Surface breaks along the coastal plain of Cirò (Ionian Calabria, southern Italy): geophysical and paleoseismological insights

Abstract

On July 28, 2004, the coastal plain of Ciro (Calabria, Ionian Sea) was interested by the sudden opening of a ~5-km-long fracture, paralleling the coastline. All the buildings, roads, and pipelines crossed by the fracture were damaged, inducing several inhabitants to abandon their houses. The crack was 1–2-cm wide, downthrowing the seaward block up to 1–2 cm. This phenomenon is known for having already hit this area at least in the past century, both concurrently or not to earthquakes with epicentre far away from Ciro. In order to investigate the subsurface geometry, and the nature of the crack, we performed 5 electrical resistivity tomography and two paleoseismological trenches across it. The two methods provided evidences for the whole displacement of the local coastal succession, with offsets of several metres cumulated during the late Holocene. Giving the radiocarbon age of samples collected in trenches, we have constrained the dates of a couple of offset events which match some paleoearthquakes caused by the Lakes Fault, in the neighbouring Sila Massif. Excluding other possible causes (i.e. anthropic or tectonic), we are inclined to interpret these phenomena as the westernmost, surficial expression of a submarine deep-seated gravity sliding.