Ground movements and sea level changes in urban areas: 5000 years of geological and archaeological record from Naples (Southern Italy)

The occasion offered by the construction of a new branch of Naples Subway and the related geotechnical and archaeological investigations permitted to reconstruct that the city shoreline underwent remarkable change during the Late Holocene highstand, when coastal evolution was influenced by vertical ground movements. In particular, for three investigation sites the density and quality of data were such to permit ???for the first time- the reconstruction of graphs of relative sea level change (RSLC) which, compared with curves of the coeval eustatic changes in the Tyrrhenian Sea, were transformed into data about the Late Holocene history of local tectonics. By averaging the data collected in all the different sites, the obtained curve of the tectonics suggests that the coastal strip of Naples underwent a subsidence rate of 1.3 mm/a from about 5 000 years BP to the XII century AD, followed by a 1m uplift during the last eight centuries (mean rate again of 1.0 mm/a). The subsidence trend is consistent with the regional tectonics, even if the mean rate was less that in other portions of the Bay of Naples graben, probably because the study area lies among the parallel, synthetic faults forming the NW margin of that depression (Magnaghi-Sebeto Fault Zone) and not on the proper hanging-wall block. The prevailing subsiding trend was probably interrupted by minor episodes of uplift not only in the last eight centuries but also during the 3rd millennium BC, likely in connection with periods in which the volcano-tectonic phenomena of the Phlegrean Fields district superimposed on the descending movements caused by the MSFZ. The three investigation sites share substantially the same tectonic behaviour in the long term, but the details of their RSLC records show also discrepancies in some centuries. If not due to errors in dating and/or in estimating palaeo-elevations, such discrepancies could be ascribed to episodes of fragmentary tectonic behaviour due, probably, to occasional reactivation of some E-W trending minor lines of the Magnaghi-Sebeto Fault Zone.