The liquefaction evidences following the 2020 Petrinja earthquake (Pannonian basin, Croatia): A full database and insights for phenomena comprehension

The 2020 MW 6.4 Petrinja (Croatia) earthquake induced extensive and diversified liquefaction and lateral spreading phenomena within ≈ 20 km radius from the epicenter. A detailed investigation from field and Unmanned Aerial Vehicle (UAV) surveys was carried out by a European researcher team (EUTeam) in the months following the mainshock. This work focuses on 61 surveyed sites: field observations were coupled with laboratory tests for soil classification and sediment composition. The adopted procedure provides an in-depth geological and geotechnical characterization of the liquefied sites in the Petrinja region. The liquefaction evidences are mainly associated to alluvial plain environments, in particular to meander paleochannels, and the ejected material is predominantly siliciclastic, made up of very rounded quartz-rich lithics. Few sites are dominated by angular carbonate rock fragments, related to liquefaction in cataclastic deposits along tectonic fractures. The ejected sediment includes a wide range of grain-size from silt to gravel. The peculiar presence of gravel in the liquefied deposits (up to 28% in some samples) confirms the need of expanding the grain-size boundaries for liquefiable coarse-grained gravelly soils. The information gathered from the post-earthquake surveys and from the sedimentological and geotechnical analysis for each studied site were compiled in organized data sheets, providing a striking instrument for in-depth earthquake studies, both for geological and geotechnical engineering purposes. The format defined for the data sheet can be functional and applicable also in liquefaction studies from different geological and depositional settings.