Alberto Armigliato

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

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.

The Tsunami of August 17, 1999 in Izmit Bay, Turkey

The Kocaeli 1999 Earthquake with an Mw = 7.4 caused major hazards throughout the NW of Turkey from Tekirdag to Bolu. Historical data indicates that some of the earthquakes around Izmit Bay have caused tsunamis. In this study, tsunami research for the Kocaeli 1999 Earthquake has been made also taking into consideration historical data. In this research more than about 70 data at 35 localities have been used to determine the tsunami evidences in the bay. Coastal observations indicated runups which were ranging from 1 to 2.5 m along the shores. However, the wave runups are more complex along the south coast due to the presence of coastal landslides (Değirmendere, Halidere, Ulasli, Karamürsel) and subsided areas (Kavakli to Yeniköy) along the shore. West of Yalova, evidence of tsunami rapidly diminished. In addition, possible tectonic mechanism has been determined by using 33 single-channel high-resolution digital seismic reflection profiles which were acquired following the Kocaeli 1999 Earthquake. As a result it has been determined that the Kocaeli Earthquake has created tsunami in Izmit Bay.

Identification of the source fault of the 1908 Messina earthquake through tsunami modelling. Is it a possible task

Abstract The 1908 Messina Straits tsunami is the last catastrophic event that hit the Italian coast. The parent earthquake may be considered one of the strongest shocks reported in Italian seismic catalogues. Several source models have been; proposed in the literature that are quite different with regard to almost all the fault parameters. The aim of this work is to evaluate whether tsunami data can add reliable inforiration for the identification of the fault mechanism of this earthquake. Tsunamis generated by two of the faults proposed in the literature were simulated via finite-element modelling in a former work (see Piatanesi et al., 1998): the results were in good agreement with the observations as regards the polarity of the first wave, whereas a relevant disagreement was found as far as run-up data were concerned. In this paper, that may be considered the continuation of the previous work, we focus on a particular fault, the one proposed by Capuano et al. (1988), and simulate the consequent tsunami adding two new features to possibly improve the agreement with run-up observations: i) an algorithm allowing for the effect of the sea bottom bathymetry on the tsunami initial condition, and ii) a heterogeneous slip distribution on the fault.

Displacements and stresses induced by a point source across a plane interface separating two elastic semi-infinite spaces: An analytical solution

The problem of computing the static deformations and stresses produced by a point source in a homogeneous infinite medium was solved by Volterra [1907] in a closed analytical form at the beginning of this century. The similar problem of computing fields generated by point sources in a homogeneous half-space bounded by a free surface was later studied by Steketee [1958a,b] and several others [see Okada, 1985, 1992], who focused on point as well as on rectangular fault sources of interest in seismology. Here the model taken into account consists of two elastic half-spaces characterized by different elastic properties (rigidity modulus μ and Poisson coefficient v ) and separated by a planar interface: assuming that a point source is active in one half-space, static deformations and stresses generated by the source in the whole space are computed. The similar problem of two half-spaces welded together was solved by Heaton and Heaton [1989], but they imposed the simplifying constraint that both materials are Poissonian ( i.e., both have the same Poisson coefficient v=0.25). The present approach, which is based on the Galerkin vector method, is general and applicable to an arbitrary point source. In this paper the computations have been carried out explicitly only for the special case of a dislocation source having the form of a strike-slip double couple. The solution is provided in a closed analytical form by means of expressions involving the source descriptors (position and intensity) as well as the elastic parameters of the heterogeneous medium. The solutions have been illustrated and discussed with special attention given to the dependence of the displacement and stress components on the elastic parameters of the model. One interesting finding concerns the limiting case when the rigidity modulus of the half-space not containing the point source is equalized to zero. Although the solution in this half-space no longer makes sense, the solution in the other reduces exactly to the one computed for a half-space with a free surface, that is, to the solutions computed by Steketee [1958a] and Okada [1985] following an alternative approach.

The 1977 Gioia Tauro Harbour (South Tyrrhenian Sea, Italy) Landslide-Tsunami: Numerical Simulation

On July 12th, 1977, waves 5 m high hit the harbour of Gioia Tauro, provoking relevant damages on facilities and infrastructures, but fortunately no casualties. The source was supposed to be a submarine mass failure, ranging 5 million m3 and occurring in very shallow water at the heads of a canyon just in front of the port. The sliding mass cut a cable at 600 m depth, meaning that it travelled more than 15 km far from the source. In this work we reconstruct the hypothesized mass failure, compute the slide dynamics and simulate the generated tsunami through numerical modelling. The results obtained fit well with the observations, strengthening the hypothesis of such a landslide as the source for the 1977 tsunami. This event can be considered as paradigmatic of a category of coastal hazards: small submarine landslides occurring close to populated coasts may pose considerable risk, even if only at a local scale.

