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The Mw 6.4 Petrinja earthquake (2020, Croatia) is among the strongest continental earthquakes that occurred in Eastern Europe for decades. In such low-strain contexts, the sparse terrestrial-monitoring (few seismic and geodetic stations) of rare but strong earthquakes often prevents a detailed analysis of their seismic source. Here, we take advantage of > 160 geodetic benchmarks and optical image correlation to obtain a dense near-field coverage of the coseismic surface displacements. The geodetic dataset is obtained by repeated measurements of benchmark networks designed for civilian purposes and constitutes a unique dataset of coseismic displacements in the near-field of the fault. The optical image correlation is based on pre-earthquake (December 2017) WorldView and post-earthquake (February 2021) Pleiades satellite images with a 50 cm resolution. We also complete these displacement fields with unwrapped coseismic interferograms based on Sentinel-1 products, except in the near field affected by decorrelation. These displacement fields are consistent and thus suitable for modeling the slip distribution of the Petrinja earthquake. The elastic inversion of the geodetic benchmarks revealed interesting characteristics of this event: the rupture occurred on a near-vertical strike-slip fault, at a shallow depth (< 10 km), with significant slip reaching the surface. It also suggests that the deformation was partly accommodated by a subparallel strand 2.5 km from the main source northward. The aim of this research is to improve the source model of Petrinja 2020earthquake sequence, with a joint inversion of the geodetic benchmarks, optical image correlation, and InSAR data. Nevertheless, the comparison of the geodetic and coseismic offsets measurement on the field, shows that > 70% of the slip is likely distributed at the surface. Moreover, the coseismic strain maps derived from the unique benchmark data set helped us to identify zones where deformation appears distributed. Finally, the new data raises questions about whether such moderate earthquakes are accompanied by subsurface off-fault deformation or residual elastic strain.

The most recent major earthquake series struck near Petrinja (December 29th 2020 M 6.2), and triggered extensive ground failures in the wider area of Petrinja, Sisak and Glina. Coseismic ground failures including subsidence dolines, liquefaction and landslides have been documented over a large area by various experts and teams. These data are stored in the newly created inventory, which is openly presented in this paper. This inventory is administered and updated by the Croatian Geological Survey, and will be available online via a Web Map Service (WMS) (www.hgi-cgs.hr). The aim of the inventory is to not only provide data for the development of susceptibility maps and more detailed exploration for possible remediation measures, but also to define the priorities for immediate action. The earthquake triggered the rapid development of dropout dolines which endanger the local populations of the villages of Mečenčani and Borojevići. This is still an ongoing process in the vicinity of the houses and therefore in-situ exploration started immediately. Liquefaction related to alluvial sediments of the Sava, Kupa and Glina rivers occurred almost exclusively in loose and pure sands, and was accompanied by sand boils, subsidence and lateral spreading. Liquefaction also presents a greater hazard because settlement of houses and river embankments occurred. Lateral spreading caused failures of river flood embankments and natural river banks. According to the data known to date, the majority of the coseismic landslides were reactivated with minor displacements. Despite that, it has been recognised that houses at the edge, or in landslide colluvium suffered greater damage than other houses located outside the landslide impact zone.

