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Geological Society, London, Special Publications; 2009; v. 322; p. 105-119;
DOI: 10.1144/SP322.4
© 2009 Geological Society of London

Articles

Evidence of slope instabilities and tsunami associated with the 3.5 ka Avellino eruption of Somma-Vesuvius volcano, Italy

Alfonsa Milia1,*, Arturo Raspini2 & Maurizio M. Torrente3

1 Istituto per l'Ambiente Marino Costiero (IAMC), CNR, Calata Porta di Massa, Porto di Napoli, 80133, Napoli, Italy
2 Istituto di Geoscienze e Georisorse (IGG), CNR, Via Giorgio La Pira, 4, 50121, Firenze, Italy
3 Dipartimento di Studi Geologici ed Ambientali (DSGA), Università del Sannio, Via Portarsa 11, 82100, Benevento, Italy

* Corresponding author (e-mail: alfonsa.milia{at}iamc.cnr.it)

The interpretation of high-resolution seismic profiles acquired close the eastern coast of the Bay of Naples offshore Somma–Vesuvius volcano, and calibrated by borehole data, allowed us to recognize a reflection-free seismic facies consisting of a volcaniclastic debris avalanche overlain by pyroclastic density current deposits. Both are associated with the 3.5 ka ‘Avellino eruption’ and are in turn overlain by a marine succession. The top of the volcaniclastic unit corresponds to a deep erosional surface covered by a coarse-grained bioclastic layer with rounded pumice. We argue that these features are related to a tsunami that was triggered by the sudden entrance into the sea of the gigantic (>1 km3) volcaniclastic unit. The onshore–offshore correlation and mapping of this volcaniclastic unit have allowed us to evaluate its distribution west of Somma–Vesuvius. The pyroclastic density current deposits terminate seaward at 40 m water depth; at the same depth the debris avalanche is affected by a gravitational instability. Over the last 3.5 ka the slump has been in constant movement, as documented by the angular unconformities at the top of the chaotic seismic unit and within the overlying marine deposits.