Composition of sands injected during the seismic crisis of May 2012 at San Carlo, Ferrara (Italy)

In May 2012 widespread sand blows formed along buried channels in the eastern sector of the Po Plain as a consequence of a seismic crisis with main shocks of Mw 6.1 and 5.9. At San Carlo (Ferrara) a trench dug a few week after the earthquakes exposed sand dikes cutting through an old Reno River channel-levee system that was diverted in the 18° century and was deposited starting from the 14° century (unit A); this sequence lie on Holocene muddy floodplain and scattered sandy channel deposits (unit B) and a Pleistocene channel sand unit (unit C). Sand inverse and direct grading, concave layering and vertical lamination coexisting along the dikes suggest multiple rhythmic opening and closing of the fracture borders that were injected and filled of slurry sand during the compression pulses end emptied during the extension phase. The pulse mechanism may have lasted for several minutes and formed well stratified structure of the sand volcanoes that formed on the top of some fractures. Sands from dikes and from the various units show well defined compositional fields from lithoarenitic to quartz-feldspar-rich compositions. Sorting related to sediment flux variations did not apparently affect the sand composition, across the sedimentary structures. Sands from the old Reno levee and channel fill (unit A) have abundant lithic fragments deriving from the erosion of Apennine sedimentary carbonate and terrigenous successions. Pleistocenic sands (unit C) are enriched in quartz and feldspars as a consequence of the different climatic weathering condition that prevailed during the last glacial stage. The Pleistocene sand were partially reworked during the Holocene (unit B). Composition of the sand filling the dikes show clear affinities with sand layer of the old Reno River channel (Unit A) and clearly differ from any sand from deeper layers (Unit B and C), which are richer in quartz and feldspar and poorer in sedimentary lithic fragments. Textural and compositional data indicate that the liquefaction processes originated from a relatively shallow source consisting of channel sands located within Unit A at 6.8.to 7.5 m depth.