Ivanščica Mt. is an inselberg in the transitional area where S-verging Southern Alpine structures overprint NNW-verging Dinaric structures (van Gelder et al. 2015, and references therein). It is composed of Triassic to Cretaceous shallow to deep-marine sedimentary succession of the Adriatic continental passive margin (Šimunić et al. 1976), overthrust by ophiolitic mélange (Babić et al. 2002). Early Cretaceous siliciclastic flysch-type deposits continuously overlaying pelagic limestones clearly indicates distal position of these successions on the continental margin. However, so far complete Jurassic succession on Mt. Ivanščica was never found, causing different interpretations of its Mesozoic history. Babić (1974) assumed the existence of Jurassic pelagic succession on top of the Late Triassic shallow-water carbonates, describing up to few meters of Early to Middle Jurassic condensed pelagic limestone overlain by Late Jurassic radiolarian cherts and pelagic limestones. Contrary, Šimunić et al. (1976) assumed shallow-marine conditions throughout the Early Jurassic followed by emersion until latest Jurassic. Our study revealed for the first time complete shallow-water to pelagic Jurassic succession on Mt. Ivanščica. Southern slopes of Ivanščica Mt. are mostly built of ophiolitic mélange, except for prominent hills built of the Late Triassic Dachstein limestone, which were so far interpreted as klippes (Šimunić et al. 1976), or olistoliths (Babić & Zupanič 1978). Our new data indicate continuous succession on top of Dachstein limestones composed of shallow-marine carbonates, represented by intraclastic-peloidal packstones to grainstones, gradually transitioning to wackestones with pelagic influence. The onset of pelagic sedimentation took place around the end of the Early Jurassic when thin bedded marls, shales, marly limestones with intercalated fine-grained calciturbidites were deposited. Higher in the succession Callovian to possibly early Tithonian radiolarian cherts are overlaid by calcarenites, late Tithonian to Early Cretaceous pelagic limestones and Early Cretaceous flysch-type deposits. Discovery of the Jurassic pelagic sediments allows for a new interpretation of structural relations on the Ivanščica Mt. In our opinion occurrence of Dachstein limestone, previously interpreted as klippes or olistoliths represent an imbricate fan. Because the Southern Alps thrust front in this area was interpreted according to these mapped klippes and nappe contact, our findings may also have an impact on the redefining of the easternmost Southern Alps thrust front. Babić, Lj. (1974). Jurassic−Cretaceous sequence of Mt. Ivanščica (North Croatia). Bull. Sci. Cons. Acad. Yugoslav. (A), 19/7−8, 180−181. Zagreb. Babić, Lj. & Zupanič, J. (1978). Mlađi mezozoik Ivanščice. In Babić, Lj. & Jelaska, V. (ured.): Fieldtrip Guidebook 3. Skupa sedimentologa Jugoslavije, 11–23. Hrvatsko Geološko Društvo, Zagreb. Babić, Lj., Hochuli P. A.. & Zupanič, J. (2002). The Jurassic ophiolitic mélange in the NE Dinarides: dating, internal structure and geotectonic implications. Eclogae Geologicae Helvetiae 95, 263–75. Šimunić, A., Pikija, M., Šimunič, Al., Šikić, K. & Milanović, M. (1976): Stratigrafsko - tektonski odnosi centralnog i istočnog dijela Ivanščice. 8. Jugosl.geol.kongres, Bled, 1974., 2, 3003-313, Ljubljana. van Gelder, I., Matenco, L., Willingshofer, E., Tomljenović, B., & Andriessen, P., Ducea, M., Beniest, A. & Gruić, A. (2015). The tectonic evolution of a critical segment of the Dinarides-Alps connection: Kinematic and geochronological inferences from the Medvednica Mountains, NE Croatia. Tectonics. 34. n/a-n/a. 10.1002/2015TC003937.

