RICERCA ITALIANA     

Analysis of risk from tsunamis in the Calabrian Arc and in the Adriatic sea

Università degli Studi di Bologna

Abstract

The project proposes a multidisciplinary approach to study the active tsunamigenic seismic faults of the Calabrian Arc (CA) and of the Adriatic Sea (AS) regions, and to identify and to analyse plausible scenarios of risk. The working team possesses the required expertise to investigate active faults from the viewpoint of their morpho-structural and seismotectonic characterisation (such as structural geology, seismology, historical and macroseismic seismology) as well as the technical knowledge needed to study tsunami generation by earthquakes and by mass failures (seismology, geotechnical geology and engineering) and to investigate tsunami impact on coasts and on coastal structures (oceanography, geomorphology). The chief motivations of this research are: 1a) the CA is the Italian region with the highest rate of large earthquakes and tsunamis, it is crossed by a system of normal faults striking as the mountain range, and by transcurrent faults cutting normally the chain; 1b) transcurrent faults and some of the normal faults continue into the sea (south Tyrrhenian and Messina strait); 1c) the main system of faults off eastern Sicily is associated with the Hyblean-Malta escarpment, and other relevant faults are near-shore and intersect the coast; 1d) the present morpho-structural knowledge of all the above faults is incomplete; 1e) there is uncertainty on the source fault of several tsunamigenic earthquakes; 1f) the potential of tsunami generation by mass failures triggered by earthquakes is never been evaluated; 1g) vulnerability and risk analysis for the coasts of Calabria and Sicily affected by tsunami cannot be further postponed; 2a) the AS has been affected by large earthquakes and tsunamis especially in the south; 2b) the seismotectonics of the Gargano region is complex and is still incompletely understood; 2c) the fault responsible for the large tsunamigenic earthquake of the 1627 is still unknown; 2d) the tsunamigenic potential of seismic sources in western mid and southern Adriatic is not known; 2e) analysis of vulnerability and risk due to tsunami threat has never been attempted for the coasts of the AS.
The activities of the project may be broken down in the same sequence of concepts for both geographical regions (CA and AS): near-shore active faults, scenario of catastrophic events, assessment of risk associated with the tsunami. The activities are detailed here below: 1) recognition of geometry and kinematics of the active tectonic features interacting with the landscape evolution by means of fieldwork and by interpretation of aerial photographs and satellite images (SPOT and LANDSAT 5 TM); 2) structural analysis of the major fault systems to discriminate regional deformation patterns from deformation due to single faults (analysis of fault scarps, of stream network, of deposits affected by faulting, of marine terraces, of paleoshorelines); 3) geomorphological analysis of coastal landscape and identification of landforms related to the action of tsunamis; 4) analysis of lagoon logs - see Ganzirri lake - and of coastal sediment cores to identify tsunami markers, also with the goal of assessing frequency and intensity of paleotsunamis; 5) analysis of seismicity and of historical tsunamigenic events; 6) association between faults and earthquakes; 7) definition of scenarios of destructive tsunamis in the CA and in the AS, including ground effects and tsunamigenic mass movements; 8) collection of high-quality bathymetric data and near-shore topographic data; 9) numerical simulations of tsunamis; 10) physical modeling of tsunamis due to mass failures; 11) assessment of tsunami effects on the coastal natural environment as well as on structures of special value; 12) analysis of tsunami risk; 13) creation of a pilot GIS system for tsunami risk mitigation. <<<

