WNW dipping directions. The results obtained upon modelling of levelling data lead to sensibly di¡erent conclusions. Mulargia and Boschi (1983) performed a trial-and-error procedure and formulated a model involving two normal faults, with a shallower northern segment dipping at low angle toward ESE and a steeper southern portion, antithetic to the former. Later, Capuano et al. (1988) and De Natale and Pingue (1991) on one side, and Boschi et al. (1989) and Valensise and Pan-

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tosti (1992) on the other side, performed independently levelling data inversion and developed rather similar source models, di¡ering mainly for the strike direction and fault dimensions. The important aspect is that both models involve east-dipping faults characterised by small dipping angles (30^40‡). The hypothesis of a source dipping mainly toward the east was con¢rmed by Valensise and Pantosti (1992, 2001) based on the geological study of the state of deformation of the 125-ka marine terrace. 5.2. Contribution of tsunami analyses Tinti et al. (1999c) and Tinti and Armigliato (2000, 2001) studied numerically the available tsunami data taking as starting fault models the Capuano et al. (1988) and Boschi et al. (1989) sources. The key observation was that if tsunami analysis is applied to the chosen faults, di⁄culties arise since the observed characteristics of the tsunami are not reproduced by the corresponding numerical simulations. Essentially, these faults give rise to tsunami waves that are larger in the north and smaller in the south of the Straits, or at best as large in the north as in the south, which does not agree at all with what is known. Here we take into examination only the model by Capuano et al. (1988), which was further re¢ned by De Natale and Pingue (1991) to account for slip heterogeneity on the fault plane. The tsunami run-up observations were inverted by Tinti et al. (1999c)

Table 1 Results of tsunami run-up and geodetic data inversion for faults C, EC and CS Fault

u1 (m)

u2 (m)

u3 (m)

u4 (m)

Ampli¢cation factors

Run-up rms (m)

Geodetic rms (m)

C (hom1) C (hom3) C (het1) C (het3) EC (hom1) EC (hom3) EC (het1) EC (het3) CS (hom1) CS (hom3) CS (het1) CS (het3)

1.5 1.5 1.5 1.5 1.13 1.13 0.565 0.565 ^ ^ ^ ^

1.5 1.5 0.75 0.75 1.13 1.13 0.565 0.565 1.5 1.5 0.75 0.75

1.5 1.5 2.25 2.25 1.13 1.13 1.695 1.695 1.5 1.5 1.65 1.5

^ ^ ^ ^ 1.13 1.13 1.695 1.695 1.5 1.5 2.1 2.25

5.8 A1 = 5.9, 5.1 A1 = 3.9, 8.4 A1 = 9.2, 7 A1 = 5.9, 6.4 A1 = 6.5, 6.4 A1 = 6.6,

4.07 2.58 3.56 2.29 3.39 2.14 2.68 1.74 3.32 2.10 2.67 1.67

0.057 0.057 0.13 0.13 0.096 0.096 0.14 0.14 0.078 0.078 0.13 0.13

A2 = 33.1, A3 = 4.1 A2 = 17.8, A3 = 4.5 A2 = 24.5, A3 = 5.8 A2 = 14.1, A3 = 5.5 A2 = 18.4, A3 = 4.6 A2 = 12.9, A3 = 4.7

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Fig. 10. Comparison between measured run-ups and computed maximum water elevations (upper panel), and between geodetic measurements and computed co-seismic vertical displacements (lower panel) for a few of the cases studied in Table 1. In the upper panel, measured run-ups are taken from Tinti and Giuliani (1983). See Fig. 8 for the numbering of both the coastal stations and the geodetic benchmarks.

and by Tinti and Armigliato (2000, 2001) through a weighted least-squares minimisation of the runup data residuals. The adopted scheme, which is thoroughly described in Tinti et al. (2001), allows to retrieve the slip value on a number of given independent sub-faults and to compute one or

more ampli¢cation factors, accounting for the growth of the tsunami waves on approaching the shallow coastal waters. Moreover, Tinti and Armigliato (2000, 2001) tested the slip distributions retrieved through run-up inversion against a subset of the levelling dataset, with the main