The Middle Triassic period represents a very dynamic time in a broader Tethyan region. Tectonic movements related to disintegration of Pangea and opening of the Neotethys gave rise to the formation of volcanic and volcaniclastic deposits. In the NW Croatia, these rocks outcrop in approximately 60 km long intra-Pannonian Mountain chain (Mts. Kalnik, Ivanščica, Strahinjščica, Kuna Gora, Desinić Gora and Ravna Gora), representing the junction between the Southern Alps and Internal Dinarides. Among these mountains, Mt. Ivanščica represents a complex structure built of shallow-marine to pelagic successions originating off the passive continental margin of Adria microplate. These deposits are found in a tectonic contact with ophiolitic mélange containing remnants of Neotethys. Middle Triassic volcanic and volcaniclastic rocks from this area include basic/intermediate to acidic effusive and pyroclastic lithologies and are interfingered with marine sediments. The specific area of Vudelja quarry, situated in the central part of the northern Ivanščica slopes, is composed mainly of mafic lithologies and their volcaniclastic derivates. Volcanic and volcaniclastic rocks of basaltic composition were studied in detail to distinguish different facies and their spatial distribution. Facies characteristics enabled the interpretation of their genesis. Three different facies were recognized: 1) effusive coherent facies; 2) pyroclastic flow facies, and 3) resedimented autoclastic facies. Effusive coherent facies is composed of hyaline basalts, with incorporated clasts of the same lithotype. This facies was formed by effusion of basaltic magma. Basaltic clasts were likely incorporated by the primary effusive flow during magma ascent, or following effusion while flowing over the fragmented basaltic material. Quenching fragmentation of a basaltic effusion in the marine environment is another possible process explaining the formation of basalt clasts. Pyroclastic flow facies is composed of plagioclase crystalloclasts, basaltic lithoclasts and scoria fragments, with the flow texture indicated by clast arrangement and glassy matrix with flow features. This facies occurs only locally and its geographical extent is limited thus indicating a small volume of the flow, characteristic for basaltic pyroclastic flows of scoria and ash type. Resedimented autoclastic facies is dominant at the outcropping quarry front and is composed of several lithotypes with various grain sizes and types of matrix. Clast dimensions vary from block to ash size. Some samples exhibit common sedimentary features such as horizontal lamination. The facies was formed by the autofragmention processes of hot basaltic magma in contact with sea water, and subsequent resedimentation of newly formed volcaniclastic particles. Due to an intense tectonic disruption, spatial organization of determined facies is not clear, though the alienation from the primary source can generally be followed from south to north. Studied locality presents a portion of Middle Triassic volcanic and volcano-sedimentary formations well known from the Southern Alps, Transdanubian Range and the Dinarides. Intense volcanic activity, related to the rifting between the future Adria microplate and southern edge of Laurasia, fed the material to a complicated pattern of sedimentary environments formed along the future Adria passive margin. Extensional tectonics created deep faults within the continental crust which might have served as conduits for submarine basaltic extrusions.

The Petrinja-Pokupsko fault-system is a NW-SE right-lateral fault system that ruptured during the 29 December 2020 Mw 6.4 earthquake (~40km south-east of Zagreb, Croatia). Field analysis revealed opening of cracks and offsets of several centimeters (3 to 40 cm) along a ~20 km long fault zone extending from the Kupa river (in the northwest) to the Petrinjčica river (in the southeast). Optical image correlation based on WorldView satellite images has been used to document the first-order near-field rupture signal. The pre-event image was acquired on 7th December 2017, and the post-event image on 15th January 2021. The first results indicate a right-lateral displacement of ≈75 cm with a small (6 earthquakes. Since the fault extends farther NW and SE, from the Vukomeričke Gorice hills to Mount Kozara (Bosnia), for a total length of about 100 km, it could generate potentially larger events. It is also noteworthy that the 2020 Petrinja event occurred only 9 months after the Zagreb March 2020 (Mw 5.3) earthquake. This event occurred on an ENE-WSW-trending thrust fault, broadly orthogonal to the right-lateral Petrinja-Pokupsko fault system, ~45 km north of Petrinja, and raises the prospect of potential interplay between strike-slip and thrust faults in moderate strain-rate intra-plate settings. To address this problem, future works will aim at constraining the geometry of this fault network and its seismogenic potential.

Earthquakes and related coseismic effects at the surface, both primary and secondary, such as liquefaction and lateral spreading, can impact humans due to induced economic or social disruptions (e.g. slope, bridge and building foundation failures, flotation of buried structures). In this respect, it results of primary interest to map liquefaction induced evidences soon after an earthquake. On the 29th December 2020, a major earthquake (Mw 6.4) occurred in Croatia, close to Petrinja, 45 km south of Zagreb, generating widespread liquefaction and lateral spreading phenomena in a radius of approximately 20 km from the epicentre. A European team of researchers (geologists and engineers), in strict collaboration with the Croatian Geological Survey, performed field reconnaissance campaigns with the aim to provide a detailed identification and characterization of the primary and secondary geological and geotechnical coseismic effects induced by the Croatian earthquakes. Specifically with reference to the liquefaction phenomena, the Working Group integrated the data collected directly in the field with those from remote survey by drone aerial photos acquired in the post-event immediate. The adopted process allowed the collection of the liquefaction record with the highest possible completeness both in terms of pattern and distribution of the phenomena. The database includes several detailed case studies typified by the following characteristics: (1) liquefaction occurring on alluvial plain sites (Kupa river, Sava river and Glina river); (2) blows made by sand and/or gravel with local presence of shells and armored mud balls; (3) lateral spreading phenomena along road and river embankments; (4) sand ejecta of different grain size and matrix, even at the same site; (5) sand and/or gravel ejecta along fault traces. The characteristics of these features are discussed with reference to the alluvial setting and tectonic context. All together, the detailed survey of these recent liquefaction features will assist to build new empirical relations, to update the existing ones and to mitigate the effects of future earthquakes recognizing liquefaction prone areas for a correct land use planning, as for seismic microzonation studies.