Principal Investigator

Stefano TINTI Università degli Studi di BOLOGNA

Research Objectives

The main goal of the project is to identify and to characterise the most important submarine active faults in the regions of the Calabrian Arc (CA) and of the Adriatic Sea (AS) with the purpose of assessing the related risk. These regions have different tectonic styles, but have similar features that give us the motivations to study them together. These are: 1) from catalogues of historical events it is known that both regions have been hit by destructive earthquakes repeatedly that in turn caused disastrous tsunamis; 2) seismic sources are placed near shore and often involve the coastal zone; 3) earthquakes are able to offset the sea floor as well as to mobilise marine or coastal sediments with consequent changes of the coastline position; 4) the involved marine basins have complex morphology with presence of structures that tend to trap tsunami energy even with formation of edge waves and to amplify their effects (e.g. Messina Straits, Gargano promontory, Strait of Otranto); 5) existence of coastal basins capable of preserving traces of paleotsunami inundations (e.g. lakes of Lesina and Varano in northern Gargano, lake of Ganzirri, close to Messina); 6) the involved areas have a paramount socio-economic value since development concentrated mainly in a narrow coastal belt for historical tradition and also for recent trends (over 60% of the economic activities of Calabria and eastern Sicily are carried out in the coastal zone where more than 50% of the population lives; similar figures hold also for the Italian regions watered by the AS).
The objectives of the projects can be illustrated schematically through the following key questions which the participants intend to give appropriate answer to. Questions are listed separately for region CA and region AS for the sake of clarity, but some are expectedly common to both.
CALABRIAN ARC
What are the source faults of the main tsunamigenic earthquakes (e.g. March 1638, January 1693, the sequence of 1783, 1905, 1908)? What is their geometry? What is their extension offshore and onland? What is their kinematics, and what relation do they have with the dynamic processes ruling the CA evolution in the Plio-Quaternary? What is the relationship between the normal faults in the chain and the transcurrent faults cutting orthogonally the mountain range? Is there any interaction between onshore along-chain faults and the offshore or near-shore faults? What is the relationship between the offshore faults associated with the Hyblean-Malta escarpment and the active faults of eastern Sicily? What is the expected return period of the large tsunamigenic earthquakes? What contribution is expected from studying geomorphological and sedimentary traces of paleotsunamis?
What is the relationship between the seismic dislocations in the lower crust and the co-seismic displacements of the upper crust and of the sediment surface layers? What is the expected amount of sea bottom dislocation resulting from the tsunamigenic faults in the main marine basins of the CA, namely eastern Sicily, the Messina strait, the gulf of Gioia and the gulf of St.Eufemia? How big is the expected offset of the coast? Are CA earthquakes able to provoke submarine landslides or to mobilise coastal segments? What are the characteristics of the scenario earthquakes and tsunamis? Are there relevant sea-surface oscillations that have the same period as tsunamis? How high is the expected tsunami wave on the coast? What are the coastal segments most exposed to tsunami attack? What is the expected maximum inundation and maximum sea withdrawal, or equivalently, what is the maximum run-up and the maximum draw-down? What are the effects and damage expected on the attacked coast? What is the tsunami action on the structures: for example, on the maritime works of the harbours, on the communication lines running along the coastline, such as roads, motorways and railways? What is the effect we can expect on a big coastal town such as Reggio Calabria? What countermeasures can be suggested to the civil protection authorities?
ADRIATIC SEA
What are the source faults of the main tsunamigenic earthquakes (e.g. July 1627 in Gargano area, April 1672 and March 1875 in central Adriatic) ? What is their geometry? What is their extension offshore and onland? What is the tsunamigenic potential of the sedimentary bodies that are found in the transition zone between the shallow, about 100-200m deep, platform in northern-central Adriatic and the southern Adriatic depression (around 1000-1200 m deep)? What impact did paleo- and recent tsunamis have on the coasts? What are the characteristics of the scenario sources and tsunamis in the central AM and in the southern AM? How high is the expected water wave on the coast? What are the coastal segments most exposed to tsunami attack? What is the expected maximum penetration and maximum sea withdrawal? What are the effects and damage expected on the attacked coast? What is the tsunami action on the structures?
It is worth and not superfluous to mention that all the above questions are problems that are open at the present status of knowledge, and moreover that some issues (i.e. those regarding tsunami impact of tsunamis on the coasts of Sicily, Calabria and Apulia) are problems, that no previous research has ever tackled before with the methods that are proposed here.
It is further worth to point out that a pilot example of GIS will be built: the system will contain and allow the access to all the results of the researches undertaken by the team of partners that are finalised to the analysis of risk from tsunamis. The GIS data will be of various nature coming from different disciplines: seismology, geology, geomorphology, oceanography, history and archaeology, use of coastal zone and urban sciences. <<<

