Liquefaction susceptibility in delta environments: the case of the Po delta area (northern Italy)

We know that, during a seismic event, liquefaction phenomena are not randomly distributed over an active or recent floodplain but are strongly dependent on the depositional evolution of the area like grain size of the sediments, depositional age shallow layering, and depth of the groundwater level. Recent unconsolidated sediments (Holocene age), deposited by fluvial or wave action, such as those found in paleo-channels and point bars in coastal areas, generally exhibit a higher frequency of liquefaction compared to sediments deposited under different conditions within the same floodplain and/or coastal zone. Based on these considerations, Youd and Perkins (1978) proposed the division of geological units into four classes of liquefaction susceptibility: low, moderate, high, and very high. They also emphasized that water-saturated geological formations, such as river channels, floodplains, deltaic regions, and dunes, are generally characterized by a high to very high susceptibility to liquefaction. The reliability of a liquefaction susceptibility map is heavily influenced by the investigation scale, as well as the density and accuracy of the geological, geomorphological, and hydrogeological data. In this paper, an area of about 560 km², situated in the eastern sector of the Po River plain (northern Italy) within the Po River delta, including part of the modern delta, has been considered and analyzed in detail to examine the likelihood of geological units to liquefaction. To achieve this, a large variety of data has been considered. Lithological, chronological (14C data), geomorphological, hydrological, hydrogeological data, satellite imagery, modern and historical topographic maps, as well as historical and archaeological evidence, were integrated with data from continuous core drilling and penetrometer tests.