Geological and geomorphological effects induced by the Emilia seismic swarm of May 2012 (northern Italy)

Since May 16th 2012, a seismic swarm has affected the central-southern sector of the Po Plain. A first main shock occurred on May 20th (M = 5.9; focal depth: 6.3 km; epicenter: N of Finale E.) and a second one on May 29th (M= 5.8; focal depth: 10.5 km; epicenter: about 12 km W of the first one). This seismic swarm has consisted of another five M>5 quakes and about 2,500 minor shocks till the end of November 2012. The subsurface is made up of marine and continental deposits overlying compressional northverging faulted folds of the Apennines (“Ferrara Folds”) which caused the quakes. The geomorphological setting is characterized by a complex drainage and ancient drainage pattern of the Rivers Po, Secchia, Panaro and Reno. The most relevant effect caused by the two stronger earthquakes was the 10-15 cm uplift of the epicentral area detected by InSAR interferometry. On the whole, more than 500 geological and geomorphological effects were recorded over an area of about 700 km2. The effects are mainly due to liquefaction with ejection of sand from ground fractures, sand-boils and wells. In some places the bottom of artificial canals showed uplifting, bulging and cracks and fractures and landslides were mapped on the banks. About 80% of the effects were induced by the May 20th main shock, while 20% of the effects were triggered by the second main quake. Many cases of liquefaction triggered by the first main quake were reactivated by the second one, with jets of water up to 1.50 m high. The material erupted reached in many cases a thickness of more than 30 cm and, inside some buildings, from pavement cracks it uplifted up to 1 m. Hydrogeological anomalies, such as strong water-table fluctuations, water level variations in some tracts of Po and Secchia rivers, emission of hot water from ground cracks and water wells, were also recorded. The co-seismic effects appear to be caused not only by the local stratigraphic characteristics but also by the network of abandoned riverbeds.