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2008-11-02 – “Paleo”-Tsunami

Ritrovate tracce di “paleo”-tsunami, riportati in basso gli articoli da Le Sienza (ITA) e Nature/news (ENG).

La storia geologica degli tsunami

Negli stati di terra sotto le sabbie costiere vi sono tracce di altri tre tsunami catastroficio che hanno interessato il Sud Est asiatico negli ultimi 2800 anni

“Molti nel Sud Est asiatico credono, o vorrebbero credere, che uno tsunami come quello del 2004 non si verificherà mai più, ma in realtà la zona ha una storia di tsunami rari e catastrofici”, dice Kruawun Jankaew, geologo della Chulalongkorn University, in Thailandia, che ha condotto con Brian Atwater, dell’Università di Washington, uno studio sulla storia geologica degli tsunami nel Sud Est asiatico, ora pubblicata su “Nature”.

I ricercatori hanno trovato dati di precedenti tsunami grazie a perforazioni in 150 siti su un isola situata a 250 chilometri dalla nota località turistica di Phuket, interessata nel 1004 dallo tsunami del 2004 generato da un terremoto sottomarino di magnitudo 9,2 verificatosi 500 chilometri più a occidente.

In venti siti i ricercatori hanno trovato strati di sabbia bianca di circa una decina di centimetri alternati da strati di scuro suolo torboso. Testimoni hanno confermato che il primo strato superiore, appena sotto la sabbia, era stato lasciato dallo tsunami del 2004.

La datazione al radiocarbonio di detriti ritrovati nel secondo strato di sabbia hanno indotto gli scienziati a stimare che un forte tsunami precedente a quello del 2004 deve essersi verificato fra il 1300 e il 1450 d.C. Altre tracce indicano due ulteriori tsunami che devono essere avvenuti fra 2500 e 2800 anni fa.

Singolarmente non si hanno resoconti scritti dello tsunami avvenuto fra il 1300 e il 1450, nonostante la zona sia stata visitata all’incirca in quel periodo da diverse spedizioni, come quelle dell’arabo Ibn Battuta e di alcune missioni esplorative militari della flotta cinese della dinastia Ming. “La ricerca dimostra che la geologia degli tsunami, recenti e passati, può aiutarci ad ampliare il catalogo ben al di là dei resoconti storici”, ha detto Atwater.

Le nuove scoperte possono essere utili anche per le regioni della regione di Cascadia, ovvia quelle che si affacciano sulla costa Pacifica nord occidentale del Nord America, dove i geologi ritengono che a intervalli di centinaia di anni si verifichino catastrofici tsunami generati dalla zona di subduzione della Cascadia. (gg)

fonte: http://lescienze.espresso.repubblica.it/articolo/La_storia_geologica_degli_tsunami/1333681

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Ancient tsunami uncovered

Indian Ocean disaster of 2004 was biggest in more than 600 years.

Geologists have found evidence for the most recent predecessor of the Indian Ocean mega-tsunami thought to have killed more than 220,000 people in 2004.

Two teams independently conclude that the last time a tsunami of similar size hit the region was around AD 1300–1400. “This is crucial information for improving disaster preparedness”, says Jörn Lauterjung, a physicist at the national German Research Centre for Geosciences in Potsdam, who co-ordinates the implementation of the German-Indonesian Tsunami Early Warning System for the Indian Ocean.

Each team analysed more than 100 sediment cores collected during fieldwork in 2006 and 2007, and found traces of several tsunamis that may have occurred during the last 2,500 years. But only the medieval event, whose age was determined by radiocarbon dating of organic material in the sediment, correlated between the two study areas near Phuket, Thailand, and in Aceh on the Indonesian island of Sumatra.

The next big hit

The results, published this week in Nature1,2, suggest that Indian Ocean tsunamis on that scale occur every 600–700 years. Models of how seismic stress builds up in the Sunda Trench, which forms a seam between two tectonic plates on the floor of the Indian Ocean, give similar frequency estimates. One of the studies also found evidence of a large tsunami in AD 780–990.

Katrin Monecke, a geologist formerly at Kent State University in Ohio, and her colleagues searched around Aceh for sediment records undisturbed by erosion or agriculture. Like the team in Thailand, the team took cores in ridges and in low marshy areas called swales, where sandy tsunami deposits are preserved between darker layers of peaty soil.

“Many people in Aceh still live in tents, and many are still in a state of shock,” says Monecke. “But when we explained what we were doing they were very interested and supportive, lending us their boats or helping us find paths in areas where no good maps are available.”

Risk worth taking?

The German-Indonesian early-warning system is intended to prevent a similar tragedy in the future. After four years of intense preparation and testing, the system will be officially launched on 11 November in Jakarta by Indonesia’s president Susilo Bambang Yudhoyono.

Lauterjung points out that the rarity of mega-tsunamis does not limit the system’s usefulness. Although the Indian Ocean accounts for only 5% of global tsunamis, smaller local events occur every few years near seismically active regions from Indonesia to Oman. On 17 July 2006, for example, a magnitude-7.7 earthquake off Indonesia triggered three-metre high waves that killed hundreds of people along the south coast of Java.

However, as it is likely that no large tsunami will strike again for centuries, many fishermen and their families will consider the risk of living close to the sea preferable to the disadvantage of moving farther inland.

In Aceh, the northernmost province of Sumatra that was most heavily hit by the 2004 tsunami, there are no written accounts or social memory of past tsunami catastrophes. This might explain why most people in the area failed to run to higher ground when they felt the shock waves, unlike those on more frequently affected islands. The mega-tsunami killed more than 150,000 people in Aceh alone.

Maintaining an early warning system for that long will be a challenge, says Stein Bondevik, a geologist and tsunami researcher at the University of Tromso in Norway.

Europe’s soft underbelly

Although tsunami warning systems are already running, or being installed, in the Indian and Pacific Oceans, experts warn that the Mediterranean coast is still unprotected, despite the fact that the Hellenic arc — an area of explosive volcanism in the eastern Mediterranean — is thought to see mega-tsunamis at roughly the same frequency as Indonesia and Thailand.

The issue will be addressed next week at a meeting in Athens, Greece, of the United Nations Eductional, Scientific and Cultural Organization’s Coordination Group for the Tsunami Early Warning and Mitigation System in the North-Eastern Atlantic, the Mediterranean and Connected Seas.

“If there was a submarine earthquake in the Aegean, who would issue a warning? Who would warn the Middle East?” says Costas Synolakis, director of the Tsunami Research Center at the University of Southern California in Los Angeles. “Four years since losing a quarter million people [in Asia] the underbelly of Europe remains blissfully unprotected.”

References

  1. Jankaew, K. et al. Nature 455, 1228–1231 (2008).
  2. Monecke, K. et al. Nature 455, 1232–1234 (2008).

source:http://www.nature.com/news/2008/081029/full/news.2008.1193.html

novembre 2, 2008 - Posted by | Articolo sc. di riferimento, Geology - Geologia, Lang. - Italiano | , ,

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