First Results

CALABRIAN ARC
Identification of the active faults, discrimination between regional and local deformation and assessment of long-term and short-term (Holocene) deformation rates, with special attention to the structures of Capo Vaticano towards the Tyrrhenian sea, to the high of Palmi-Bagnara and to the structures of the Peloritani range bordering the Ionian sea. Identification of possible paleotsunamis through paleoseismological investigations and analysis of stratigraphic anomalies. Recognition of coastal mass failures associated with large earthquakes. Seismic characterisation of the region with association of structures to seismicity pattern. Acquisition of geomorphological, bathymetric data as well as data on the urban, commercial and industrial structures in areas that are exposed to tsunamis, finalised to the numerical simulations of tsunamis and to the computations of inundation. Delineation of disaster scenarios. Numerical simulations of the scenario events. Maps of maximum inundation expected in relevant areas. Assessment of tsunami risk for the selected scenarios.
ADRIATIC SEA
Identification of active interplate and intraplate sources. Characterisation of their tsunamigenic potential. Re-examination of the catalogues of tsunamis, especially for the eastern coasts of the basin. Identification of possible paleotsunamis by means of geomorphological investigations, coring and dating of core samples. Determination of frequency and intensity of tsunamis, by combining data on paleotsunamis and on historical events. Collection of bathymetric and topographic data, finalised to the numerical simulations of tsunamis. Delineation of disaster scenarios. Numerical simulations of the scenario tsunamis. Maps of maximum inundation expected in relevant areas. Assessment of tsunami risk for the selected scenarios.
SEISMIC SOURCES
Calculation of the coseismic deformation by means of analytical and semianalytical models for layered media. Numerical code to build meshes with tetrahedral elements. Numerical code to compute coseismic deformations in finite-element 3D grids. Algorithm to assign conditions on the "crustal" boundary of the finite-element 2D grid through an iterative procedure.
TSUNAMI MODELLING
Algorithm to compute run-up to be developed within the tsunami simulation code based on 2D finite elements. Test against analytical cases and through 1D codes. Code to calculate the impact of waves against structures. 3D module for the generation of tsunamis induced by mass movements and integration of the code into the 2D code for tsunami propagation.
GIS
Implementation of a pilot GIS consisting of data items structured in multiple levels (geographic, geomorphological, seismotectonic, demographic, land use and anthropization data) up to the level including the estimates of risk associated to tsunamis. <<<

