Modelling long term ground deformation by a submerged archaeological site: the case of Basiluzzo island.

Abstract

We estimate the long-term vertical deformation of Basiluzzo island, located in the volcanic arc of the Aeolian Islands, Italy, by inferring the subsidence of a Roman age submerged wharf dated 2000 ± 50 years ago.We model the crustal deformation that produced this displacement as a cooling magma chamber emplaced during the formation of the island. This is the first attempt to model crustal deformation using the subsidence of a submerged archaeological structure. Nowadays the top of the wharf, which can be considered an unconventional leveling benchmark, is located near Punta Levante, at an average depth of 3.20 ± 0.10mbelow actual sea level, and it is still in good conservation. Its present location is due to combined effect of sea level rise and volcanic and tectonic activity that occurred since it was built. Taking into account the architectural features of the wharf and that the mean sea level rise of the Mediterranean sea has been estimated as 0.45 m during the last 2000 years [Flemming andWebb, 1986],we estimate a total subsidence of 3.75 ± 0.10m, at a rate of about 1.87 mm yr 1 [Anzidei et al., 2002]. We propose a possible mechanism for the long-term subsidence, which occurred on a timescale of thousands of years, by considering the Earth’s crust as a Maxwell body. We assume that the magmatic source is located directly beneath Basiluzzo dome and underwent progressive solidification and subsequent volume reduction since its emplacement 50,000 years ago.