The earthquake with magnitude ML=6.2 that occurred on 29th December 2020 has caused significant material damage to objects and infrastructure in the towns of Petrinja, Sisak,Glina and the surrounding area. According to the satellite interferometry data, the coseismic and postseismic deformation area covers around 500 square kilometers. The existing geodetic benchmarks have been set in the affected towns, and their coordinates have been determined based on previous GPS campaigns. The GPS network was set up and adjusted at the State Geodetic Administration's request for geodetic monitoring of infrastructure and cadastral projects. These points are not primarily intended for high accuracy measurements at the level of a few millimeters, so their accuracy and the absolute shift concerning geodynamic processes in the region should be taken into account. Nevertheless, the data obtained by their observation after the earthquake can provide valuable information about the horizontal and vertical displacements with a certain level of confidence. The field survey has detected disappearance of a large number of benchmarks and some valuable information has been lost. Still, 58 points were found and observed and it has been concluded that 52 points are reliable and can be used for future research. Because the network of benchmarks is not developed in rural areas, there is a gap in the distribution of benchmarks in affected area. Therefore, the additional data was collected using the benchmarks established for the engineering and cadastral projects and studies. From a total of 67 points that have been found and observed, 42 points will be used. Along with the data collected in urban areas, there will be a total of 94 benchmarks. The accuracy of the geodetic benchmark measurements is at the centimeter level, while the values of deformation are at the level of a few decimeters. Therefore, the obtained data can be used to better assess the displacement recorded during the 29 December 2020 event. In the future, field research will focus on finding additional benchmarks to reach a better spatial distribution.

Mt. Ivanščica is one of inselbergs in the Internal Dinarides (NW Croatia) in the transitional area with Southern Alps. In this area, NNW-verging Dinaric structures are overprinted by S-verging Alpine structures. Mt. Ivanščica is composed of Mesozoic shallow to deep-marine sedimentary succession of the passive continental margin of Adriatic plate, which was facing the Neotethys ocean, overthrust by ophiolitic mélange. Here, we aim to present new preliminary structural data from pelagic sediments of Ivanščica Mt. in attempt to better understand tectonic history of this part of Internal Dinarides.Mesozoic succession of Mt. Ivanščica is composed of Triassic clastic, volcanic and carbonate rocks overlain by Upper Triassic to Lower Jurassic shallow-marine carbonates. These are overlain by Jurassic pelagic carbonates and cherts followed by Tithonian−Valanginian pelagic “Aptychus Limestones”. The uppermost part of this succession is composed of Lower Cretaceous Oštrc fm., which conformably overlies the “Aptychus Limestone”. The Oštrc fm. is characterized by turbidites with ophiolitic detritus and represents syn-orogenic deposits presumed as formed in a front of advancing ophiolitic nappe(s).The focus of our investigation is primarily on structural characteristics of the “Aptichus Limestones” and the Oštrc fm. The character of the contact between the “Aptychus Limestones” and underlying Upper Triassic to Lower Jurassic carbonates is still uncertain. According to Šimunić et al. (1982) “Aptychus limestones” unconformably overlays Triassic carbonates in periclinal geometry, while Babić (1974) suggests continuous condensed pelagic sedimentation throughout the Jurassic. In contrast with previous observations and interpretations, our observations suggest a tectonic contact, characterized by significantly different orientation of bedding and locally marked by fault gauge (clay) seams.Structural analysis shows numerous gentle to open asymmetric folds in the “Aptychus Limestones” and closed chevron folds and isoclinal folds in overlaying Oštrc fm. Chevron folds and open to gentle asymmetric folds indicate NW vergence in present day orientation with fold axis parallel to the strike of the contact with underlying unit. Although different in shape and size, these folds are likely