Timescale

24 months

National and international background

PRESENT KNOWLEDGE ON THE CALABRIAN ARC
The Calabrian Arc (CA) is a structure belonging to the Mediterranean orogenic belt, and connecting the Maghrebides to the Appennines. The geodynamical evolution of this region is complex. Geological and geophysical data suggest that in the last 0.7 Myrs the geodynamical evolution of the CA is characterised by vertical motion (Pirazzoli et al., 1997; Stewart et al., 1997; Bordoni and Valensise, 1998; Antonioli et al., 2003), as well as by movements on the transform faults. The focal mechanisms of recent and historical earthquakes show an extensional mode of deformation, both parallel and perpendicular to the arc (Frepoli and Amato, 2000). Geological data also indicate that extension acts along NE-SW and NW-SE faults since the middle Pleistocene, modelling graben-like structures trending both NE-SW and NW-SE (Tortorici et al., 1995). The main regional feature in this area is given by a prominent normal fault that runs more or less continuously for a total length of 370 km along the inner side of the CA, extending across the Strait of Messina along the Ionian coast of Sicily as far as the Hyblean Plateau. The distinct fault segments are characterised by a very young morphology and control both the major mountain fronts of the region (Catena Costiera, Sila, Serre, Aspromonte, Peloritani, Hyblean Plateau) and the coastline of the southern Calabria and eastern Sicily. Morphological observations and stratigraphic data indicate that different fault segments are characterised by long-term (since Middle Pleistocene) vertical slip rates ranging between 0.5 and 1.2 mm/yr (Tortorici et al., 1995; Monaco et al., 1997; Stewart et al., 1997). Values of about 2.0 mm/yr characterise the fault segments located in the volcanic district of the Aeolian archipelago and at Mt.Etna (Monaco et al., 1997). The system of normal faults is formed by several faults striking, from north to south, N-S (Crati Valley fault system), NE-SW (Serre and Cittanova faults), then ENE-WSW and finally NE-SW (Reggio Calabria fault) (Tortorici et al., 1995; Monaco and Tortorici, 2000). Also present are faults striking E-W, namely the Crati Valley and the Lamezia grabens. From the catalogues of the Italian earthquakes and tsunamis (Boschi et al., 1995; Tinti and Maramai, 1996; Boschi et al., 1999; CSTI, 2001) there is evidence that Tyrrhenian Calabria and the Messina Strait are the Italian regions with the highest occurrence rate of disastrous earthquakes and tsunamis, and that most of the shocks shaking this area in the last four centuries have also generated a tsunami: the most relevant ones occurred in March 1638, in February and March 1783 (5 events), in 1905 and 1908. The last destructive event is the earthquake-tsunami of 28 December 1908, that caused over 80,000 deaths. It is also proven that large earthquakes are associated with mass failures and collapses that, in turn, can generate tsunamis (e.g. the seismic crisis of 1783 and the 6 February tsunami causing more than 1500 casualties on the Marina of Scilla, in Calabria). What are the source faults of all these earthquakes? It is still unclear. Monaco and Tortorici (2000) state that most such earthquakes are shallow and are located in the hanging walls of the different Quaternary normal fault segments, while for the 1905 and 1908 events they suggest faults continuing into the sea. That identifying the fault capable of producing a tsunami is a very difficult task is proven by the emblematic case of the 1908 event: even today there is no general consensus on the characteristics of its genetic fault, in spite of the intense research carried out by several groups in the last 20 years with analysis of geological, geodetic and seismological data (De Natale, 1991; Pino and Valensise, 2001; Amoruso et al., 2002). Seismic quiescence for medium-to-large size events in the last 1000 years has been observed along different fault segments located both in northern Calabria and eastern Sicily (Monaco and Tortorici, 1995). This implies that further studies of these fault segments must be carried out in order to define the possible largest earthquake associated with each fault segment and its recurrence interval. Available structural and seismological data collected in the last years on different portions of this region have represented constraints for the formulation of large-scale seismotectonic models, that, though differing in several fundamental aspects, show the existence of concomitant dynamical processes acting at different crustal depths and at different space-time scales. With the exception of few studies (seismic sequences of 1783 in southern Calabria (Jacques et al., 2001), of 1693 (Bianca et al., 1999), of 1638 (Galli and Bosi, 2003) available information is not sufficient for defining the effective seismic and tsunamigenic potential of active faulting.
PRESENT KNOWLEDGE ON THE ADRIA MICROPLATE
Motion and deformation of the Adriatic microplate can be interpreted in the frame of the ongoing collision between Africa and Eurasia (Meletti et al., 2000). The Apennines to the west, the Alps to the north, and the mountain chains from Dinarides to the Hellenides to the east form the plate borders. The plate plays a key role in the kinematic processes that involve the Balkan and Italian areas. In fact the Adria plate is the foreland of the periadriatic chains and its characteristics, typical of an ancient and rigid continental plate, influence the tectonic evolution of the younger orogens that surround it (Doglioni et al., 1994). The relationships between Adria and the African plate are still subject of debate. In particular, it is still controversial whether there is structural continuity with the Africa plate or rather a discontinuity running along the Otranto Strait or along a belt crossing the middle Adriatic. Seismicity concentrates at the plate margins: mainly a dextral transpressive and compressive regime in the eastern margin from Greece to Croatia (Slejko et al., 1999), thrust and transcurrent faulting in central Italian Adriatic (Romagna and Marche), normal and strike-slip earthquakes in the Gargano area. The distribution of the