Floodplain evolution and its influence on liquefaction clustering: The case study of March 2021 Thessaly, Greece, seismic sequence

The March 2021 Thessaly, Central Greece, seismic sequence characterized by an Mw = 6.3 mainshock and five Mw > 5.0 aftershocks triggered extensive liquefaction in the floodplain of the Piniada Valley and localized phenomena along the riverbanks of the Titarissios River. In order to document these secondary effects, we carried out immediate post event surveys based on remote sensing techniques and field-based reconnaissance. The initial desktop-based approach succeeded in delineating in advance the extensive liquefaction surface manifestations and in saving time for the subsequent field survey. The field work was performed with ground-based surveys, and UAV aerial campaigns. The longest reported linear liquefaction feature was 26.7 m, while the total sand blow covered an area of 0.0325 km2. Over one of the liquefaction manifestations, we performed parallel 2D electrical resistivity tomography profiles in order to reconstruct the subsoil stratigraphy. The produced 3D resistivity model successfully detects and illustrates the subsurface disruption following the generation of liquefaction. From the comparison between the spatial distribution of liquefaction phenomena and the geomorphological conditions in the area, the direct influence of the historical evolution of Pinios River in clustering liquefaction phenomena was highlighted. Thus, it is confirmed that a detailed investigation of the historical and late Holocene evolution of a floodplain i.e., location of ox-bow lakes, meanders and former river channels, can identify sites prone to liquefaction. This could result in a reduction of the area where detailed engineering geology investigations (e.g., boreholes with in-situ tests) are required for the evaluation of the liquefaction hazard.