Stratigraphic control on earthquake-induced liquefaction: A case study from the Central Po Plain (Italy)

Abstract

Studies on earthquake-induced liquefaction tied to high-resolution stratigraphic analysis have been rarely undertaken. We report the results of a multidisciplinary study from the Quistello–Moglia area, in the central Po Plain (northern Italy). In this region, combined stratigraphic, sedimentological, geotechnical, and geochemical data allowed assessment of liquefaction potential and identification of the primary source for liquefaction, following the second main shock (Mw~6) of the 2012 Po Plain earthquake. Using Cone Penetration Test (CPT)-based simplified procedures for liquefaction hazard evaluation, we assessed the highest liquefaction potential of Holocene, fluvial-channel and related (crevasse/levee) fine sand-silt facies encased in thick, mud-prone floodplain and swamp successions. The liquefaction potential, and the intensity of the manifestations induced on the ground surface, decreased for the vertically-amalgamated, sheet-like Pleistocene sandy fluvial units encountered at depths greater than 13m. Floodplain and swamp deposits were virtually non-liquefiable. In the Quistello area, the compositional characterization of sands that were liquefied and extruded during the 2012 earthquake reveals the diagnostic geochemical fingerprint of sediment carried by the Po River, as opposed to the Apennine composition of surficial sediments. These data rule out proximity of liquefied layers to the surface, and attest the buried, meandering Po River system at depths of ~7–10m most likely representing the source for the liquefied sand that vented to the surface. Similarly, at Moglia, liquefied sands were likely sourced from loose and saturated, ribbon-shaped, fluvial sand bodies encased in mud, though at shallower (4–7m) depths. Pronounced liquefaction phenomena in alluvial plain systems are commonly believed to be associated primarily with elongate topographic ridges following paleo-river courses. Here, we document that under favorable stratigraphic conditions liquefaction may also occur away from surficial channel–levee systems, in areas dominated superficially by overbank fines. Combining subsurface stratigraphic analysis with geotechnical data, thus, is critical to investigate liquefaction patterns and delineate liquefaction hazard zones.