formed during the same tectonic event while their geometry is controlled by differences in rheological properties. Isoclinal folds occurring exclusively at the contact with ophiolitic mélange are characterized by E-W oriented fold axis and S dipping axial surfaces which is in a contrast with aforementioned folds. Thus, we assume that these folds originated from another, presumably older tectonic event. Bedding in Triassic dolomites uniformly dips towards the SE. Local occurrence of condensed pelagic limestones and radiolarian cherts is interpreted, as rheologically weak horizon ideal to form a décollement that, at least locally, could be interpreted to mark a thrust fault.Formation of isoclinal folds in the Oštrc fm. and the tectonic contact with ophiolitic mélange is preliminarily attributed to the Aptian-Albian nappe stacking known from the Internal Dinarides. In addition, we assume that the pelagic succession of the “Aptychus Limestones” together with the overlying Oštrc fm. and the ophiolitic mélange are thrusted over the Upper Triassic to Liassic carbonates sometime later, possibly during the final stage of Neotethys closure in the Internal Dinarides.

After the earthquake of 29/12/2020 in Petrinja (ML6.2, ImaxVIII-IX EMS), an attempt was made to characterize the active structure associated with the earthquake. As a first step towards this goal, we performed a geomorphological analysis in order to contribute to the identification and characterization of the surface expression of the active Pokupsko dextral strike-slip fault. We focused on the area between the southernmost parts of Vukomeričke Gorice and the southernmost parts of Hrastovica Mountain, where the NW-SE striking Pokupsko fault has slipped during and after the recent earthquake (Ganas et al., 2021). Using available 1 : 5 000 scale topographic maps and various 10 m resolution digital elevation model visualizations, we mapped lineaments that could represent relatively recently active fault segments. We used a quantitative approach to perform stream sinuosity analysis (e.g., Leopold et al., 1964; Zamolyi et al., 2010) on major streams crossing the structure to identify distinct changes in channel patterns that may be associated with vertical movement along the predominantly strike-slip fault. We observed changes in the shape of the valleys, especially the changes in width, height, and direction. By summarizing various geomorphological indicators of active fault segmentation at the surface with available geological data (Pikija, 1987) and so far limited field observations, we provide insights into the structure of the Pokupsko fault.Preliminary results show good agreement between lineament mapping, changes in valley shape, changes in the stream sinuosity index, and (to some extent) previously mapped faults. In addition, some of the changes in stream sinuosity correspond to locations where coseismic surface ruptures occurred during the December 29 earthquake (Budić et al., this session; Pollak et al., 2021). Results suggest that the several-kilometer-wide zone of uplifted Neogene deposits results from the dextral-transpressive structure, which at the surface consists of a series of subparallel fault strands branching off the main fault that runs along the SE slopes of the Hrastovica Mountain. The SW-most fault strands are associated with significant changes in the shape of the valleys: the wide valleys of Petrinjčica, Utinja and Šanja change to narrow and deeply incised as they cross the uplifted structure. Paleocene and Eocene rocks, which otherwise underlie the Neogene, outcrop in the NE parts of the fluvial breakthrough valleys, indicating the uplift of the Hrastovica Mountain. Topographic data show a decrease of the mountain range elevation towards the SW. This evidence suggests that the main fault runs on the NE side of the mountain, strikes NW-SE and dips steeply towards the SW. The fault strike deviates between Župić and Farkašić. The fault plane solution for the December 29 earthquake suggests a nearly pure strike-slip fault, while geomorphic evidence strongly indicates areas of active uplift along the fault, further supported by the general antiformal structure. We interpret this as an indication of either a general current transpressional character of the fault or as local kinematic variations due to segmentation and changes in the strike of the fault; further analyses are pending.