Tsunami and Rapid Catastrophic Environmental Change. The Hazard Along the Italian Coastal Area
by
Franco Ortolani
Dipartimento di Pianificazione e Scienza del Territorio, Università di Napoli "Federico II"
Coauthors: S. Pagliuca, CNR-ISPAIM; G. D'Agostino, Dipartimento di Pianificazione e Scienza del Territorio, Università di Napoli "Federico II"

A research about the geoenvironmental impact of the 26 December 2004 tsunami, evaluating local morphology, run-up and waves provenance, has been carried out. In the northern part of Sumatra, in Thailand and in the oriental zone of the Sri Lanka, the long wave of the tsunami has been of over 1500 ms of amplitude, with a middle height from about 5 ms to over 10 ms.

Marine water's volume flowed on the earth has been enormous and the sandy water endowed with notable speed and discharge rapidly transformed into debris flow. On thousand meters the coast have been invaded, in few ten seconds, from about 2.500.000 to 5.000.000 of cubic meters of water, with a discharge of about 50.000-100.000 mc/sec, equal to 5-10 times the course of the Po River when it is in flood.

A normal big wave that invests the Mediterranean coast during a violent sea storm has a middle amplitude of about 20 m and a height of about 5 ms. The volume of water invading the coast is about 50 cubic meters in about 2 seconds with a discharge of about 25 mc/sec for every meter of beach. The wave of the tsunami, for every meter of beach, has poured on the emerged area about 5000-10000 meters cubes of water with a varying discharge from around 25 to 50 mc/sec for an inclusive period between 60 and 120 seconds. The most frequent geoenvironmental impacts has been: Destruction of the beaches predominantly constituted by bioclastic sand; Soil Erosion; Soil and water table salinisation; Accumulation of sand and brackish mud on the emerged earth; Partial destruction of the coral barriers.

The disastrous tsunami that has devastated the coasts of oriental south Asia has also lifted a general interest in Italy. Till now the available data underline that also the Italian coasts at greater risk (affected by many disastrous tsunamis during the last 900 years) are not protected by planning finalized to prevent and to limit the damages, especially during the bathing period. Within the research it has been simulated the impact possible today in the same places (Messina Strait) affected by the 1908 tsunami. Remembering the images of the effects of the 1908 tsunami and of the recent event of Indian Ocean, the seriousness of the connected risk to tsunami is evident.

A recent research allowed us to reconstruct the cause of the disastrous 1908 tsunami that followed the 1908 earthquake that devastated the provinces of Reggio Calabria and Messina provoking about ten thousand of victims. On the base of accurate historical researches has emerged that the tsunami affected the coasts of the epicentral area about 10 minutes after the earthquake. Considering the brief distance among the two banks of the Messina Strait (2 km) and that the seismogenic structure has been situated in the submerged area, it is impossible that the coseismic deformation directly caused the tsunami. In few second, in fact, the waves would have invaded the Sicilian and Calabrian coasts. We concluded that the cause of the tsunami must have been a big and rapid submarine landslide happened south of Reggio Calabria in the zone where the run-up has been maximum (more than 10 meters).

The 30 December 2002 Stromboli tsunami and the December 1908 Messina Strait tsunami permitted to reconstruct the speed of tsunami wave propagation along the Italian coasts; it results of about 100 km/h for the Stromboli tsunami and of about 120 km/h for that of Messina Strait. Such values are significantly lower than the speed of propagation of the oceanic tsunamis like that of December 26th 2004, characterized by a speed of about 600 km/h.

About 70 tsunami affected the Italian coast during the last 900 years. They affected the following areas: 14 between the Liguria and southeastern France; 23 between the Strait of Messina, oriental Sicily, southern Calabria Aeolian Islands; 10 along the Adriatic coasts; 9 in the Gulf of Naples; 3 in Tuscany; 2 in northern Sicily (Palermo-Cefalù); 2 in southern Sicily (Sciacca); 1 in ionic northern Calabria; 1 in the Lazio. This research has allowed to map the coast that till now have been interested by anomalous and sudden movements of the sea; it's possible to underline that the most greater number of events has been provoked by submarine landslides affecting the unstable edges of the continental shelf.

On the base of the data till now available it is possible to delineate the Italian coastal areas exposed to major tsunami hazard.

Date received: July 20, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-42.