Environmental effects and seismogenic source characterization of the December 2020 earthquake sequence near Petrinja, Croatia

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