Characterisation at different scales of earthquake-induced soil liquefaction along the Kupa river (Croatia)

The Pokupsko earthquake in Oct. 1909 (Ms 5.7) and, more recently, the Petrinja earthquake in Dec. 2020 (Mw 6.4), both occurring along the same Petrinja-Pokupsko Fault system, induced soil liquefaction phenomena in the alluvial plain of the Kupa river (Croatia). While surface evidence of liquefaction was limited in the 1909 event, the 2020 earthquake triggered more extensive and well-developed liquefaction features along the riverbanks. These features included sand blows, fissures, ground settlements, and lateral spreading, highlighting the increased susceptibility of the alluvial deposits to seismic shaking in the 2020 event. 

The use of remote sensing techniques enhances the understanding of the spatial distribution of liquefaction occurrences and their subsequent impacts. 
Drone surveys and a high-resolution Digital Elevation Model (0.5 m resolution) reveal that liquefaction is concentrated within the lowest Holocene terrace and specific areas shaped by fluvial processes. Of particular note is the higher density of sand blows observed in the convex sections of river meanders and the increased lateral spreading at the inflection zones of meanders, where point bar formations become tightened.
Lateral spreading along the Kupa river can also be mapped and quantified from optical image correlation of high-resolution aerial images (30 cm resolution) taken before and after the 2020 Petrinja earthquake. These data show that lateral spreading locally exceeds 1 m of displacement toward the river and is generally confined to within 200 m of the riverbanks. 

On a smaller scale, geological and geotechnical surveys in different sites along the Kupa river point to other conditions influencing liquefaction occurrences and their effects. Soundings and sampling have shown that all sand blows originate from point bars buried between 3 m and 6 m below the surface covered by silty sediments. The soil strength and the thickness of this covering sediment layer are key parameters controlling the occurrence of sand blows. In addition, OSL and 14C dating indicate possible paleo-liquefaction for a sand dyke sealed below the upper part of the aforementioned cover. 

This combined approach facilitates the identification and detailed characterization of the areas most susceptible to liquefaction in 2020 and historically along the Kupa river. However, in these regions, the surface cover may impede the emergence of liquefied soils to the surface or mask their presence.