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goal of investigating the possibility of ¢nding a solution able to reproduce satisfactorily both kinds of data. In addition to the original fault by Capuano et al. (1988) (fault C in panel (f) of Fig. 9), Tinti and Armigliato (2000, 2001) took into account possible southward extension (fault EC) and southward shift (fault CS) of the seismic source. Table 1 summarises the results of the inversion for the three faults C, EC and CS, while Fig. 10 shows curves referring to selected cases treated in Table 1. Both homogeneous (hom) and heterogeneous (het) slip distributions on the fault have been investigated. The slip values on each subfault were allowed to vary within pre¢xed intervals, namely from 0.5 to 1.5 times the average value of the slip in the uniform case (1.5 m). Cases with one or three ampli¢cation factors were examined. When three ampli¢cation factors are used, they refer to the Calabrian (A1 ) and Sicilian (A2 ) coasts of the Straits, respectively, and to the remaining stations (A3 ). So, for example, code ‘EC (het3)’ in Table 1 means that the extended version of fault C is examined in the tsunami inversion procedure, with heterogeneous slip distribution and three ampli¢cation factors. Note further that Table 1 extends and completes the discussion performed in Tinti and Armigliato (2000). Moreover, the complete set of levelling measures is used here to compute the geodetic rms in the last column of Table 1. The ¢rst conclusion we arrive at is that it is di⁄cult to ¢nd a single source matching simultaneously both tsunami and levelling data. Fault C with homogeneous slip produces the best agreement with the levelling data but the worst matching with maximum tsunami wave amplitudes, especially in Sicily. The mis¢t with the tsunami measures is sensibly reduced when a heterogeneous slip distribution is introduced, with the highest slip patch being predicted for the southernmost portion of the fault. Note that this is not in agreement with the results obtained by De Natale and Pingue (1991) by inverting levelling data alone: in that study, the highest slip patch was found in correspondence with the central portion of the fault (see also Pino et al., 2000). However, Table 1 clearly shows that the tsunami data in-

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version leads to a slip concentration in the southern part of the source in each of the studied con¢gurations in which heterogeneity is allowed, and should be then regarded as a stable and reliable feature. We further observe that this discrepancy could be imputed to the di¡erent spatial coverage of the levelling and tsunami measurements. No levelling data are available in Sicily south of Messina, while post-event tsunami surveys allowed data collection along the entire Sicilian coast, and in particular on the portion facing the Straits (see Fig. 8). A further reduction of the mis¢t with respect to tsunami data results from the introduction of three ampli¢cation factors, with minimum values of 1.74 m and 1.67 m obtained for cases EC (het3) and CS (het3), that is for fault C being extended or shifted to the south. But no similar improvement in the matching between measured and computed geodetic data is obtained: to the contrary, improving the run-up rms generally implies increasing the geodetic rms. The relevant conclusion that a sensible improvement on the run-up rms is obtained if the original fault C is extended or shifted to the south is in agreement with our previous studies and with the very recent results obtained by Amoruso et al. (2002). Their analysis, based on the joint inversion of levelling data and P-wave polarities, led to a fault model with strike and dip angles and focal mechanism similar the those obtained in the previous studies dealing with levelling data, but characterised by a high slip concentration in a region extending to the south well beyond the southern limit of the Calabrian coast. The above discussion suggests that several problems remain open concerning the Messina 1908 earthquake and tsunami. Concerning the tsunami analysis, one possible future research direction could be the application of the tsunami inversion approach adopted for the models by Capuano et al. (1988) and by Boschi et al. (1989) also to other faults proposed in the literature on the basis of di¡erent datasets (see Fig. 9). A second possible task could be represented by the enlargement of the domain adopted in the previous studies for the ¢nite-element tsunami simulations, shown in Fig. 8: in particular, the

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entire eastern coast of Sicily should be included in the computational domain, since additional experimental run-up measures are available in localities placed beyond the southern limit of the currently adopted domain. Indeed, this would require a heavier computational e¡ort, since a more extended and reasonably dense ¢nite-element grid should be built. But the future discussion should regard also other important aspects, and primarily the models used to compute static displacements. The analytical model (Okada, 1992) commonly used in the inversion of levelling data is based on the adoption of a homogeneous and perfectly elastic halfspace bounded by a £at free surface as a representation of the Earth’s upper crust. In areas characterised by large topographic and bathymetric gradients, like the Messina Straits and southeastern Sicily (see Fig. 3), the assumption of a £at free surface seems questionable and can lead to distorted results. In particular, Tinti and Armigliato (2002) developed and applied a 2-D hybrid approach to study the e¡ect of the irregular topography of the Earth’s free surface on the coseismic displacements produced by buried faults. The approach was applied to both theoretical and real topographies, and signi¢cant discrepancies were found between the surface displacement components computed accounting for or neglecting the topographic features : more precisely, one can expect that neglecting topography in inverse modelling could lead to biased estimates especially of the dip of the fault and of its depth. The same 2-D approach has been applied by Armigliato and Tinti (2002) to the Messina Straits area : their preliminary investigations have con¢rmed the main results found by Tinti and Armigliato (2002) and have also drawn the attention to the consequences that an incorrect modelling of the initial sea-bottom deformation produced by a submarine earthquake can have on the tsunami generation mechanism, and hence on the entire tsunami evolution. The extension of this kind of approach to three dimensions could be a very powerful tool toward a more realistic earthquake-induced tsunami modelling and would allow for further improved tsunami hazard assessment along the Italian coasts.

