No abstract

Summary On 29 December 2020, a shallow earthquake of magnitude Mw 6.4 struck northern Croatia, near the town of Petrinja, more than 24 hours after a strong foreshock (Ml 5). We formed a reconnaissance team of European geologists and engineers, from Croatia, Slovenia, France, Italy and Greece, rapidly deployed in the field to map the evidence of coseismic environmental effects. In the epicentral area, we recognized surface deformation, such as tectonic breaks along the earthquake source at the surface, liquefaction features (scattered in the fluvial plains of Kupa, Glina and Sava rivers), and slope failures, both caused by strong motion. Thanks to this concerted, collective and meticulous work, we were able to document and map a clear and unambiguous coseismic surface rupture associated with the main shock. The surface rupture appears discontinuous, consisting of multi-kilometer en échelon right stepping sections, along a NW-SE striking fault that we call the Petrinja-Pokupsko Fault (PPKF). The observed deformation features, in terms of kinematics and trace alignments, are consistent with slip on a right lateral fault, in agreement with the focal solution of the main shock. We found mole tracks, displacement on faults affecting natural features (e. g. drainage channels), scarplets, and more frequently breaks of anthropogenic markers (roads, fences). The surface rupture is observed over a length of ∼13 km from end-to-end, with a maximum displacement of 38 cm, and an average displacement of ∼10 cm. Moreover, the liquefaction extends over an area of nearly 600 km² around the epicenter. Typology of liquefaction features include sand blows, lateral spreading phenomenon along the road and river embankments, as well as sand ejecta of different grain size and matrix. Development of large and long fissures along the fluvial landforms, current or ancient, with massive ejections of sediments is pervasive. These features are sometimes accompanied by small horizontal displacements. Finally, the environmental effects of the earthquake appear to be reasonably consistent with the usual scaling relationships, in particular the surface faulting. This rupture of the ground occurred on or near traces of a fault that shows clear evidence of Quaternary activity. Further and detailed studies will be carried out to characterize this source and related faults in terms of future large earthquakes potential, for their integration into seismic hazard models.

In May 2012, two earthquakes (Mw 6.1 and 5.9) affected the Po Plain, Italy. The strongest shock produced extensive secondary effects associated with liquefaction phenomena. Few weeks after the earthquakes, an exploratory trench was excavated across a levee of the palaeo‐Reno reach, where a system of aligned ground ruptures was observed. The investigated site well preserves the geomorphic expression of a fluvial body that mainly formed in the fifteenth to sixteenth centuries as historical sources and radiometric data testify. In the trench several features pinpointed the occurrence of past liquefaction events: (i) dikes filled with overpressured injected sand and associated with vertical displacements have no correspondence with the fractures mapped at the surface; (ii) thick dikes are buried by the plowed level or even by fluvial deposits; (iii) although some of the 2012 ground fractures characterized by vertical displacement and opening occurred in correspondence of thick dikes observed in the trench, sand and water ejection did not occur; (iv) some seismites (load casts) were observed in the trench well above the 2012 water level. The results strongly suggest that shaking has locally occurred in the past producing a sufficient ground motion capable of triggering liquefaction phenomena prior to, and likely stronger than, the May 2012 earthquake. Historical seismicity documents three seismic events that might have been able to generate liquefaction in the broader investigated area. Based on the analysis of their macroseismic fields, the 17 November 1570 Ferrara earthquake is the most likely causative event of the observed palaeoliquefactions.

Our research is aimed at estimating the vertical deformation affecting late Quaternary units accumulated into the foreland basin of the Northern Apennines chain. Beneath the study alluvial plain, compressive fault-fold structures are seismically active. We reconstructed the stratigraphic architecture and the depositional evolution of the alluvial deposits, which accumulated in the first 40 m of subsurface, through the last 45,000 years, from before the Last Glacial Maximum to the present. A 58 km-long stratigraphic profile was correlated from the foothill belt near Bologna to the vicinity of the Po River. The analysis of the profile documents subsidence movements through the last 12,000 years, exceeding − 18 m in syncline areas, with subsidence rates of at least 1.5 m/ka. Anticlines areas experienced a much lower subsidence than the syncline ones.

The composition and texture of liquefied sands represent an important tool for the recognition of buried source layers and for a better understanding of earthquake-induced liquefaction mechanisms. The earthquakesimulating field experiment (blast test) carried out in 2016 in fluvial sediments of the Emilia plain induced subsurface liquefaction and the surface expulsion of sand as sand blows. The area was widely affected by liquefaction phenomena during the Mw 6.1 Emilia earthquake (2012). The grain size and composition of sand blows ejected during the blast test have been compared with various horizons of buried fluvial sediments as deep as 20 m, and with sands from two trenches in the blast site representative of co-seismic 2012 liquefied sands. The sands from the cores show a clear trend from shallow lithoarenitic to deeper quartz-feldspar-rich compositions. The sands at shallow depth (up to 7.7 m) are the most lithoarenitic, with fine-grained sedimentary rock fragments (shales, siltstones, and limestone) as the dominant lithic type. Lithic fragments derive mostly from erosion of sedimentary terrigenous and carbonate successions of Apenninic affinity. In contrast, deeper sands (at depth . 7.7 m) are enriched in quartz and feldspars and impoverished in lithic fragments, which are similar in character to Po River sands. The composition of ejected sands largely overlap that of the shallow litharenitic Apenninic sands, indicating that liquefaction processes affected mainly sand layers at relatively shallow depth (5.9-7.7 m). Textural parameters show that silty sands and silts characterized by relatively high content of fines can also liquefy. This is in contrasts to most of the literature, where fine-grained sediments are considered as incapable of generating the high pore pressures commonly associated with liquefaction. This result should be considered when estimating the liquefaction as a potential hazard. Moreover, we observe that there is a selective loss of fines in the clastic dikes and sand volcanoes relative to the source beds, indicating that the liquefaction process appears to preferentially select the diameters of the grains that reach the ground surface, probably following the generated excess pore-water pressure. This may have caused the segregation and dispersion of the fine silt-clay content, producing highly sorted sand boils. This effect is easlit observable in both the blast-induced sand boils, and the co-seismic 2012 dikes and sand boils ejected in the same area.