Adria microplate seismicity shows that the most relevant sources are placed in the coastal belt (Slejko et al., 1999) and that further sources, though more modest and with lower activity rate, may be also found offshore (Console et al., 1993; Del Gaudio et al., 2002). In the tsunami catalogues of the Mediterranean regions (Tinti and Maramai, 1996; Soloviev et al., 2000) evidence is found that the Adriatic and the northern Ionian basins were affected by several tsunamis, some of which even of large size. The catalogues of the historical tsunamis available today cover a time interval of about 2000 years, but they are largely incomplete in the first centuries. Practically, it can be stated that information on the major events is complete in the last two centuries on the eastern side coasts of Adria and in the last four centuries on the Italian coasts. To the east, the most active areas are the Ionian Islands (Greece), Albania and Montenegro. As for Italy, the most affected coastal segments are the central Adriatic part (Romagna, Marche) and the Apulia region. It has to be pointed out that the most disastrous tsunami of the Adriatic sea included in catalogues is the 1627 Gargano case. This tsunami, for which the available documentation is rather good, flooded deeply a long stretch of coast between San Nicandro, the Fortore mouth and the Lesina lake. Recent geological studies on coastal deposits (Gianfreda et al., 2001; De Martino et al., 2003) have found evidence of flooding in northern Gargano, that could be attributed to high-energy tsunamis that are not mentioned in catalogues and that would have taken place in centuries prior to 1600.
Further effects of impact of tsunamis have been recognised along the southern coast of Apulia. They were attibuted to the 1667 and 1743 events along the Adriatic coast and to the 1456 event along the Ionian coast (Mastronuzzi and Sansò, 2000 and 2004). These are findings of relevance as regards the evaluation of risk associated to tsunami activity. Tsunamis in historical times hit areas that were almost deserted. The present state of the distribution of population and of the industrial plants in the coastal zone is such that the occurrence of tsunamis would produce by far more severe damage than in the past (Mastronuzzi et al., 1989; Caldara et al., 1998).
COASTAL TSUNAMIS
European and regional tsunami catalogues (Soloviev et al., 2000; Tinti and Maramai, 1996; Tinti et al., 2004) show that the most active regions in the Mediterranean are the CA, the Corinthian Gulf (Greece) and the Marmara Sea, and that nearly all sources are far no more than some tens of kilometres from the shoreline. Near-shore tsunami sources pose difficult scientific questions concerning their source and their effects: 1) their source is very complex, usually depending not only on tectonic dislocations associated with the parent earthquake, but also on additional sea floor displacements caused by ancillary faults and on sea wave excitation due to coastal and offshore slope failures (see the Riangkroko slump with over 20m run-up caused by the Flores 1992 earthquake); 2) since the threatened coastlines are within or next to the source region, the complexity of the source influences the tsunami wave from its inception to its disastrous impact against the coasts and must be taken in proper account and properly modelled, though it can be neglected for far tsunamis, often associated with point- or Okada-like sources. The research on tsunamis in Europe has been performed so far developing three main investigation lines: 1) historical reconstruction, 2) numerical simulations to infer the source, 3) numerical simulations to study tsunami propagation. The first line led to identifing criteria for a unified catalogue of the European tsunamis (Tinti, 1993), to an Italian tsunami catalogue, in agreement with those criteria (Tinti and Maramai, 1996; Tinti et al., 2004), and to a preliminary version of an European catalogue now still under revision (Tinti et al., 2001a). Although in principle the same waveform inversion techniques can be applied to seismic and tsunami records, in practice this is not allowed for tsunamis due to the endemic scarcity and low-quality of tide-gauge records, apart from some rare exceptions (Piatanesi et al., 2001). Indeed, especially for historical tsunamis, a different approach is needed to take into account semi-quantitative data such as first arrivals polarities (ebb or flood) and maximum wave height observed on land. This new technique was first successfully introduced to study the tsunamigenic 1992 Nicaragua earthquake (Piatanesi et al., 1996) and applied with variants to study important tsunamigenic Italian earthquakes such as the 1627 Gargano, the 1693 Augusta and the 1905 Calabrian events (Tinti et al., 1997; Piatanesi and Tinti, 1998a; Tinti et al., 2001b; Piatanesi and Tinti, 2002). Tsunami propagation is influenced by bathymetry and by complexities of marine basins. To study waves travelling in the open sea, finite-difference simulation codes are adequate and convenient since they do not require large amounts of computer resources. But they are unsatisfactory to handle small-scale irregularities of the bathymetry or of the coastline: to this scope unstructured grids and finite-element methods are more suitable (Tinti et al., 1994) that allow one to study adequately wave amplification and energy trapping (Piatanesi and Tinti, 1998b). Near-shore earthquakes often destabilise sediments, determining slides and coastal failures, as is well documented for many cases. This occurred also during the last large Italian tsunamigenic earthquake (Messina, 1908) that caused sliding of long coastal segments into the sea in the strait. Mass movements are able to set in motion tsunamis in addition to earthquakes. Landslide-induced tsunamis have to be studied by codes different from those suitable for tsunamis of seismic origin, since the dynamics of slide itself has to be calculated. Such a model was developed and used to study the tsunami induced by a 200,000 cubic meters landslide from the flank of Vulcano in 1988 (Tinti et al., 1999) and to investigate the scenario of a lateral collapse of Stromboli (Tinti et al., 2003). <<<