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.
A detailed sedimentological features of the Slatinski plaz section with a transition from the deep-water Anisian (Illyrian) Strelovec Formation to the Ladinian shallow-water Contrin Formation have been presented. The Strelovec Formation is divided into five different lithostratigraphic units that are characterised by dark hemipelagic and pelagic thin-bedded limestones and dolomitic mudstones that are frequently intercalated with deposits of gravitymass flows, slump and slide to fine-grained low-density turbidity origin. The Strelovec Formation was deposited in a hydrodynamically quiet, pelagic deeper-water anoxic environment, most probably on a gentle platform slope. In the upper part of the formation, the presence of olistolith blocks of shallow-water limestones indicates the closing of the prograding shallow-water platform wedge. Massive dolostones of Unit 6 mark the complete filling of the basin and the beginning of the shallow-water sedimentation of the Contrin Formation in the Early Ladinian. Izvle~ekV članku predstavljamo detajlni sedimentološki profil Slatinski plaz, v katerem lahko opazujemo prehod iz anizijske (ilirske) globljevodne Strelovške formacije v ladinijsko plitvomorsko Contrinsko formacijo. Strelovško formacijo smo razdelili na pet litostratigrafskih enot, za katere je značilno menjavanje temnih hemipelagičnih tankoplastnatih apnencev in dolomitov (mudstone), med katerimi so pogosti sedimenti gravitacijskih tokov (od zdrsov do zdrobnozrnatih turbiditov). Strelovška formacija se je odlagala v hidrodinamsko mirnem, pelagičnem, globljevodnem anoksičnem okolju, najverjetneje na blagem pobočju. V zgornjem delu formacije se pojavijo olistolitni bloki plitvovodnih apnencev, ki nakazujejo približevanje progradacijskega klina napredujoče platforme. Šesta litostratigrafska enota vsebuje plivovodne kamnine zgodnjeladinijske Contrinske formacije, ki kažejo, da se je globljevodni bazen v tem času že popolnoma zasul. GEOLOGIJA 59/1, 23-34, Ljubljana 2016 http://dx
We present the compilation of a new database of active faults in Slovenia, aiming at introducing geological data for the first time as input for a new national seismic hazard model. The area at the junction of the Alps, the Dinarides, and the Pannonian Basin is moderately seismically active. About a dozen Mw > 5.5 earthquakes have occurred across the national territory in the last millennium, four of which in the instrumental era. The relative paucity of major earthquakes and low to moderate fault slip rates necessitate the use of geologic input for a more representative assessment of seismic hazard. Active fault identification is complicated by complex regional structural setting due to overprinting of different tectonic phases. Additionally, overall high rates of erosion, denudation and slope mass movement processes with rates up to several orders of magnitude larger than fault slip rates obscure the surface definition of faults and traces of activity, making fault parametrization difficult. The presented database includes active, probably active and potentially active faults with trace lengths >5 km, systematically compiled and cataloged from a vast and highly heterogeneous dataset. Input data was mined from published papers, reports, studies, maps, unpublished reports and data from the Geological Survey of Slovenia archives and dedicated studies. All faults in the database are fully parametrized with spatial, geometric, kinematic and activity data with parameter descriptors including data origin and data quality for full traceability of input data. The input dataset was compiled through an extended questionnaire and a set of criteria into a homogenous database. The final database includes 96 faults with 240 segments and is optimized for maximum compatibility with other current maps of active faults at national and EU levels. It is by far the most detailed and advanced map of active faults in Slovenia.
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