Polygenetic sand volcanoes: On the features of liquefaction processes generated by a single event (2012 Emilia Romagna 5.9 Mw earthquake, Italy)

One of the most important geological effects generated by the seismic events of Emilia Romagna (Italy, 2012) was the liquefaction. The first earthquake took place on 20th May (5.9 Mw) and generated liquefaction in the locality of San Carlo, mainly recorded as large fissure sand volcanoes. Detailed stratigraphic and sedimentology analyses of these sand bodies indicated the assemblage of rhythmic normal graded beds of sand for many individual sand volcanoes. This is interpreted as repeated individual sand extrusions generating polygenetic sand blows for a single earthquake. This original analysis, made for first time in a case of extensive seismic liquefaction, indicates that the different pulses sand extrusion occurred after seismic shaking, and therefore no related to the cyclic seismic shearing stimulated by surface seismic waves. Underground liquefaction of shallow sandy layers and ground cracking, assisted by relevant lateral spreading, promoted water and sand ejections (E–W direction). Localised and repeated ground subsidence and bulging supported by the ejection of the underlying fluidized sediments feedback transient ground undulations and repeated sand ejections. The unstable undulating behaviour of the ground reinforced and sustained the liquefaction process through time after the seismic shaking, leading the occurrence of polygenetic sand volcanoes. In summary, the seismic liquefaction is the starting point of the process, although only if these sand ejections could promote the instability of the ground since underground sand depletions feedback the liquefaction process through time. The process stops when the hydrostatic balance is recovered in the affected zone.