6. Open problems and future perspectives The cases illustrated show that the identi¢cation of the sources of the earthquakes that have generated large tsunamis is a di⁄cult task and that the joint contribution of various disciplines such as geology, seismology and tsunami modelling is needed in order to cast light on the solution, though sometimes even an interdisciplinary approach is unsuccessful and is unable to lead us to unambiguous results. The questions that seem to be still open can be summarised separately for the cases that have been dealt with in the paper. (1) The 1627 tsunamigenic earthquake in the Gargano area was catastrophic and caused a large tsunami. The seismic fault that is the best candidate as the source of this earthquake is the Apricena^Sannicandro lineament (see Fig. 1) whose trace is quite clear from LANDSAT images (Salvi et al., 1999). This is in agreement with the reconstruction of the macroseismic ¢eld (Panza et al., 1991), with tsunami simulations (Tinti et al., 1997) and with the mechanism that can be inferred from morphological analyses showing the uplift of the northern block (Salvi et al., 1999). This convergence of results depicts a scenario that is su⁄ciently well de¢ned, though some properties of the fault (such as width, depth, dip, and the maximum co-seismic slip) remain unknown and should be determined by future research. But what seems more urgent here is to use this fault to run earthquake and tsunami models in order to evaluate the impact of the seismic and the water waves on houses and coastal structures. We observe that the area is a very famous and £ourishing tourist area with the coastal belt characterised by alternation of high rocky cli¡s and sandy beaches that in the peak summer season are populated by tens of thousands of tourists, which increases dramatically the exposure of the region to the attack of a tsunami. (2) The identi¢cation of the source of the 11 January 1693 eastern Sicily earthquake has not been satisfactorily solved yet. Tsunami analyses carried out so far can be conveniently used to rule out some hypothetical genetic faults (such as the SRL and EBT78 faults depicted in Fig. 5), but have failed to point to a source that

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is totally consistent with tsunami observations. Therefore, research in future years should have the priority objective of casting new light on the parent fault, orienting e¡orts especially to marine exploration of the sea bottom o¡shore eastern Sicily which means both planning and execution of new ad-hoc surveys and processing/reprocessing of data collected in the area by the numerous scienti¢c cruises carried out in past years (see e.g. Argnani and Bonazzi, 2002). Notwithstanding the uncertainties in the source, running scenario tsunami cases have proven to be useful to understand the impact of the waves on some relevant coastal structures (see e.g. the damping e¡ect of the jetties and breakwaters of the Augusta harbour found by Tinti et al., 2002b) and should be extended. (3) The 1908 tsunamigenic earthquake source is certainly placed under the Messina Straits, and has caused subsidence of the sea £oor. Uncertainties in the source position still remain, but the discrepancies among di¡erent studies are not dramatic. Tsunami analyses had the merit to show that the source extends under the Ionian Sea south of the Straits (Tinti and Armigliato, 2000, 2001), with a further con¢rmation coming from very recent inversion studies on seismic signals and geodetic data (Amoruso et al., 2002). Re¢nements of these studies are certainly welcome (see e.g. the contributions expected by more sophisticated models to compute the sea£oor displacements produced by a buried fault accounting for complicated local topography, Armigliato and Tinti, 2002), but what seems more urgent is running detailed scenario simulations to compute the e¡ect of tsunami in the Straits and on the coast of Sicily, since here the coastal zone has a paramount economic and social relevance, and the global value in terms of human lives and properties exposed to the menace of a tsunami is enormous. The lack of knowledge on the tsunami source that has been pointed out above is certainly even more acute for other historic cases that can be found in the Italian catalogue. In several circumstances, however, our ignorance on the source is not so drastically dramatic as to hamper a meaningful application of the concept of a scenario.

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Indeed, in case of uncertainty the scenarios corresponding to all possible sources could be run. Then a wise conservative principle suggests that tsunami countermeasures could be based for any given locality on the worst case, that is the maximum water elevation, maximum water penetration or maximum water force, etc. resulting from the set of the various scenarios. This methodology should be applied to compute thematic maps related to hazard, vulnerability and risk, such as inundation maps, for all coastal segments and sites of Calabria, Sicily and Apulia that are most exposed to the attacks of tsunami. No such maps exist so far for any single piece of the Italian coast, which means that ¢lling this gap should be a high-priority task for the tsunami scientists of Italy in the coming years.

Acknowledgements This work was ¢nanced by the Gruppo Nazionale di Difesa dai Terremoti of the Istituto Nazionale di Geo¢sica e Vulcanologia (INGV) of Rome, Italy.

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