Numerical Simulation of the BIG’95 Debris Flow and of the Generated Tsunami

Underwater mass movements along continental margins constitute a relevant threat for coastal communities. In the Mediterranean Sea this issue is particularly critical, both for the concentration of potential sources and for the density of population living along the coast. One of the biggest event recently mapped by submarine geophysical surveys, named BIG’95, is located in the Balearic Sea, between the homonymous islands and the Eastern coast of Spain: it occurred around 11 kyrs BP, involving a volume of around 26 km3 over an area of 2,200 km2, from 200 m depth down to 1,600 m and over: the looser fraction of the mass was driven along the Valencia Channel, NE-ward, more than 110 km far from the source area. The BIG’95 event is here simulated through numerical codes, adopting some reasonable simplifications on the slide morphology and dynamics. The tsunamigenic impulse is then evaluated and the propagation of the wave simulated over a computational domain covering the Western Mediterranean Sea. Results show that an event of these characteristics can generate catastrophic tsunami along the coasts of the Balearic Sea, reaching also North Africa, Corsica and Sardinia coasts with relevant waves.

Investigations on the Possible Source of the 2002 Landslide Tsunami in Rhodes, Greece, Through Numerical Techniques

The island of Rhodes (Aegean Sea, Greece) has been repeatedly hit by tsunamis in the past due to the numerous tsunamigenic sources present in the area, most of which are seismic. Here an investigation is made on the most recent event that occurred on March 24, 2002 in the northeast of the island: unexpected waves affected a 2 km long coastal segment, overtopping part of the sea-wall (3–4 m high) that runs along the littoral road a few km away from the city of Rhodes. Data on the tsunami are poor. Due to the lack of evidence of seismological or meteorological causes, the hypothesis of a landslide source for the tsunami is here explored by means of numerical codes implementing models both for the landslide dynamics and the tsunami propagation. The reconstructed failing mass is located at about 100 m sea depth with a volume of 30 million m3. A sensitivity analysis is further run by varying the source volume, to evaluate the effects on the tsunami impact.

Chapter 15 – Geoethical and Social Aspects of Warning for Low-Frequency and Large-Impact Events like Tsunamis

Real-time warning of large-impact events like tsunamis, that may attack the nearest coast soon after the generation, implies that there is no time to collect enough data before tsunami arrival to provide accurate estimates of the wave height and tsunami duration, which may result in issuing inaccurate alarm messages to the population. Assessments of tsunami hazard and analyses of tsunami risk, that are at the basis of long-term prevention policies, can be affected by large uncertainties as well, since they suffer from systematic scarcity of real observations because large tsunamis are very infrequent events. This paper discusses the role of geoscientists in providing predictions and the related uncertainties. It is stressed that through academic education geoscientists are formed more to improve their understanding of processes and the quantification of uncertainties, but are often unprepared to communicate their results in a way useful for society. Filling this gap is crucial for improving the way geosciences and society handle natural hazards and devise effective means of protection.

Tsunami Hazard in Southern Italy

Tsunamis are hazardous natural events that are generated by earthquakes, by mass impact in the sea waters and by volcanic eruptions. Most tsunami sources are in the Pacific, but tsunamis have repeatedly affected even the Italian coasts. This paper is devoted to tsunamis in southern Italy, but in the first part it also provides some basic elementary notions on tsunamis for all those readers who are not too familiar with these phenomena. One important concept to have in mind is that tsunamis can be catastrophic. An instance of great disaster is the Sanriku tsunami on 2 March 1933 that was generated by a submarine earthquake with magnitude Mw = 8.4 (Kanamori 1977) and Mt = 8.3 (Abe 1979) occurring in the Pacific some 300 km from the northern coast of Honshu, the main island of Japan. Waves larger than 20 m (29.3 m at Shirahama in Ryori Bay) devastated several coastal Japanese towns, killing more than 3 000 people. The same area was devastated by another catastrophic tsunami on 15 June 1896 (Mt= 8.6 according to Abe 1979) that originated in the same source region and hit the coast with waves that reached the maximum extraordinary height of 38.2 m at Shirahama, causing more than 25 000 deaths (Iida 1984).