Sand composition as a tool for liquefaction phenomena assessment

The petrographic composition of sands is a useful tool for the assessment of the complex phenomena related to co‐seismic liquefaction, such as sand blows, dykes, sand volcanoes and may also help in identifying ancient earthquake effects. Fluvial sand composition studies have a particular significance in the late Pleistocene–Holocene Po Plain, where distinct compositional fields characterize modern sands from different streams, as well as older sediments1 . Several key petrographic components provide diagnostic features to distinguish sand bodies buried beneath the floodplain. The composition of sands ejected during the 2012 Mw 6.1 Emilia earthquake from several sites in the Emilia plain (Modena and Ferrara provinces)2 has been compared with buried sands from cores and trenches at different depths (down to 50 m) in order to identify the source layers which originated the liquefaction phenomena. The sands from the cores show a clear trend from lithoarenitic to quartz‐feldspar‐rich compositions. The sands at shallow depth (down to 7 m) are the most lithoarenitic, with sedimentary fine‐grained rock fragments (limestone, shale and siltstone) as the dominant lithic type. Lithic fragments derive mostly from the erosion of sedimentary terrigenous and carbonate successions of Apenninic affinity referable to different fluvial systems (Secchia and Reno rivers). These shallow sands are well distinguishable from the deeper sands (at depth > 7 m), which show compositions slightly enriched in quartz and feldspars and impoverished in lithic fragments suggesting affinity with the Po river sands or older sands deposited during the last Glacial Maximum.   In all examined sites the composition of the ejected sands largely overlap that of the shallow late Holocene Apenninic sands, indicating that liquefaction processes affected mainly sand layers at depth of 6‐7.5 m. The study shows that composition of sands is crucial for a better understanding of earthquake‐ induced liquefaction mechanisms, in particular to identify the source layer of the sand blows and, more generally, for the recognition of critical layers, which may be prone to hazardous sand liquefaction phenomena.